Increasing anthropogenic nitrogen(N)inputs has profoundly altered soil microbial necromass carbon(MNC),which serves as a key source of soil organic carbon(SOC).Yet,the response pattern of MNC and its contribution to S...Increasing anthropogenic nitrogen(N)inputs has profoundly altered soil microbial necromass carbon(MNC),which serves as a key source of soil organic carbon(SOC).Yet,the response pattern of MNC and its contribution to SOC across a wide range of N addition rates,remain elusive.In a temperate grassland with six years'consecutive N addition spanning seven rates(0-50 g N/(m^(2)·year))in Inner Mongolia,China,we explored the responses of soil MNC and its contribution to SOC.The soil MNC showed a hump-shaped pattern to increasing N addition rates,with the N saturation threshold at 18.07 g N/(m^(2)·year).The soil MNC was driven by nematode abundance and the ratio of bacterial to fungal biomass below the N threshold,and by plant biomass allocation pattern and diversity above the N threshold.The contribution of soil MNC to SOC declined with increasing N addition rates,and was mainly regulated by the ratio of MNC to mineral-associated organic carbon and plant diversity and the ratio of bacterial to fungal biomass.In addition,the soil MNC and SOC differentially responded to N addition and were mediated by disparate biological and geochemical mechanisms,leading to the decoupled MNC production from SOC formation.Together,in this N-enriched temperate grassland,the soilmicrobial necro-mass production tends to be insufficient as a general explanation linking SOC formation.This study expands the mechanistic comprehension of the connections between external N input and soil carbon sequestration.展开更多
Tall clonal grasses commonly display competitive advantages with nitrogen(N)enrichment.However,it is currently unknown whether the height is derived from the vegetative or reproductive module.Moreover,it is unclear wh...Tall clonal grasses commonly display competitive advantages with nitrogen(N)enrichment.However,it is currently unknown whether the height is derived from the vegetative or reproductive module.Moreover,it is unclear whether the height of the vegetative or reproductive system regulates the probability of extinction and colonization,and determines species diversity.In this study,the impacts on clonal grasses were studied in a field experiment employing two frequencies(twice a year vs.monthly)crossing with nine N addition rates in a temperate grassland,China.We found that the N addition decreased species frequency and increased extinction probability,but did not change the species colonization probability.A low frequency of N addition decreased species frequency and colonization probability,but increased extinction probability.Moreover,we found that species reproductive height was the best index to predict the extinction probability of clonal grasses in N-enriched conditions.The low frequency of N addition may overestimate the negative effect from N deposition on clonal grass diversity,suggesting that a higher frequency of N addition is more suitable in assessing the ecological effects of N deposition.Overall,this study illustrates that reproductive height was associated with the clonal species extinction probability under N-enriched environment.展开更多
Nitrogen deposition has dramatically altered biodiversity and ecosystem functioning on the earth; however, its effects on soil bacterial community and the underlying mechanisms of these effects have not been thoroughl...Nitrogen deposition has dramatically altered biodiversity and ecosystem functioning on the earth; however, its effects on soil bacterial community and the underlying mechanisms of these effects have not been thoroughly examined. Changes in ecosystems caused by nitrogen deposition have traditionally been attributed to increased nitrogen content. In fact, nitrogen deposition not only leads to increased soil total N content, but also changes in the NIL^-N content, NO3--N content and pH, as well as changes in the heterogeneity of the four indexes. The soil indexes for these four factors, their heterogeneity and even the plant community might be routes through which nitrogen deposition alters the bacterial community. Here, we describe a 6-year nitrogen addition experiment conducted in a typical steppe ecosystem to investigate the ecological mechanism by which nitrogen deposition alters bacterial abundance, diversity and composition. We found that various characteristics of the bacterial community were explained by different environmental factors. Nitrogen deposition