Amid global climate change, rising levels of nitrogen(N) deposition have attracted considerable attention for their potential effects on the carbon cycle of terrestrial ecosystems. The desert steppes are a crucial yet...Amid global climate change, rising levels of nitrogen(N) deposition have attracted considerable attention for their potential effects on the carbon cycle of terrestrial ecosystems. The desert steppes are a crucial yet vulnerable ecosystem in arid areas, but their response to the combination of N addition and precipitation(a crucial factor in arid areas) remains underexplored. This study systematically explored the impact of N addition and precipitation on net ecosystem exchange(NEE) in a desert steppe in northern China. Specifically, we conducted a 2-a experiment from 2022 to 2023 with eight N addition treatments in the Urat desert steppe of Inner Mongolia Autonomous Region, China, to examine changes in NEE and explore its driving factors. The structural equation model(SEM) and multiple regression model were applied to determine the relationship of NEE with plant community characteristics and soil physical-chemical properties. Statistical results showed that N addition has no significant effect on NEE.However, it has a significant impact on the functional traits of desert steppe plant communities. SEM results further revealed that N addition has no significant effect on NEE in the desert steppe, whereas annual precipitation can influence NEE variations. The multiple regression model analysis indicated that plant functional traits play an important role in explaining the changes in NEE, accounting for 62.15% of the variation in NEE. In addition, plant height, as an important plant functional trait indicator, shows stronger reliability in predicting the changes in NEE and becomes a more promising predictor. These findings provide valuable insights into the complex ecological mechanisms governing plant community responses to precipitation and nutrient availability in the arid desert steppes, contributing to the improved monitoring and prediction of desert steppe ecosystem responses to global climate change.展开更多
The carbon （C） sequestration potential of turfgrass systems has been investigated and demonstrated from several studies. The role of these ecosystems in continental and Mediterranean climates though, is not yet clea...The carbon （C） sequestration potential of turfgrass systems has been investigated and demonstrated from several studies. The role of these ecosystems in continental and Mediterranean climates though, is not yet clearly understood because environmental limiting factors and management intensities can strongly influence the overall C budget. The aim of the present study is to improve the understanding of the mechanisms underlying C fluxes in a turfgrass ecosystem and to assess its C sequestration potential by estimating the annual C budget. NEE （Net Ecosystem Exchange） of turfgrass was calculated in its seasonal variation over one year, and compared between areas characterized by different degrees of maintenance. The C sequestration potential of the turfgrass was investigated in a golf course near Verona （Italy）, adopting a small-chamber enclosure approach. The measurements of gas exchanges between biosphere and atmosphere, permitted to estimate the NEE, as a function of different management intensities. The intensity of management seems to have influence on its C balance. This study needs further research to understand which maintenance variables are determinant on turfgrass C sequestration.展开更多
In 2022, an eddy covariance site was established in a young oil palm plantation in southeast Dangbo, Bénin, to study the exchange of CO2, energy, and water vapor. This study aims to present the first one-year ana...In 2022, an eddy covariance site was established in a young oil palm plantation in southeast Dangbo, Bénin, to study the exchange of CO2, energy, and water vapor. This study aims to present the first one-year analysis of seasonal dynamics in energy balance components and net ecosystem exchange above this type of ecosystem in Africa. The first results show that on average during the 2023 year, 55% of net radiation is consumed into actual evapotranspiration, demonstrating the significant amount of latent heat flux in the energy balance, as expected at this tropical humid site. The sensible heat flux was substantial, ranging between 60 and 200 W·m−2, while net radiation varied between 440 and 650 W·m−2. Carbon uptake and net release of CO2 into the atmosphere were permanent at the site. However, the CO2 uptake increases more when rainy events become regular. On average, the mean nighttime CO2 flux was ~8 µmol·m−2·s−1, while during the daytime it was ~−20 µmol·m−2·s−1.展开更多
Aims Boreal forests play an important role in the global carbon cycle.Compared with the boreal forests in North America and Europe,relatively few research studies have been conducted in Siberian boreal forests.Knowled...Aims Boreal forests play an important role in the global carbon cycle.Compared with the boreal forests in North America and Europe,relatively few research studies have been conducted in Siberian boreal forests.Knowledge related to the role of Siberian forests in the global carbon balance is thus essential for a full understanding of global carbon cycle.Methods This study investigated the net ecosystem exchange(NEE)during growing season(May-September)in an eastern Siberian boreal larch forest for a 3-year period in 2004-2006 with contrasting meteorological conditions.Important FindingsThe study found that the forest served as a carbon sink during all of the 3 studied years;in addition,the meteorological conditions essentially influenced the specific annual value of the strength of the carbon sinks in each year.Although 2005 was the warmest year and much wetter than 2004,2005 also featured the greatest amount of ecosystem respiration,which resulted in a minimum value of NEE.The study also found that the phenological changes observed during the three study years had a relatively small effect on annual NEE.Leaf expansion was 26 days earlier in 2005 than in the other 2 years,which resulted in a longer growing season in 2005.However,the NEE in 2005 was counterbalanced by the large rate of ecosystem respiration that was caused by the higher temperatures in the year.This study showed that meteorological variables had larger influences on the interannual variations in NEE for a Siberian boreal larch forest,as compared with phenological changes.The overall results of this study will improve our understanding of the carbon balance of Siberian boreal larch forests and thus can help to forecast the response of these forests to future climate change.展开更多
<span style="font-family:Verdana;">The eddy covariance technique is an accurate and direct tool to measure the Net Ecosystem Exchange (NEE) of carbon dioxide. However, sometimes conditions are not amen...<span style="font-family:Verdana;">The eddy covariance technique is an accurate and direct tool to measure the Net Ecosystem Exchange (NEE) of carbon dioxide. However, sometimes conditions are not amenable to measurements using this technique. Thus, different methods have been developed to allow gap-filling and quality assessment of eddy covariance data sets. In this study first, two different Artificial Neural Networks (ANNs) approaches, the Multi-layer Perceptron (MLP) trained by the Back-Propagation (BP) algorithm, and the Radial Basis Function (RBF), were used to fill missing NEE data measured above rain-fed maize at the University of Nebraska-Lincoln Agricultural Research and Development Center near Mead, Nebraska. The gap-filled data were then compared by different statistical indices to gap-filled data obtained with the technique suggested by Suyker and Verma in 2005 [S&V method], and the ANN approach presented by Papale in 2003. The results showed that the RBF network was able to find better fits for missing values compared to the MLP (BP) network and S&V method. In addition, unlike the S&V method, which depends on different gap-filling procedures over the year;the structure of RBF and MLP (BP) networks was constant. However, data analysis indicated Papale’s approach gave better fits than the RBF and MLP (BP) methods. Thus, based on this work, Papale’s approach is the best method to estimate the missing data;though the applied statistical indices, which were used for model evaluation, show little difference between Papale’s approach and the RBF and MLP (BP).</span>展开更多
The estimation of carbon exchange between ecosystems and the atmosphere suffers unavoidable data gaps in eddy-covariance technique, especially for short-living and fast-growing croplands. In this study we developed a ...The estimation of carbon exchange between ecosystems and the atmosphere suffers unavoidable data gaps in eddy-covariance technique, especially for short-living and fast-growing croplands. In this study we developed a modified gap-filling scheme introducing a leaf area index factor as the vegetation status information based on the conventional light response function for two East-Asian cropland sites (rice and potatoes). This scheme’s performance is comparable to the conventional time window scheme, but has the advantage when the gaps are large compared to the total length of the time series. To investigate how the time binning approach performs for fast-growing croplands, we tested different widths of the time window, showing that a four-day window for the potato field and an eight-day time window for the rice field perform the best. The insufficiency of the conventional temperature binning approach was explained as well as the influence of vapor pressure deficit. We found that vapor pressure deficit plays a minor role in both the potato and the rice fields under Asian monsoon weather conditions with the exception of the early pre-monsoon growing stage of the potatoes. Consequently, we recommend using the conventional time-window scheme together with our new leaf-light response function to fill data gaps of net ecosystem exchange in fast-growing croplands.展开更多