decreased bacterial abundance that is positively related to soil pH value. In addition, nitrogen addition decreased bacterial diversity, which is negatively related to soil total N content and positively related to soil NOa--N heterogeneity. Finally, nitrogen.addition altered bacterial composition that is significantly related to soil NH4＋-N content. Although nitrogen deposition significantly altered plant biomass, diversity and composition, these characteristics of plant community did not have a significant impact on processes of nitrogen deposition that led to alterations in bacterial abundance, diversity and composition. Therefore, more sensitive molecular technologies should be adopted to detect the subtle shifts of microbial community structure induced by the changes of plant community upon nitrogen deposition.展开更多
Ammonia （NH3） emission and redeposition play a major role in terrestrial nitrogen （N） cycles and can also cause environmental problems, such as changes in biodiversity, soil acidity, and eutrophication. Previous f...Ammonia （NH3） emission and redeposition play a major role in terrestrial nitrogen （N） cycles and can also cause environmental problems, such as changes in biodiversity, soil acidity, and eutrophication. Previous field grazing experiments showed inconsistent （positive, neutral, and negative） NH3 volatilization from soils in response to varying grazing intensities. However, it remains unclear whether, or to what extent, NH3 emissions from soil are affected by increasing grazing intensities in Inner Mongolian grasslands. Using a 5-year grazing experiment, we investigated the relationship between NH3 volatilization from soil and grazing pressure （0.0, 3.0, 6.0, and 9.0 sheep/hm2） from June to September of 2009 and 2010 via the vented-chamber method. The results show that soil NH3 volatilization was not significantly different at different grazing intensities in 2009, although it was higher at the highest stocking rate during 2010. There was no significant linear relationship between soil NH3 volatilization rates and soil NH4^-N, but soil NH3 volatilization rates were significantly related to soil water content and air temperature. Grazing intensities had no significant influence on soil NH3 volatilization. Soil NH3 emissions from June to Sep- tember （grazing period）, averaged over all grazing intensities, were 9.6±0.2 and 19.0±0.2 kg N/hm2 in 2009 and 2010, respectively. Moreover, linear equations describing monthly air temperature and precipitation showed a good fit to changes in soil NH3 emissions （r=0.506, P=0.014）. Overall, grazing intensities had less influence than that of climatic factors on soil NH3 emissions. Our findings provide new insights into the effects of grazing on NH3 volatili- zation from soil in Inner Mongolian grasslands, and have important implications for understanding N cycles in grassland ecosystems and for estimating soil NH3 emissions on a regional scale.展开更多
Anthropogenic environmental changes may affect community assembly through mediating both deterministic(e.g.,competitive exclusion and environmental filtering)and stochastic processes(e.g.,birth/death and dispersal/col...Anthropogenic environmental changes may affect community assembly through mediating both deterministic(e.g.,competitive exclusion and environmental filtering)and stochastic processes(e.g.,birth/death and dispersal/colonization).It is traditionally thought that environmental changes have a larger mediation effect on stochastic processes in structuring soil microbial community than aboveground plant community;however,this hypothesis remains largely untested.Here we report an unexpected pattern that nitrogen(N)deposition has a larger mediation effect on stochastic processes in structuring plant community than soil microbial community(those<2 mm in diameter,including archaea,bacteria,fungi,and protists)in the Eurasian steppe.We performed a ten-year nitrogen deposition experiment in a semiarid grassland ecosystem in Inner Mongolia,manipulating nine rates(0–50 g N m^(-2)per year)at two frequencies(nitrogen added twice or 12 times per year)under two grassland management strategies(fencing or mowing).We separated the compositional variation of plant and soil microbial communities caused by each treatment into the deterministic and stochastic components with a recently-developed method.As nitrogen addition rate increased,the relative importance of stochastic component of plant community first increased and then decreased,while that of soil microbial community first decreased and then increased.On the whole,the relative importance of stochastic component was significantly larger in plant community(0.552±0.035;mean±standard error)than in microbial community(0.427±0.035).Consistently,the proportion of compositional variation explained by the deterministic soil and community indices was smaller for plant community(0.172–0.186)than microbial community(0.240–0.767).Meanwhile,as nitrogen addition rate increased,the linkage between plant and microbial community composition first became weaker and then became stronger.The larger stochasticity in plant community relative to microbial community assembly suggested that more stochastic strategies(e.g.,seeds addition)should be adopted to maintain above-than below-ground biodiversity under the pressure of nitrogen deposition.展开更多
Background Nitrogen(N)addition profoundly alters soil phosphatase activities and induces widespread microbial phosphorus(P)limitation in grassland ecosystems.However,the long-term effects of different rates and chemic...Background Nitrogen(N)addition profoundly alters soil phosphatase activities and induces widespread microbial phosphorus(P)limitation in grassland ecosystems.However,the long-term effects of different rates and chemical forms of N addition on soil acid phosphatase(ACP)activity and P availability remain unclear.Here,based on a 10-year N addition experiment,we investigated the effects of a range of N addition rates(0–50 g N m^(-2)yr^(-1))applied as either(NH_(4))_(2)SO_(4)or urea on ACP activity in a northern China meadow steppe,with and without mowing management.Results We found that ACP activity remained unaffected by N addition until certain thresholds were reached(<10 g N m^(-2)yr^(-1)for[(NH_(4))_(2)SO_(4)]and<20 g N m^(-2)yr^(-1)for urea),beyond which it declined sharply.(NH_(4))_(2)SO_(4)exerted a stronger inhibitory effect on ACP activity compared to urea,with the former primarily acting through soil acidification and the latter through a decrease in bacterial diversity.In contrast,mowing regimes did not significantly alter ACP activity under either N form.Conclusions Our findings underscore the necessity of accounting for both N addition rates and N forms when assessing atmospheric N deposition impacts on soil phosphatase activity.These results inform the development of management strategies to mitigate declines in soil enzyme activity and enhance the long-term sustainability of grassland ecosystems.展开更多
Terrestrial species are predicted to migrate northward under global warming conditions,yet little is known about the direction and magnitude of change in microbial distribution patterns.In this continental-scale study...Terrestrial species are predicted to migrate northward under global warming conditions,yet little is known about the direction and magnitude of change in microbial distribution patterns.In this continental-scale study with more than 1600 forest soil samples,we verify the existence of core microbiota and lump them into a manageable number of eco-clusters based on microbial habitat preferences.By projecting the abundance differences of eco-clusters between future and current climatic conditions,we observed the potential warming-driven migration of the core microbiota under warming,partially verified by a field warming experiment at Southwest China.Specifically,the species that favor low p H are potentially expanding and moving northward to medium-latitudes(25°–45°N),potentially implying that warm temperate forest would be under threat of soil acidification with warming.The eco-cluster of high-p H with high-annual mean temperature(AMT)experienced significant abundance increases at middle-(35°–45°N)to high-latitudes(>45°N),especially under Representative Concentration Pathway(RCP)8.5,likely resulting in northward expansion.Furthermore,the eco-cluster that favors low-soil organic carbon(SOC)was projected to increase under warming scenarios at low-latitudes(<25°N),potentially an indicator of SOC storage accumulation in warmer areas.Meanwhile,at high-latitudes(>45°N)the changes in relative abundance of this eco-cluster is inversely related with the temperature variation trends,suggesting microbes-mediated soil organic carbon changes are more responsive to temperature variation in colder areas.These results have vital implications for the migration direction of microbial communities and its potential ecological consequences in future warming scenarios.展开更多