On the Tibetan Plateau, the alpine meadow is the most widespread vegetation type. The alpine meadow has a low biological productivity and low vegetation coverage in the growing season. The daytime NEE between the atmo...On the Tibetan Plateau, the alpine meadow is the most widespread vegetation type. The alpine meadow has a low biological productivity and low vegetation coverage in the growing season. The daytime NEE between the atmosphere and the alpine meadow ecosystem was influenced by solar radiation. To analyze the characteristics of change in NEE and to calculate the parameters related to photosynthesis and respiration in different solar radiation environments, the NEE measurements were taken in Damxung from July to August in 2003, 2004, 2005 and 2006 using the eddy covariance technique. Solar radiation was grouped into three levels according to the net radiation, which was more than 155 W m-2 d-1 on clear days, 144±5 W m-2 d-1 on partly cloudy days and less than 134 W m-2 d-1 on cloudy days. The diurnal relationships between NEE and PAR varied with differences in solar radiation, which was a rectangular hyperbola form on clear days, two different concave curves on partly cloudy days and an irregular triangle form on cloudy days. The mean CO2 absorption rate showed a decreasing trend with increasing solar radiation. The daytime absorption maximum occurred around 10：00 on clear days with an average of slightly less –0.2 mg m-2 d-1, around 11：00 on partly cloudy days with an average of about –0.2 mg m-2 d-1, and around 12：00 on cloudy days with an average of about –0.25 mg m-2 d-1. As solar radiation increased, the Amax and the Q10 decreased. However, the R10 increased and the maximum of the α occurred on partly cloudy days. The optimum net solar radiation was about 134–155 W m-2 d-1, which induced a PAR of about 1800-2000 μmol m-2 s-1 and soil temperature at a depth of 5 cm of about 14℃. Therefore, on the Tibetan Plateau, the alpine meadow ecosystem will have a higher carbon absorption potential while solar radiation decreases in the future.展开更多
The environmental impact of aerosols is currently a hot issue that has received worldwide attention. Lacking simultaneous observations of aerosols and carbon flux, the understanding of the aerosol radiative effect of ...The environmental impact of aerosols is currently a hot issue that has received worldwide attention. Lacking simultaneous observations of aerosols and carbon flux, the understanding of the aerosol radiative effect of urban agglomeration on the net ecosystem carbon exchange(NEE) is restricted. In 2009-2010, an observation of the aerosol optical property and CO_(2) flux was carried out at the Dongguan Meteorological Bureau Station(DMBS) using a sun photometer and eddy covariance systems. The different components of photosynthetically active radiation(PAR),including global PAR(GPAR), direct PAR(DPAR), and scattered PAR(FPAR), were calculated using the Santa Barbara DISORT Atmospheric Radiative Transfer(SBDART) model. The effects of PAR on the NEE between land-atmosphere systems were investigated. The results demonstrated that during the study period the aerosol optical depth(AOD)reduced the DPAR by 519.28±232.89 μmol photons · m^(-2)s^(-1), but increased the FPAR by 324.93±169.85μmol photons ·m^(-2)s^(-1),ultimately leading to 194.34±92.62 μmol photons · m^(-2)s^(-1);decrease in the GPAR. All the PARs(including GPAR,DPAR, and FPAR) resulted in increases in the NEE(improved carbon absorption), but the FPAR has the strongest effect with the light use efficiency(LUE) being 1.12 times the values for the DPAR. The absorption of DPAR by the vegetation exhibited photo-inhibition in the radiation intensity > 600 photons · m^(-2)s^(-1);in contrast, the absorptions of FPAR did not exhibit apparent photo-inhibition. Compared with the FPAR caused by aerosols, the DPAR was not the primary factor affecting the NEE. On the contrary, the increase in AOD significantly increased the FPAR, enhancing the LUE of vegetation ecosystems and finally promoting the photosynthetic CO_(2) absorption.展开更多
Meteorological elements and CO_(2) fluxes over alpine meadow ecosystem were observed continuously from 2004 to 2005 in Damxung Alpine Meadow Flux Station,China Flux Network.Based on the eddy covariance CO_(2) fluxes a...Meteorological elements and CO_(2) fluxes over alpine meadow ecosystem were observed continuously from 2004 to 2005 in Damxung Alpine Meadow Flux Station,China Flux Network.Based on the eddy covariance CO_(2) fluxes and meteorological data obtained,the relationships among the CO_(2) fluxes,the cloud amount,and the meteorological factors in alpine meadow ecosystem were explored and analyzed.Some conclusions can be drawn from the discussion with previous researches as following:(1)the cloud amount can affect the net ecosystem CO_(2) exchange(NEE)of alpine meadow on Tibetan Plateau;(2)the soil temperature sensitive to the cloud amount,is a major environmental controlling factor for NEE,and closely relates to the maximum of NEE.In the moming period with large cloud amount,the NEE reaches its maximum when the clearness index ranges from 0.5 to 0.7;yet in the afternoon it comes to the maximum with the index from 0.2 to 0.35.The span of soil temperature covers from 12 to 15℃as the NEE at its highest;(3)the scatterplots between NEE and photosynthetic available radiation(PAR)was a significant inverse triangle in the clear day,two different kinds of concave curves in the cloudy day,and strongly convergent rectangular hyperbola in the overcast day.These differences were controlled by the changes of light radiation and soil temperature.展开更多
Alpine meadows,alpine wetlands,and alpine desert steppes are the three typical vegetation types on the Qinghai-Tibet Plateau.The complex terrain and harsh climatic conditions across this region lead to considerable di...Alpine meadows,alpine wetlands,and alpine desert steppes are the three typical vegetation types on the Qinghai-Tibet Plateau.The complex terrain and harsh climatic conditions across this region lead to considerable diversification in the vegetation growth environment,resulting in substantial spatial heterogeneity in ecosystem carbon flux and its controlling mechanisms.Using eddy covariance data collected from March to August 2019,this study examined the responses of carbon and water fluxes in different ecosystems on the Tibetan Plateau to typical hydrometeorological factors,focusing on Net Ecosystem CO□Exchange(NEE)and Evapotranspiration(ET).The results indicate that:1)The Longbao alpine wetland primarily acted as a carbon sink from May to August,while serving as a carbon source from March to April.In the Maqin alpine meadow,it functioned as a carbon sink during June and July but acted as a carbon source in March,April,May,and August.The Tuotuohe alpine desert strppe was predominantly a net carbon sink from March to August.Overall,after the entire growing season(March to August),the Longbao alpine wetlands,Maqin alpine meadow,and Tuotuohe alpine desert steppe all showed net carbon sink properties,with net CO_(2)uptakes of 236.12 g/m^(2),291.45 g/m^(2),and 290.28 g/m^(2),respectively.2)The importance of meteorological factors to NEE varies with scale and ecosystem type,with global radiation(Rg)being the most critical factor influencing NEE variation.Volumetric soil water content(Soil_VWC)and soil temperature(Soil_T)had a positive effect on NEE at Maqin alpine meadow and Tuotuohe alpine desert steppe,while higher values of these variables showed a negative contribution.Furthermore,the sensitivity of NEE to Soil_T at Longbao alpine wetland and Tuotuohe alpine desert steppe was greater than its sensitivity to air temperature(Tair).3)The effect of Gross Primary Productivity(GPP)on NEE in alpine desert steppes is significantly greater than in alpine meadows.Both Ecosystem Respiration(Reco)and NEE were substantially limited by GPP,with 84%of GPP in alpine wetlands contributing to Reco and 16%to NEE;92%of GPP in alpine meadows contributing to Reco and 8%to NEE;and 40%of GPP in high-altitude desert grasslands contributing to Reco and 60%to NEE.4)The strong correlation between NEE and evapotranspiration suggests that water availability is the primary factor controlling changes in the carbon and water budgets of alpine ecosystems.展开更多
Peatland ecosystems play an important role in the global carbon cycle because they act as a pool or sink for the carbon cycle. However, the relationship between seasonality effect factors and net ecosystem CO<sub&g...Peatland ecosystems play an important role in the global carbon cycle because they act as a pool or sink for the carbon cycle. However, the relationship between seasonality effect factors and net ecosystem CO<sub>2</sub> exchange (NEE) remains to be clarified, particularly for the non-growing season. Here, based on the eddy covariance technique, NEE in the peatland ecosystem of Central China was examined to measure two years’ (2016 and 2017) accumulation of carbon dioxide emissions with contrasting seasonal distribution of environmental factors. Our results demonstrate the cumulative net ecosystem CO<sub>2</sub> emissions during the study period was in the first non-growing season 2.94 ± 4.83 μmolCO<sub>2</sub> m<sup><span style="color:#4F4F4F;font-family:" font-size:14px;white-space:normal;background-color:#ffffff;"="">-</span>2<span style="white-space:nowrap;">.</span></sup>s<sup><span style="color:#4F4F4F;font-family:" font-size:14px;white-space:normal;background-color:#ffffff;"="">-</span>1</sup> with the lowest values in the same year in first growing season was <span style="color:#4F4F4F;font-family:" font-size:14px;white-space:normal;background-color:#ffffff;"="">-</span>2.79 ± 4.92 μmolCO<sub>2</sub> m<sup><span style="color:#4F4F4F;font-family:" font-size:14px;white-space:normal;background-color:#ffffff;"="">-</span>2</sup><span style="font-family:" font-size:13.3333px;white-space:normal;"=""><span style="white-space:nowrap;"><sup>.</sup></span></span>s<sup><span style="color:#4F4F4F;font-family:" font-size:14px;white-space:normal;background-color:#ffffff;"="">-</span>1</sup>. The results indicate the effect of seasonal variations of NEE can be directly reflected in daily and seasonal variations in growth and respiration of peatland ecosystem by environmental parameters over different growing stages.展开更多