Because of the aggravated agricultural fertilization and combustion of fossil fuels,the nitrogen(N)deposition rate has increased from the pre-industrial levels of about 0.1–0.3 to as high as 10 g N m^(-2)year^(-1)in ...Because of the aggravated agricultural fertilization and combustion of fossil fuels,the nitrogen(N)deposition rate has increased from the pre-industrial levels of about 0.1–0.3 to as high as 10 g N m^(-2)year^(-1)in some developed countries,and it is predicted to increase similarly over the next 50 years in many developing countries(Galloway et al.,2004).展开更多
Background Caloric value is an important indicator of grassland ecosystem function,but the response of caloric value to nitrogen(N)addition and mowing is still unclear.We explored the adaptive changes of plant caloric...Background Caloric value is an important indicator of grassland ecosystem function,but the response of caloric value to nitrogen(N)addition and mowing is still unclear.We explored the adaptive changes of plant caloric value and energy standing crop along a N addition gradient after six-year NH_(4)NO_(3) addition and mowing treatments in an Inner Mongolian temperate meadow steppe in northern China.Results We found that the response of plant caloric value to N addition at different organizational levels was diverse.The caloric value of legumes increased linearly with N addition rates.The caloric value of grasses exhibited a non-linear response trend,initially increasing followed by saturation or decrease,with a N response threshold present.Due to the dominance of grass species,the caloric value at the community level followed a similar pattern to that of the grasses along the N addition gradient.Under mowing,the caloric value of plants at each organizational level increased and usually mowing enhanced the N response threshold.Amongst these,the N response threshold of Leymus chinensis increased from 3.302 to 5.443 g N m^(−2) yr^(−1),grasses increased from 4.414 to 5.746 g N m^(−2) yr^(−1),and community increased from 5.373 to 9.216 g N m^(−2) yr^(−1).Under non-mowing treatment,the N response thresholds of the most dominant species,Leymus chinensis,and community energy standing crop were 10.001 and 15.119 g N m^(−2) yr^(−1),respectively.Under mowing,the energy standing crops showed a linear increasing trend.Conclusions N response thresholds of plant caloric value and energy standing crop vary at different organizational levels(community>functional group>species).The results reveal varying regulatory capabilities of plants on the ecological environment at different organizational levels.These findings enhance our understanding of plant-environment interactions in grassland ecosystems under N deposition from an energy perspective,which is of great significance to clarify the response mechanism of grassland ecosystem structure and function to N deposition.展开更多
Estimation of gross primary production(GPP)from remote sensing data is an important approach to study regional or global carbon cycle.However,for a given algorithm,it usually has its limitation on applications to a wi...Estimation of gross primary production(GPP)from remote sensing data is an important approach to study regional or global carbon cycle.However,for a given algorithm,it usually has its limitation on applications to a wide range of vegetation types and/or under diverse environmental conditions.This study was conducted to compare the performance of two remote sensing GPP algorithms,the MODIS GPP and the vegetation photosynthesis model(VPM),in a semiarid temperate grassland ecosystem.Methods The study was conducted at a typical grassland site in Ujimuqin of Inner Mongolia,North China,over 2 years in 2006 and 2007.Environmental controls on GPP measured by the eddy covariance(EC)technique at the study site were first investigated with path analysis of meteorological and soil moisture data at a daily and 8-day time steps.The estimates of GPP derived from the MODIS GPP and the VPM with site-specific inputs were then compared with the values of EC measurements as ground truthing at the site.Site-specific emax(a)was estimated by using rectangular hyperbola function based on the 7-day flux data at 30-min intervals over the peak period of the growing season(May to September).Important Findings Between the two remote sensing GPP algorithms and various estimates of the fraction of absorbed photosynthetic active radiation(FPAR),the VPM based on FPAR derived from the enhanced vegetation index(EVI)works the best in predicting GPP against the ground truthing of EC GPP.A path analysis indicates that the EC GPP in this semiarid temperate grassland ecosystem is controlled predominantly by both soil water and temperature.The site water condition is slightly better simulated by the moisture multiplier in the VPM than in the MODIS GPP algorithm,which is a most probable explanation for a better performance of the VPM than MODIS GPP algorithm in this semiarid grassland ecosystem.展开更多