Rice-wheat （R-W） rotation systems are ubiquitous in South and East Asia, and play an important role in modulating the carbon cycle and climate. Long-term, continuous flux measurements help in better understanding th...Rice-wheat （R-W） rotation systems are ubiquitous in South and East Asia, and play an important role in modulating the carbon cycle and climate. Long-term, continuous flux measurements help in better understanding the seasonal and interannual variation of the carbon budget over R-W rotation systems. In this study, measurements of CO2 fluxes and meteorological variables over an R-W rotation system on the North China Plain from 2007 to 2010 were analyzed. To analyze the abiotic factors regulating Net Ecosystem Exchange （NEE）, NEE was partitioned into gross primary production （GPP） and ecosystem respiration. Nighttime NEE or ecosystem respiration was controlled primarily by soil temperature, while daytime NEE was mainly determined by photosythetically active radiation （PAR）. The responses of nighttime NEE to soil temperature and daytime NEE to light were closely associated with crop development and photosynthetic activity, respectively. Moreover, the interannual variation in GPP and NEE mainly depended on precipitation and PAR. Overall, NEE was negative on the annual scale and the rotation system behaved as a carbon sink of 982 g C m 2 per year over the three years. The winter wheat field took up more CO2 than the rice paddy during the longer growing season, while the daily NEE for wheat and rice were -2.35 and -3.96 g C m-2, respectively. After the grain harvest was subtracted from the NEE, the winter wheat field became a moderately strong carbon sink of 251-334 g C m-2 per season, whereas the rice paddy switched to a weak carbon sink of 107-132 per season.展开更多
Decreasing wind speed is one aspect of global climate change as well as global warming, and has become a new research orientation in recent decades. The decrease is especially evident in places with frequent perennial...Decreasing wind speed is one aspect of global climate change as well as global warming, and has become a new research orientation in recent decades. The decrease is especially evident in places with frequent perennially high wind speeds. We simulated decreased wind speed by using a steel-sheet wind shield in a temperate grassland in Inner Mongolia to examine the changes in physical environmental variables, as well as their impacts on the photosynthesis of grass leaves and net ecosystem exchange （NEE）. We then used models to calculate the variation of boundary layer conductance （BLC） and its impact on leaf photosynthesis, and this allowed us to separate the direct effects of wind speed reduction on leaf photo- synthesis （BLC） from the indirect ones （via soil moisture balance）. The results showed that reduced wind speed primarily resulted in higher moisture and temperature in soil, and indirectly affected net assimilation and water use efficiency of the prevalent bunch grass Stipa krylovii. Moreover, the wind-sheltered plant community had a stronger ability to sequester carbon than did the wind-exposed community during the growing season.展开更多
Drought may impact the net ecosystem exchange of CO2 （NEE） between grassland ecosystems and the atmosphere during growth seasons. Here, carbon dioxide exchange and controlling factors in alpine grassland under droug...Drought may impact the net ecosystem exchange of CO2 （NEE） between grassland ecosystems and the atmosphere during growth seasons. Here, carbon dioxide exchange and controlling factors in alpine grassland under drought stress in the hinterland of Tibetan Plateau （Damxung, Tibet, China） were investigated. Data were obtained using the covariance eddy technique in 2009. Severe drought stress appeared in the early growing season （May to early July） and September. Drought conditions during the early growing season limited grass production and the green leaf area index （GLAD increased slowly, with an obvious decline in June. When encountering severe water stress, diurnal patterns of NEE in the growth season altered with a peak carbon release around 16：00 h or a second carbon uptake period before sunset. NEE variations in daytime related most closely with O other than PAR when daily averaged @〈0.1 m3 m 3. Seasonal patterns of gross primary production （GPP） and NEE were also influenced by drought： the maximum and minimum of daily-integrated NEE were 0.9 g C m-2 d-1 on 3 July 2009, and -1.3 g C m-2 d-1 on 12 August 2009 with a GPP peak （-2.3 g C m-2 d-1） on the same day, respectively. Monthly NEE from May to July remained as carbon release and increased gradually; peak values of monthly NEE and GPP both appeared in August, but that of ecosystem respiration （R^co） was reached in July. Annual NEE, GPP and Reco of the alpine grassland ecosystem were 52.4, -158.1 and 210.5 g C m-2, respectively. Therefore, the grassland was a moderate source of COs to the atmosphere in this dry year. Interannual variation in NEE was likely related to different water conditions in the growing season. The three greatest contributors to seasonal variation in NEE, GPP and R^co respectively were GLAI〉Ta〉O, GLAI〉O〉PPT, and Ta〉GLAI〉PAR. Seasonality of GLAI explained 60.7% and 76.1% of seasonal variation in NEE and GPP, respectively. GPP or NEE was more sensitive than Reco to variation in GLAI, and ecosystem water conditions.展开更多
CO2 flux was measured continuously in a wheat and maize rotation system of North China Plain using the eddy covariance technique to study the characteristic of CO2 exchange and its response to key environmental factor...CO2 flux was measured continuously in a wheat and maize rotation system of North China Plain using the eddy covariance technique to study the characteristic of CO2 exchange and its response to key environmental factors. The results show that nighttime net ecosystem exchange (NEE) varied exponentially with soil temperature. The temperature sensitivities of the ecosystem (Q10) were 2.94 and 2.49 in years 2002-2003 and 2003-2004, respectively. The response of gross primary productivity (GPP) to photosynthetically active radiation (PAR) in the crop field can be expressed by a rectangular hyperbolic function. Average Amax andαfor maize were more than those for wheat. The values ofαincreased positively with leaf area index (LAI) of wheat. Diurnal variations of NEE were significant from March to May and from July to September, but not remarkable in other months. NEE, GPP and ecosystem respiration (Rec) showed significantly seasonal variations in the crop field. The highest mean daily CO2 uptake rate was -10.20 and -12.50 gC·m-2·d-1 in 2003 and 2004, for the maize field, respectively, and -8.19 and -9.50 gC·m-2·d-1 in 2003 and 2004 for the wheat field, respectively. The maximal CO2 uptake appeared in April or May for wheat and mid-August for maize. During the main growing seasons of winter wheat and summer maize, NEE was controlled by GPP which was chiefly influenced by PAR and LAI. Rec reached its annual maximum in July when Rec and GPP contributed to NEE equally. NEE was dominated by Rec in other months and temperature became a key factor controlling NEE. Total NEE for the wheat field was -77.6 and -152.2 gC·m-2·a-1 in years 2002-2003 and 2003-2004, respectively, and -120.1 and -165.6 gC·m-2·a-1 in 2003 and 2004 for the maize field, respectively. The cropland of North China Plain was a carbon sink, with annual -197.6 and -317.9 gC·m-2·a-1 in years 2002-2003 and 2003-2004, respectively. After considering the carbon in grains, the cropland became a carbon source, which was 340.5 and 107.5 gC·m-2·a-1 in years 2002-2003 and 2003-2004, respectively. Affected by climate and filed managements, inter-annual carbon exchange varied largely in the wheat and maize rotation system of North China Plain.展开更多
In this study, the diurnal and seasonal variations of CO2 fluxes in a subtropical mixed evergreen forest in Ningxiang of Hunan Province, part of the East Asian monsoon region, were quantified for the first time. The f...In this study, the diurnal and seasonal variations of CO2 fluxes in a subtropical mixed evergreen forest in Ningxiang of Hunan Province, part of the East Asian monsoon region, were quantified for the first time. The fluxes were based on eddy covariance measurements from a newly initiated flux tower. The relationship between the CO2 fluxes and climate factors was also analyzed. The results showed that the target ecosystem appeared to be a clear carbon sink in 2013, with integrated net ecosystem CO2exchange（NEE）, ecosystem respiration（RE）, and gross ecosystem productivity（GEP） of-428.8, 1534.8 and1963.6 g C m^-2yr^-1, respectively. The net carbon uptake（i.e. the-NEE）, RE and GEP showed obvious seasonal variability,and were lower in winter and under drought conditions and higher in the growing season. The minimum NEE occurred on12 June（-7.4 g C m^-2d^-1）, due mainly to strong radiation, adequate moisture, and moderate temperature; while a very low net CO2 uptake occurred in August（9 g C m^-2month^-1）, attributable to extreme summer drought. In addition, the NEE and GEP showed obvious diurnal variability that changed with the seasons. In winter, solar radiation and temperature were the main controlling factors for GEP, while the soil water content and vapor pressure deficit were the controlling factors in summer. Furthermore, the daytime NEE was mainly limited by the water-stress effect under dry and warm atmospheric conditions, rather than by the direct temperature-stress effect.展开更多