Aims We aimed to quantify the variation of leafδ^(13)C along an arid and semi-arid grassland transect in northern China.We also evaluated the effects of environmental factors(i.e.precipitation,temperature and altitud...Aims We aimed to quantify the variation of leafδ^(13)C along an arid and semi-arid grassland transect in northern China.We also evaluated the effects of environmental factors(i.e.precipitation,temperature and altitude)on the spatial variation of leafδ^(13)C in northern grasslands and Tibetan Plateau,China.Method We sampled leaves of plant species belonging to three herb genera(Stipa spp.,Leymus spp.and Cleistogenes spp.)and three shrub genera(Caragana spp.,Reaumuria spp.and Nitraria spp.)for carbon isotope analysis from 50 locations along a 3200-km arid and semiarid grassland transect in northern China.Leafδ^(13)C data in Tibetan Plateau and northern grasslands in China were also compiled from studies in literature.Important Findings Along the transect,leafδ^(13)C for C_(3)plants ranged from−28.0‰to−23.3‰,and from−16.3‰to−13.8‰for C_(4)plant Cleistogenes spp..The change in leafδ^(13)C ranged from−0.26‰to−3.51‰with every 100 mm increase of annual precipitation,and leafδ^(13)C of shrubs(Nitraria spp.,Reaumuria spp.and Caragana spp.)responded more markedly to climatic factors(precipitation and temperature)than that of herbs(Stipa spp.,Leymus spp.and Cleistogenes spp.),indicating higher sensitivity of shrubδ^(13)C to climatic changes.The most important factor regulating spatial variations of leafδ^(13)C in Tibetan Plateau was altitude,while it was precipitation in northern grasslands.Our results suggested that shrubs are more adapted to increasing drought in arid and semi-arid grassland.Controls of environmental factors on leafδ^(13)C depended on the most limiting factors in arid grassland(precipitation)and Tibetan grasslands(atmospheric CO_(2)concentration).展开更多
Aims Nitrogen(N)enrichment caused by human activities threatens bio-diversity and alters plant community composition and structure.It has been found that heavy and infrequent N inputs may over-estimate species extinct...Aims Nitrogen(N)enrichment caused by human activities threatens bio-diversity and alters plant community composition and structure.It has been found that heavy and infrequent N inputs may over-estimate species extinction,but it remains unclear whether plant community structure will equally respond to frequent reactive N enriched conditions.Methods We independently manipulated the rates and the frequencies of N addition in a temperate steppe,northern China,between 2008 and 2013.Important Findings We found that plant community structure changes,measured by‘Euclidean distance’involving species richness,composition and productivity,were significantly positively related to increasing N enrichment rates rather than frequencies.Changes in aboveground net primary productivity(ANPP),plant species richness and shifts in dominant species were observed.Community ANPP increased with N enrichment,whereas species richness reduced.The frequency of N enrichment increased species richness but had no impacts on community ANPP and the relative ANPP of the two dominant spe-cies,C3 perennial bunchgrass Stipa grandis and C3 perennial rhi-zome grass Leymus chinensis.The ANPP and relative ANPP of the two dominant species were significantly negatively correlated with each other.Moreover,changes in the relative ANPP of S.grandis was negatively associated with the changes in community structure.After 5 years’treatment,direct influence of the frequency of N en-richment on plant community structure was not observed,but the effects of the rate of N enrichment were apparent.Our results sug-gested that further study in various ecosystems and with long-term and well-controlled comparisons the frequency vs.the rate of N enrichment may still be needed.展开更多