Xinjiang is the largest semi-arid and arid region in China, and drip irrigation under plastic mulch is widely used in this water-limited area. Quantifying carbon and water fluxes as well as investigating their environ...Xinjiang is the largest semi-arid and arid region in China, and drip irrigation under plastic mulch is widely used in this water-limited area. Quantifying carbon and water fluxes as well as investigating their environ- mental drivers over cotton fields is critical for understanding regional carbon and water budgets in Xinjiang, the largest cotton production basin of China. In this study, an eddy covariance （EC） technique was used to measure the carbon and water fluxes of cotton field under drip irrigation with plastic mulch in the growing seasons of 2009, 2010, 2012 and 2013 at Wulanwusu Agrometeorological Experiment Station, a representative oasis cropland in northern Xinjiang. The diurnal patterns of gross primary production （GPP）, net ecosystem exchange （NEE） and evapotran-spiration （ET） showed obviously sinusoidal variations from June to September, while the diurnal ecosystem respiration （Res） was stable between daytime and nighttime. The daytime hourly GPP and ET displayed asymptotic rela-tionships with net solar radiation （Rnet）, while showed concave patterns with raising vapor pressure deficit （VPD） and air temperature （Ta）. The increases in hourly GPP and ET towards the maximum occurred over half ranges of VPD and Ta. The seasonal variations of GPP, NEE and ET were close to the cotton phenology, which almost reached the peak value in July. The cumulative GPP averaged 816.2±55.0 g C/m^2 in the growing season （from April to October）, and more than half of GPP was partitioned into NEE （mean value of -478.6±41.4 g C/m^2）. The mean seasonal ET was 501.3±13.9 mm, and the mean water use efficiency （WUE） was 1.0＋0.1 （mg C/g H2O）/d. The agro-ecosystem behaved as a carbon sink from squaring to harvest period, while it acted as a carbon source before the squaring time as well as after the harvest time.展开更多
The Arctic ecosystem, especially High Arctic tundra, plays a unique role in the global carbon cycle because of amplified warming in the region. However, relatively little research has been conducted in High Arctic tun...The Arctic ecosystem, especially High Arctic tundra, plays a unique role in the global carbon cycle because of amplified warming in the region. However, relatively little research has been conducted in High Arctic tundra compared with other global ecosystems. In the present work, summertime net ecosystem exchange （NEE）, ecosystem respiration （ER）, and photosynthesis were investigated at six tundra sites （DM1-DM6） on Ny-A.lesund in the High Arctic. NEE at the tundra sites varied between a weak sink and strong source （-3.3 to 19.0 mg CO2·m-2.h-1）. ER and gross photosynthesis were 42.8 to 92.9 mg CO2·m-2·h-1 and 54.7 to 108.7 mg CO2·m-2·h-1, respectively. The NEE variations showed a significant correlation with photosynthesis rates, whereas no significant correlation was found with ecosystem respiration, indicating that NEE variations across the region were controlled by differences in net uptake of CO2 owing to photosynthesis, rather than by variations in ER. A Qm value of 1.80 indicated weak temperature sensitivity of tundra ER and its response to future global warming. NEE and gross photosynthesis also showed relatively strong correlations with C/N ratio. The tundra ER, NEE, and gross photosynthesis showed variations over slightly waterlogged wetland tundra, mesic and dry tundra. Overall, soil temperature, nutrients and moisture can be key effects on CO2 fluxes, ecosystem respiration, and NEE in the High Arctic.展开更多
Previous studies have shown that carbon dioxide fluxes vary considerably among Arctic environments and it is important to assess these differences in order to develop our understanding of the role of Arctic tundra in ...Previous studies have shown that carbon dioxide fluxes vary considerably among Arctic environments and it is important to assess these differences in order to develop our understanding of the role of Arctic tundra in the global carbon cycle. Although many previous studies have examined tundra carbon dioxide fluxes, few have concentrated on elevated terrain(hills and ridge tops) that is exposed to harsh environmental conditions resulting in sparse vegetation cover and seemingly low productivity. In this study we measured carbon dioxide(CO2) exchange of four common tundra communities on the crest of an esker located in the central Canadian low-Arctic. The objectives were to quantify and compare CO2 fluxes from these communities, investigate responses to environmental variables and qualitatively compare fluxes with those from similar communities growing in less harsh lowland tundra environments. Measurements made during July and August 2010 show there was little difference in net ecosystem exchange(NEE) and gross ecosystem production(GEP) among the three deciduous shrub communities, Arctous alpina, Betula glandulosa and Vaccinium uliginosum, with means ranging from -4.09 to -6.57 μmol·m^-2·s^-1 and -7.92 to -9.24 μmol·m^-2·s^-1, respectively. Empetrum nigrum communities had significantly smaller mean NEE and GEP(-1.74 and -4.08 μmol·m^-2·s^-1, respectively). Ecosystem respiration(ER) was similar for all communities(2.56 to 3.03 μmol·m^-2·s^-1), except the B. glandulosa community which had a larger mean flux(4.66 μmol·m^-2·s^-1). Overall, fluxes for these esker-top communities were near the upper range of fluxes reported for other tundra communities. ER was related to soil temperature in all of the communities. Only B. glandulosa GEP and ER showed sensitivity to a persistent decline in soil moisture throughout the study. These findings may have important implications for how esker tops would be treated in construction of regional carbon budgets and for predicting the impacts of climate change on Arctic tundra future carbon budgets.展开更多
Background:The accurate estimation of carbon-water flux is critical for understanding the carbon and water cycles of terrestrial ecosystems and further mitigating climate change.Model simulations and observations have...Background:The accurate estimation of carbon-water flux is critical for understanding the carbon and water cycles of terrestrial ecosystems and further mitigating climate change.Model simulations and observations have been widely used to research water and carbon cycles of terrestrial ecosystems.Given the advantages and limitations of each method,combining simulations and observations through a data assimilation technique has been proven to be highly promising for improving carbon-water flux simulation.However,to the best of our knowledge,few studies have accomplished both parameter optimization and the updating of model state variables through data assimilation for carbon-water flux simulation in multiple vegetation types.And little is known about the variation of the performance of data assimilation for carbon-water flux simulation in different vegetation types.Methods:In this study,we assimilated leaf area index(LAI)time-series observations into a biogeochemical model(Biome-BGC)using different assimilation algorithms(ensemble Kalman filter algorithm(EnKF)and unscented Kalman filter(UKF))in different vegetation types(deciduous broad-leaved forest(DBF),evergreen broad-leaved forest(EBF)and grassland(GL))to simulate carbon-water flux.Results:The validation of the results against the eddy covariance measurements indicated that,overall,compared with the original simulation,assimilating the LAI into the Biome-BGC model improved the carbon-water flux simulations(R^(2)increased by 35%,root mean square error decreased by 10%;the sum of the absolute error decreased by 8%)but more significantly,improved the water flux simulations(R^(2)increased by 31%,root mean square error decreased by 18%;the sum of the absolute error decreased by 16%).Among the different forest types,the data assimilation techniques(both EnKF and UKF)achieved the best performance towards carbon-water flux in EBF(R^(2)increased by 44%,root mean square error decreased by 24%;the sum of the absolute error decreased by 28%),and the performances of EnKF and UKF showed slightly different when simulating carbon fluxes.Conclusion:We suggest that to reduce the uncertainty in global carbon-water flux quantification,forthcoming data assimilation treatment should consider the vegetation types where the data assimilation experiments are carried out,the simulated objectives and the assimilation algorithms.展开更多
基金supported by the Major Science and Technology Project of Inner Mongolia Autonomous Region (2024JBGS0011-02)Foundation for Innovative Research Groups in Basic Research of Gansu Province (25JRRA490)+1 种基金Youth Innovation Promotion Association of the Chinese Academy of Sciences (2022437)National Natural Science Foundation of China (42207538)。
文摘Amid global climate change, rising levels of nitrogen(N) deposition have attracted considerable attention for their potential effects on the carbon cycle of terrestrial ecosystems. The desert steppes are a crucial yet vulnerable ecosystem in arid areas, but their response to the combination of N addition and precipitation(a crucial factor in arid areas) remains underexplored. This study systematically explored the impact of N addition and precipitation on net ecosystem exchange(NEE) in a desert steppe in northern China. Specifically, we conducted a 2-a experiment from 2022 to 2023 with eight N addition treatments in the Urat desert steppe of Inner Mongolia Autonomous Region, China, to examine changes in NEE and explore its driving factors. The structural equation model(SEM) and multiple regression model were applied to determine the relationship of NEE with plant community characteristics and soil physical-chemical properties. Statistical results showed that N addition has no significant effect on NEE.However, it has a significant impact on the functional traits of desert steppe plant communities. SEM results further revealed that N addition has no significant effect on NEE in the desert steppe, whereas annual precipitation can influence NEE variations. The multiple regression model analysis indicated that plant functional traits play an important role in explaining the changes in NEE, accounting for 62.15% of the variation in NEE. In addition, plant height, as an important plant functional trait indicator, shows stronger reliability in predicting the changes in NEE and becomes a more promising predictor. These findings provide valuable insights into the complex ecological mechanisms governing plant community responses to precipitation and nutrient availability in the arid desert steppes, contributing to the improved monitoring and prediction of desert steppe ecosystem responses to global climate change.
文摘The carbon （C） sequestration potential of turfgrass systems has been investigated and demonstrated from several studies. The role of these ecosystems in continental and Mediterranean climates though, is not yet clearly understood because environmental limiting factors and management intensities can strongly influence the overall C budget. The aim of the present study is to improve the understanding of the mechanisms underlying C fluxes in a turfgrass ecosystem and to assess its C sequestration potential by estimating the annual C budget. NEE （Net Ecosystem Exchange） of turfgrass was calculated in its seasonal variation over one year, and compared between areas characterized by different degrees of maintenance. The C sequestration potential of the turfgrass was investigated in a golf course near Verona （Italy）, adopting a small-chamber enclosure approach. The measurements of gas exchanges between biosphere and atmosphere, permitted to estimate the NEE, as a function of different management intensities. The intensity of management seems to have influence on its C balance. This study needs further research to understand which maintenance variables are determinant on turfgrass C sequestration.
文摘In 2022, an eddy covariance site was established in a young oil palm plantation in southeast Dangbo, Bénin, to study the exchange of CO2, energy, and water vapor. This study aims to present the first one-year analysis of seasonal dynamics in energy balance components and net ecosystem exchange above this type of ecosystem in Africa. The first results show that on average during the 2023 year, 55% of net radiation is consumed into actual evapotranspiration, demonstrating the significant amount of latent heat flux in the energy balance, as expected at this tropical humid site. The sensible heat flux was substantial, ranging between 60 and 200 W·m−2, while net radiation varied between 440 and 650 W·m−2. Carbon uptake and net release of CO2 into the atmosphere were permanent at the site. However, the CO2 uptake increases more when rainy events become regular. On average, the mean nighttime CO2 flux was ~8 µmol·m−2·s−1, while during the daytime it was ~−20 µmol·m−2·s−1.
基金The National Science Foundation of China(41301020)National Key Basic Research Program of China(2013CB956604)Core Research for Evolutional Science and Technology of the Japan Science and Technology.
文摘Aims Boreal forests play an important role in the global carbon cycle.Compared with the boreal forests in North America and Europe,relatively few research studies have been conducted in Siberian boreal forests.Knowledge related to the role of Siberian forests in the global carbon balance is thus essential for a full understanding of global carbon cycle.Methods This study investigated the net ecosystem exchange(NEE)during growing season(May-September)in an eastern Siberian boreal larch forest for a 3-year period in 2004-2006 with contrasting meteorological conditions.Important FindingsThe study found that the forest served as a carbon sink during all of the 3 studied years;in addition,the meteorological conditions essentially influenced the specific annual value of the strength of the carbon sinks in each year.Although 2005 was the warmest year and much wetter than 2004,2005 also featured the greatest amount of ecosystem respiration,which resulted in a minimum value of NEE.The study also found that the phenological changes observed during the three study years had a relatively small effect on annual NEE.Leaf expansion was 26 days earlier in 2005 than in the other 2 years,which resulted in a longer growing season in 2005.However,the NEE in 2005 was counterbalanced by the large rate of ecosystem respiration that was caused by the higher temperatures in the year.This study showed that meteorological variables had larger influences on the interannual variations in NEE for a Siberian boreal larch forest,as compared with phenological changes.The overall results of this study will improve our understanding of the carbon balance of Siberian boreal larch forests and thus can help to forecast the response of these forests to future climate change.
文摘<span style="font-family:Verdana;">The eddy covariance technique is an accurate and direct tool to measure the Net Ecosystem Exchange (NEE) of carbon dioxide. However, sometimes conditions are not amenable to measurements using this technique. Thus, different methods have been developed to allow gap-filling and quality assessment of eddy covariance data sets. In this study first, two different Artificial Neural Networks (ANNs) approaches, the Multi-layer Perceptron (MLP) trained by the Back-Propagation (BP) algorithm, and the Radial Basis Function (RBF), were used to fill missing NEE data measured above rain-fed maize at the University of Nebraska-Lincoln Agricultural Research and Development Center near Mead, Nebraska. The gap-filled data were then compared by different statistical indices to gap-filled data obtained with the technique suggested by Suyker and Verma in 2005 [S&V method], and the ANN approach presented by Papale in 2003. The results showed that the RBF network was able to find better fits for missing values compared to the MLP (BP) network and S&V method. In addition, unlike the S&V method, which depends on different gap-filling procedures over the year;the structure of RBF and MLP (BP) networks was constant. However, data analysis indicated Papale’s approach gave better fits than the RBF and MLP (BP) methods. Thus, based on this work, Papale’s approach is the best method to estimate the missing data;though the applied statistical indices, which were used for model evaluation, show little difference between Papale’s approach and the RBF and MLP (BP).</span>
基金funding provided by University of Innsbruckpart of the International Research Training Group TERRECO (Grant No. GRK 1565/1) funded by the Deutsche Forschungsge-meinschaft (DFG) at the University of Bayreuth, Germany and the Korean Research Foundation (KRF) at Kangwon National University, Chuncheon, South Korea
文摘The estimation of carbon exchange between ecosystems and the atmosphere suffers unavoidable data gaps in eddy-covariance technique, especially for short-living and fast-growing croplands. In this study we developed a modified gap-filling scheme introducing a leaf area index factor as the vegetation status information based on the conventional light response function for two East-Asian cropland sites (rice and potatoes). This scheme’s performance is comparable to the conventional time window scheme, but has the advantage when the gaps are large compared to the total length of the time series. To investigate how the time binning approach performs for fast-growing croplands, we tested different widths of the time window, showing that a four-day window for the potato field and an eight-day time window for the rice field perform the best. The insufficiency of the conventional temperature binning approach was explained as well as the influence of vapor pressure deficit. We found that vapor pressure deficit plays a minor role in both the potato and the rice fields under Asian monsoon weather conditions with the exception of the early pre-monsoon growing stage of the potatoes. Consequently, we recommend using the conventional time-window scheme together with our new leaf-light response function to fill data gaps of net ecosystem exchange in fast-growing croplands.
基金The Major Basic Research Development Program of China, No.2010CB833501 No.2010CB951704+1 种基金Knowledge Innovation Program of Chinese Academy of Sciences, No.KSCXZ-YW-N-44Knowledge Inno-vation Program of IGSNRR, CAS, No.Y0V00230ZZ
文摘On the Tibetan Plateau, the alpine meadow is the most widespread vegetation type. The alpine meadow has a low biological productivity and low vegetation coverage in the growing season. The daytime NEE between the atmosphere and the alpine meadow ecosystem was influenced by solar radiation. To analyze the characteristics of change in NEE and to calculate the parameters related to photosynthesis and respiration in different solar radiation environments, the NEE measurements were taken in Damxung from July to August in 2003, 2004, 2005 and 2006 using the eddy covariance technique. Solar radiation was grouped into three levels according to the net radiation, which was more than 155 W m-2 d-1 on clear days, 144±5 W m-2 d-1 on partly cloudy days and less than 134 W m-2 d-1 on cloudy days. The diurnal relationships between NEE and PAR varied with differences in solar radiation, which was a rectangular hyperbola form on clear days, two different concave curves on partly cloudy days and an irregular triangle form on cloudy days. The mean CO2 absorption rate showed a decreasing trend with increasing solar radiation. The daytime absorption maximum occurred around 10：00 on clear days with an average of slightly less –0.2 mg m-2 d-1, around 11：00 on partly cloudy days with an average of about –0.2 mg m-2 d-1, and around 12：00 on cloudy days with an average of about –0.25 mg m-2 d-1. As solar radiation increased, the Amax and the Q10 decreased. However, the R10 increased and the maximum of the α occurred on partly cloudy days. The optimum net solar radiation was about 134–155 W m-2 d-1, which induced a PAR of about 1800-2000 μmol m-2 s-1 and soil temperature at a depth of 5 cm of about 14℃. Therefore, on the Tibetan Plateau, the alpine meadow ecosystem will have a higher carbon absorption potential while solar radiation decreases in the future.
基金National Key R&D Program of China(2019YFC0214605)Key-Area R&D Program of Guangdong Province (2020B1111360003)+4 种基金Provincial Natural Science Foundation of Guangdong (2021A1515011494)Science and Technology Innovation Team Plan of Guangdong Meteorological Bureau (GRMCTD202003)Open Project of the Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration,Nanjing University of Information Science and Technology (KDW 1803)Scientific and Technological Innovation Team Project of Guangzhou Joint Research Center of Atmospheric Sciences,China Meteorological Administration (201704)Science and Technology Research Project of Guangdong Meteorological Bureau (GRMC2018M01)。
文摘The environmental impact of aerosols is currently a hot issue that has received worldwide attention. Lacking simultaneous observations of aerosols and carbon flux, the understanding of the aerosol radiative effect of urban agglomeration on the net ecosystem carbon exchange(NEE) is restricted. In 2009-2010, an observation of the aerosol optical property and CO_(2) flux was carried out at the Dongguan Meteorological Bureau Station(DMBS) using a sun photometer and eddy covariance systems. The different components of photosynthetically active radiation(PAR),including global PAR(GPAR), direct PAR(DPAR), and scattered PAR(FPAR), were calculated using the Santa Barbara DISORT Atmospheric Radiative Transfer(SBDART) model. The effects of PAR on the NEE between land-atmosphere systems were investigated. The results demonstrated that during the study period the aerosol optical depth(AOD)reduced the DPAR by 519.28±232.89 μmol photons · m^(-2)s^(-1), but increased the FPAR by 324.93±169.85μmol photons ·m^(-2)s^(-1),ultimately leading to 194.34±92.62 μmol photons · m^(-2)s^(-1);decrease in the GPAR. All the PARs(including GPAR,DPAR, and FPAR) resulted in increases in the NEE(improved carbon absorption), but the FPAR has the strongest effect with the light use efficiency(LUE) being 1.12 times the values for the DPAR. The absorption of DPAR by the vegetation exhibited photo-inhibition in the radiation intensity > 600 photons · m^(-2)s^(-1);in contrast, the absorptions of FPAR did not exhibit apparent photo-inhibition. Compared with the FPAR caused by aerosols, the DPAR was not the primary factor affecting the NEE. On the contrary, the increase in AOD significantly increased the FPAR, enhancing the LUE of vegetation ecosystems and finally promoting the photosynthetic CO_(2) absorption.
基金Under the auspices of the Major Basic Research Development Program of China(Grant no.2005CB422005)the Knowledge Innovation Program of Chinese Academy of Sciences(Grant no.KSCXZ-YW-N-44)
文摘Meteorological elements and CO_(2) fluxes over alpine meadow ecosystem were observed continuously from 2004 to 2005 in Damxung Alpine Meadow Flux Station,China Flux Network.Based on the eddy covariance CO_(2) fluxes and meteorological data obtained,the relationships among the CO_(2) fluxes,the cloud amount,and the meteorological factors in alpine meadow ecosystem were explored and analyzed.Some conclusions can be drawn from the discussion with previous researches as following:(1)the cloud amount can affect the net ecosystem CO_(2) exchange(NEE)of alpine meadow on Tibetan Plateau;(2)the soil temperature sensitive to the cloud amount,is a major environmental controlling factor for NEE,and closely relates to the maximum of NEE.In the moming period with large cloud amount,the NEE reaches its maximum when the clearness index ranges from 0.5 to 0.7;yet in the afternoon it comes to the maximum with the index from 0.2 to 0.35.The span of soil temperature covers from 12 to 15℃as the NEE at its highest;(3)the scatterplots between NEE and photosynthetic available radiation(PAR)was a significant inverse triangle in the clear day,two different kinds of concave curves in the cloudy day,and strongly convergent rectangular hyperbola in the overcast day.These differences were controlled by the changes of light radiation and soil temperature.
基金supported in part by the Fundamental Research Project of the Science and Technology Department of the Qinghai Province(Grant No.2025-ZJ-739)the National Natural Science Foundation of China(Grant No.U21A2021)+1 种基金the Open Fund of Greenhouse Gas and Carbon Neutral Key Laboratory of Qinghai Province(Grant No.ZDXM-2023-3)the Key Projects of Qinghai Meteorological Bureau(Grant No.QXZD2024-08)。
文摘Alpine meadows,alpine wetlands,and alpine desert steppes are the three typical vegetation types on the Qinghai-Tibet Plateau.The complex terrain and harsh climatic conditions across this region lead to considerable diversification in the vegetation growth environment,resulting in substantial spatial heterogeneity in ecosystem carbon flux and its controlling mechanisms.Using eddy covariance data collected from March to August 2019,this study examined the responses of carbon and water fluxes in different ecosystems on the Tibetan Plateau to typical hydrometeorological factors,focusing on Net Ecosystem CO□Exchange(NEE)and Evapotranspiration(ET).The results indicate that:1)The Longbao alpine wetland primarily acted as a carbon sink from May to August,while serving as a carbon source from March to April.In the Maqin alpine meadow,it functioned as a carbon sink during June and July but acted as a carbon source in March,April,May,and August.The Tuotuohe alpine desert strppe was predominantly a net carbon sink from March to August.Overall,after the entire growing season(March to August),the Longbao alpine wetlands,Maqin alpine meadow,and Tuotuohe alpine desert steppe all showed net carbon sink properties,with net CO_(2)uptakes of 236.12 g/m^(2),291.45 g/m^(2),and 290.28 g/m^(2),respectively.2)The importance of meteorological factors to NEE varies with scale and ecosystem type,with global radiation(Rg)being the most critical factor influencing NEE variation.Volumetric soil water content(Soil_VWC)and soil temperature(Soil_T)had a positive effect on NEE at Maqin alpine meadow and Tuotuohe alpine desert steppe,while higher values of these variables showed a negative contribution.Furthermore,the sensitivity of NEE to Soil_T at Longbao alpine wetland and Tuotuohe alpine desert steppe was greater than its sensitivity to air temperature(Tair).3)The effect of Gross Primary Productivity(GPP)on NEE in alpine desert steppes is significantly greater than in alpine meadows.Both Ecosystem Respiration(Reco)and NEE were substantially limited by GPP,with 84%of GPP in alpine wetlands contributing to Reco and 16%to NEE;92%of GPP in alpine meadows contributing to Reco and 8%to NEE;and 40%of GPP in high-altitude desert grasslands contributing to Reco and 60%to NEE.4)The strong correlation between NEE and evapotranspiration suggests that water availability is the primary factor controlling changes in the carbon and water budgets of alpine ecosystems.
文摘Peatland ecosystems play an important role in the global carbon cycle because they act as a pool or sink for the carbon cycle. However, the relationship between seasonality effect factors and net ecosystem CO<sub>2</sub> exchange (NEE) remains to be clarified, particularly for the non-growing season. Here, based on the eddy covariance technique, NEE in the peatland ecosystem of Central China was examined to measure two years’ (2016 and 2017) accumulation of carbon dioxide emissions with contrasting seasonal distribution of environmental factors. Our results demonstrate the cumulative net ecosystem CO<sub>2</sub> emissions during the study period was in the first non-growing season 2.94 ± 4.83 μmolCO<sub>2</sub> m<sup><span style="color:#4F4F4F;font-family:" font-size:14px;white-space:normal;background-color:#ffffff;"="">-</span>2<span style="white-space:nowrap;">.</span></sup>s<sup><span style="color:#4F4F4F;font-family:" font-size:14px;white-space:normal;background-color:#ffffff;"="">-</span>1</sup> with the lowest values in the same year in first growing season was <span style="color:#4F4F4F;font-family:" font-size:14px;white-space:normal;background-color:#ffffff;"="">-</span>2.79 ± 4.92 μmolCO<sub>2</sub> m<sup><span style="color:#4F4F4F;font-family:" font-size:14px;white-space:normal;background-color:#ffffff;"="">-</span>2</sup><span style="font-family:" font-size:13.3333px;white-space:normal;"=""><span style="white-space:nowrap;"><sup>.</sup></span></span>s<sup><span style="color:#4F4F4F;font-family:" font-size:14px;white-space:normal;background-color:#ffffff;"="">-</span>1</sup>. The results indicate the effect of seasonal variations of NEE can be directly reflected in daily and seasonal variations in growth and respiration of peatland ecosystem by environmental parameters over different growing stages.
基金supported by the China Meteorological Administration (Grant No.GYHY201006024)the National Key Basic Research Program (Grant Nos.2010CB428502 and 2012CB417203)+2 种基金the Chinese Academy of Sciences Strategic Priority Research Program (Grant No.XDA05110101)the National Natural Science Foundation of China (Grant Nos.40975009 and 41405018)the State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry,Institute of Atmospheric Physics,Chinese Academy of Sciences (Grant No.LAPC-KF-2009-02)
文摘Rice-wheat （R-W） rotation systems are ubiquitous in South and East Asia, and play an important role in modulating the carbon cycle and climate. Long-term, continuous flux measurements help in better understanding the seasonal and interannual variation of the carbon budget over R-W rotation systems. In this study, measurements of CO2 fluxes and meteorological variables over an R-W rotation system on the North China Plain from 2007 to 2010 were analyzed. To analyze the abiotic factors regulating Net Ecosystem Exchange （NEE）, NEE was partitioned into gross primary production （GPP） and ecosystem respiration. Nighttime NEE or ecosystem respiration was controlled primarily by soil temperature, while daytime NEE was mainly determined by photosythetically active radiation （PAR）. The responses of nighttime NEE to soil temperature and daytime NEE to light were closely associated with crop development and photosynthetic activity, respectively. Moreover, the interannual variation in GPP and NEE mainly depended on precipitation and PAR. Overall, NEE was negative on the annual scale and the rotation system behaved as a carbon sink of 982 g C m 2 per year over the three years. The winter wheat field took up more CO2 than the rice paddy during the longer growing season, while the daily NEE for wheat and rice were -2.35 and -3.96 g C m-2, respectively. After the grain harvest was subtracted from the NEE, the winter wheat field became a moderately strong carbon sink of 251-334 g C m-2 per season, whereas the rice paddy switched to a weak carbon sink of 107-132 per season.
基金supported by the National Science Foundation of China (Nos. 41171445 and 41321001)the State Key Laboratory of Earth Surface Processes and Resources Ecology (No. 2012-TDZY-31)the National Program on Key Basic Research Project (No. 2014CB954303)
文摘Decreasing wind speed is one aspect of global climate change as well as global warming, and has become a new research orientation in recent decades. The decrease is especially evident in places with frequent perennially high wind speeds. We simulated decreased wind speed by using a steel-sheet wind shield in a temperate grassland in Inner Mongolia to examine the changes in physical environmental variables, as well as their impacts on the photosynthesis of grass leaves and net ecosystem exchange （NEE）. We then used models to calculate the variation of boundary layer conductance （BLC） and its impact on leaf photosynthesis, and this allowed us to separate the direct effects of wind speed reduction on leaf photo- synthesis （BLC） from the indirect ones （via soil moisture balance）. The results showed that reduced wind speed primarily resulted in higher moisture and temperature in soil, and indirectly affected net assimilation and water use efficiency of the prevalent bunch grass Stipa krylovii. Moreover, the wind-sheltered plant community had a stronger ability to sequester carbon than did the wind-exposed community during the growing season.
基金National Basic Research Program of China(No.2010CB833500)National Natural Science Foundation of China(Grant No.41171044)
文摘Drought may impact the net ecosystem exchange of CO2 （NEE） between grassland ecosystems and the atmosphere during growth seasons. Here, carbon dioxide exchange and controlling factors in alpine grassland under drought stress in the hinterland of Tibetan Plateau （Damxung, Tibet, China） were investigated. Data were obtained using the covariance eddy technique in 2009. Severe drought stress appeared in the early growing season （May to early July） and September. Drought conditions during the early growing season limited grass production and the green leaf area index （GLAD increased slowly, with an obvious decline in June. When encountering severe water stress, diurnal patterns of NEE in the growth season altered with a peak carbon release around 16：00 h or a second carbon uptake period before sunset. NEE variations in daytime related most closely with O other than PAR when daily averaged @〈0.1 m3 m 3. Seasonal patterns of gross primary production （GPP） and NEE were also influenced by drought： the maximum and minimum of daily-integrated NEE were 0.9 g C m-2 d-1 on 3 July 2009, and -1.3 g C m-2 d-1 on 12 August 2009 with a GPP peak （-2.3 g C m-2 d-1） on the same day, respectively. Monthly NEE from May to July remained as carbon release and increased gradually; peak values of monthly NEE and GPP both appeared in August, but that of ecosystem respiration （R^co） was reached in July. Annual NEE, GPP and Reco of the alpine grassland ecosystem were 52.4, -158.1 and 210.5 g C m-2, respectively. Therefore, the grassland was a moderate source of COs to the atmosphere in this dry year. Interannual variation in NEE was likely related to different water conditions in the growing season. The three greatest contributors to seasonal variation in NEE, GPP and R^co respectively were GLAI〉Ta〉O, GLAI〉O〉PPT, and Ta〉GLAI〉PAR. Seasonality of GLAI explained 60.7% and 76.1% of seasonal variation in NEE and GPP, respectively. GPP or NEE was more sensitive than Reco to variation in GLAI, and ecosystem water conditions.
基金This study was jointly sponsored by the Knowledge Innovation Project of the Chinese Academy of Sciences (Grant No. KZCX1-SW-01-01A) the National Natural Science Fund for Overseas Outstanding Youth (Grant No. 40328001) the Ministry of Science and Technology of China (Grant No.2002CB4125001).
文摘CO2 flux was measured continuously in a wheat and maize rotation system of North China Plain using the eddy covariance technique to study the characteristic of CO2 exchange and its response to key environmental factors. The results show that nighttime net ecosystem exchange (NEE) varied exponentially with soil temperature. The temperature sensitivities of the ecosystem (Q10) were 2.94 and 2.49 in years 2002-2003 and 2003-2004, respectively. The response of gross primary productivity (GPP) to photosynthetically active radiation (PAR) in the crop field can be expressed by a rectangular hyperbolic function. Average Amax andαfor maize were more than those for wheat. The values ofαincreased positively with leaf area index (LAI) of wheat. Diurnal variations of NEE were significant from March to May and from July to September, but not remarkable in other months. NEE, GPP and ecosystem respiration (Rec) showed significantly seasonal variations in the crop field. The highest mean daily CO2 uptake rate was -10.20 and -12.50 gC·m-2·d-1 in 2003 and 2004, for the maize field, respectively, and -8.19 and -9.50 gC·m-2·d-1 in 2003 and 2004 for the wheat field, respectively. The maximal CO2 uptake appeared in April or May for wheat and mid-August for maize. During the main growing seasons of winter wheat and summer maize, NEE was controlled by GPP which was chiefly influenced by PAR and LAI. Rec reached its annual maximum in July when Rec and GPP contributed to NEE equally. NEE was dominated by Rec in other months and temperature became a key factor controlling NEE. Total NEE for the wheat field was -77.6 and -152.2 gC·m-2·a-1 in years 2002-2003 and 2003-2004, respectively, and -120.1 and -165.6 gC·m-2·a-1 in 2003 and 2004 for the maize field, respectively. The cropland of North China Plain was a carbon sink, with annual -197.6 and -317.9 gC·m-2·a-1 in years 2002-2003 and 2003-2004, respectively. After considering the carbon in grains, the cropland became a carbon source, which was 340.5 and 107.5 gC·m-2·a-1 in years 2002-2003 and 2003-2004, respectively. Affected by climate and filed managements, inter-annual carbon exchange varied largely in the wheat and maize rotation system of North China Plain.
基金supported by the National Natural Science Foundation of China (Grant Nos. 41305066 and 91125016)the Special Funds for Public Welfare of China (Grant No. GYHY201306045)
文摘In this study, the diurnal and seasonal variations of CO2 fluxes in a subtropical mixed evergreen forest in Ningxiang of Hunan Province, part of the East Asian monsoon region, were quantified for the first time. The fluxes were based on eddy covariance measurements from a newly initiated flux tower. The relationship between the CO2 fluxes and climate factors was also analyzed. The results showed that the target ecosystem appeared to be a clear carbon sink in 2013, with integrated net ecosystem CO2exchange（NEE）, ecosystem respiration（RE）, and gross ecosystem productivity（GEP） of-428.8, 1534.8 and1963.6 g C m^-2yr^-1, respectively. The net carbon uptake（i.e. the-NEE）, RE and GEP showed obvious seasonal variability,and were lower in winter and under drought conditions and higher in the growing season. The minimum NEE occurred on12 June（-7.4 g C m^-2d^-1）, due mainly to strong radiation, adequate moisture, and moderate temperature; while a very low net CO2 uptake occurred in August（9 g C m^-2month^-1）, attributable to extreme summer drought. In addition, the NEE and GEP showed obvious diurnal variability that changed with the seasons. In winter, solar radiation and temperature were the main controlling factors for GEP, while the soil water content and vapor pressure deficit were the controlling factors in summer. Furthermore, the daytime NEE was mainly limited by the water-stress effect under dry and warm atmospheric conditions, rather than by the direct temperature-stress effect.
基金supported by the West Light Foundation of the Chinese Academy of Sciences (XBBS201110)the National Natural Science Foundation of China (41101101)the Chinese Academy of Sciences Key Deployment Project (KZZDEW-08-02-02)
文摘Xinjiang is the largest semi-arid and arid region in China, and drip irrigation under plastic mulch is widely used in this water-limited area. Quantifying carbon and water fluxes as well as investigating their environ- mental drivers over cotton fields is critical for understanding regional carbon and water budgets in Xinjiang, the largest cotton production basin of China. In this study, an eddy covariance （EC） technique was used to measure the carbon and water fluxes of cotton field under drip irrigation with plastic mulch in the growing seasons of 2009, 2010, 2012 and 2013 at Wulanwusu Agrometeorological Experiment Station, a representative oasis cropland in northern Xinjiang. The diurnal patterns of gross primary production （GPP）, net ecosystem exchange （NEE） and evapotran-spiration （ET） showed obviously sinusoidal variations from June to September, while the diurnal ecosystem respiration （Res） was stable between daytime and nighttime. The daytime hourly GPP and ET displayed asymptotic rela-tionships with net solar radiation （Rnet）, while showed concave patterns with raising vapor pressure deficit （VPD） and air temperature （Ta）. The increases in hourly GPP and ET towards the maximum occurred over half ranges of VPD and Ta. The seasonal variations of GPP, NEE and ET were close to the cotton phenology, which almost reached the peak value in July. The cumulative GPP averaged 816.2±55.0 g C/m^2 in the growing season （from April to October）, and more than half of GPP was partitioned into NEE （mean value of -478.6±41.4 g C/m^2）. The mean seasonal ET was 501.3±13.9 mm, and the mean water use efficiency （WUE） was 1.0＋0.1 （mg C/g H2O）/d. The agro-ecosystem behaved as a carbon sink from squaring to harvest period, while it acted as a carbon source before the squaring time as well as after the harvest time.
基金supported by the National Natural Science Foundation of China (Grant nos.41576181 and 41176171)Specialized Research Fund for the Doctoral Program of Higher Education (Grant no.20123402110026)
文摘The Arctic ecosystem, especially High Arctic tundra, plays a unique role in the global carbon cycle because of amplified warming in the region. However, relatively little research has been conducted in High Arctic tundra compared with other global ecosystems. In the present work, summertime net ecosystem exchange （NEE）, ecosystem respiration （ER）, and photosynthesis were investigated at six tundra sites （DM1-DM6） on Ny-A.lesund in the High Arctic. NEE at the tundra sites varied between a weak sink and strong source （-3.3 to 19.0 mg CO2·m-2.h-1）. ER and gross photosynthesis were 42.8 to 92.9 mg CO2·m-2·h-1 and 54.7 to 108.7 mg CO2·m-2·h-1, respectively. The NEE variations showed a significant correlation with photosynthesis rates, whereas no significant correlation was found with ecosystem respiration, indicating that NEE variations across the region were controlled by differences in net uptake of CO2 owing to photosynthesis, rather than by variations in ER. A Qm value of 1.80 indicated weak temperature sensitivity of tundra ER and its response to future global warming. NEE and gross photosynthesis also showed relatively strong correlations with C/N ratio. The tundra ER, NEE, and gross photosynthesis showed variations over slightly waterlogged wetland tundra, mesic and dry tundra. Overall, soil temperature, nutrients and moisture can be key effects on CO2 fluxes, ecosystem respiration, and NEE in the High Arctic.
基金Funding for this research to PML was from the Natural Science and Engineering Research Council of Canada(NSERC)ABC was supported through the Northern Scientific Training Programan NSERC Undergraduate Student Research Award。
文摘Previous studies have shown that carbon dioxide fluxes vary considerably among Arctic environments and it is important to assess these differences in order to develop our understanding of the role of Arctic tundra in the global carbon cycle. Although many previous studies have examined tundra carbon dioxide fluxes, few have concentrated on elevated terrain(hills and ridge tops) that is exposed to harsh environmental conditions resulting in sparse vegetation cover and seemingly low productivity. In this study we measured carbon dioxide(CO2) exchange of four common tundra communities on the crest of an esker located in the central Canadian low-Arctic. The objectives were to quantify and compare CO2 fluxes from these communities, investigate responses to environmental variables and qualitatively compare fluxes with those from similar communities growing in less harsh lowland tundra environments. Measurements made during July and August 2010 show there was little difference in net ecosystem exchange(NEE) and gross ecosystem production(GEP) among the three deciduous shrub communities, Arctous alpina, Betula glandulosa and Vaccinium uliginosum, with means ranging from -4.09 to -6.57 μmol·m^-2·s^-1 and -7.92 to -9.24 μmol·m^-2·s^-1, respectively. Empetrum nigrum communities had significantly smaller mean NEE and GEP(-1.74 and -4.08 μmol·m^-2·s^-1, respectively). Ecosystem respiration(ER) was similar for all communities(2.56 to 3.03 μmol·m^-2·s^-1), except the B. glandulosa community which had a larger mean flux(4.66 μmol·m^-2·s^-1). Overall, fluxes for these esker-top communities were near the upper range of fluxes reported for other tundra communities. ER was related to soil temperature in all of the communities. Only B. glandulosa GEP and ER showed sensitivity to a persistent decline in soil moisture throughout the study. These findings may have important implications for how esker tops would be treated in construction of regional carbon budgets and for predicting the impacts of climate change on Arctic tundra future carbon budgets.
基金supported by the National Natural Science Foundation of China(No.41301451).
文摘Background:The accurate estimation of carbon-water flux is critical for understanding the carbon and water cycles of terrestrial ecosystems and further mitigating climate change.Model simulations and observations have been widely used to research water and carbon cycles of terrestrial ecosystems.Given the advantages and limitations of each method,combining simulations and observations through a data assimilation technique has been proven to be highly promising for improving carbon-water flux simulation.However,to the best of our knowledge,few studies have accomplished both parameter optimization and the updating of model state variables through data assimilation for carbon-water flux simulation in multiple vegetation types.And little is known about the variation of the performance of data assimilation for carbon-water flux simulation in different vegetation types.Methods:In this study,we assimilated leaf area index(LAI)time-series observations into a biogeochemical model(Biome-BGC)using different assimilation algorithms(ensemble Kalman filter algorithm(EnKF)and unscented Kalman filter(UKF))in different vegetation types(deciduous broad-leaved forest(DBF),evergreen broad-leaved forest(EBF)and grassland(GL))to simulate carbon-water flux.Results:The validation of the results against the eddy covariance measurements indicated that,overall,compared with the original simulation,assimilating the LAI into the Biome-BGC model improved the carbon-water flux simulations(R^(2)increased by 35%,root mean square error decreased by 10%;the sum of the absolute error decreased by 8%)but more significantly,improved the water flux simulations(R^(2)increased by 31%,root mean square error decreased by 18%;the sum of the absolute error decreased by 16%).Among the different forest types,the data assimilation techniques(both EnKF and UKF)achieved the best performance towards carbon-water flux in EBF(R^(2)increased by 44%,root mean square error decreased by 24%;the sum of the absolute error decreased by 28%),and the performances of EnKF and UKF showed slightly different when simulating carbon fluxes.Conclusion:We suggest that to reduce the uncertainty in global carbon-water flux quantification,forthcoming data assimilation treatment should consider the vegetation types where the data assimilation experiments are carried out,the simulated objectives and the assimilation algorithms.