Aims One major goal of modern community ecology is to understand how deterministic and stochastic processes combine to drive community assembly.However,little empirical knowledge is known about how their relative impo...Aims One major goal of modern community ecology is to understand how deterministic and stochastic processes combine to drive community assembly.However,little empirical knowledge is known about how their relative importance varies between common and rare species.Methods We exploited two 30-year data sets of plant communities in a temperate steppe using two different methods.One is a null model method,and the other is a recently developed direct-calculation method.Important Findings We found that stochastic processes tended to be more important in influencing rare than common species.This finding suggests that stochastic forces may play a more important role in structuring communities with more rare species,providing a possible solution to the debate on the varied importance of deterministic and stochastic processes among different communities.展开更多
Soil microbial biomass is critical for biogeochemical cycling and serves as precursor for carbon(C)sequestration.The anthropogenic nitrogen(N)input has profoundly changed the pool of soil microbial biomass.However,tra...Soil microbial biomass is critical for biogeochemical cycling and serves as precursor for carbon(C)sequestration.The anthropogenic nitrogen(N)input has profoundly changed the pool of soil microbial biomass.However,traditional N deposition simulation experiments have been exclusively conducted through infrequent N addition,which may have caused biased effects on soil microbial biomass compared with those under the natural and continuous N deposition.Convincing data are still scarce about how the different N addition frequencies affect soil microbial biomass.By independently manipulating the frequencies(2 times vs.12 times N addition yr^(–1))and the rates(0–50 g N m^(−2) yr^(−1))of N addition,our study aimed to examine the response of soil microbial biomass C(MBC)to different N addition frequencies with increasing N addition rates.Soil MBC gradually decreased with increasing N addition rates under both N addition frequencies,while the soil MBC decreased more at low frequency of N addition,suggesting that traditional studies have possibly overestimated the effects of N deposition on soil microbial biomass.The greater soil microbial biomass loss with low N frequency resulted from the intensifed soil acidifcation,higher soil inorganic N,stronger soil C and N imbalance,less net primary production allocated to belowground and lower fungi to bacteria ratio.To reliably predict the effects of atmospheric N deposition on soil microbial functioning and C cycling of grassland ecosystems in future studies,it is necessary to employ both the dosage and the frequency of N addition.展开更多
Aims We aimed to improve the understanding of the carbon and nutri-ent physiological responses and adaptation of semi-arid grassland plants to environmental changes.Methods We investigated plant leaf non-structural ca...Aims We aimed to improve the understanding of the carbon and nutri-ent physiological responses and adaptation of semi-arid grassland plants to environmental changes.Methods We investigated plant leaf non-structural carbohydrate(NSC=solu-ble sugars+starch),nitrogen(N)and phosphorus(P)levels in an Inner Mongolian semi-arid grassland community treated with water,N and P additions for 8 years.Two dominant grasses(Agropyron cris-tatum(L.)Gaertn.,Stipa krylovii Roshev.)and two forbs(Artemisia frigida Willd.,Potentilla bifurca L.)were analyzed.Important Findings Water addition decreased plant leaf N and P concentrations,whereas N and P addition increased them,indicating that the semi-arid grassland studied suffers from a shortage of N and P sup-ply.Both N and P addition decreased the levels of soluble sugars,starch and thus also NSC in plant leaves,which may be attributed to(i)increased carbohydrate consumption associated with a higher growth rate,and(ii)a dilution effect of greater plant size under N and P addition.Water addition tended to increase the leaf NSC levels both in the grasses(+9.2%)and forbs(+0.6%only),which may be a result of increased photosynthesis of plants with increased water availability.Under conditions of ambient and increased water supply in the present study,N addition resulted in an N/P ratio of>16 in the grasses but a significantly lower N/P ratio of<11 in the forb species.This finding implies that growth of the two grass spe-cies will be limited mainly by P availability but the forbs will still be mainly limited by N supply if N deposition,alone or in combination with summer precipitation,continues to increase as predicted in Inner Mongolia.展开更多
Aims Ecosystem carbon models often require accurate net ecosystem exchange of CO_(2)(NEE)light-response parameters,which can be derived from the Michaelis–Menten equation.These parameters include maximum net ecosyste...Aims Ecosystem carbon models often require accurate net ecosystem exchange of CO_(2)(NEE)light-response parameters,which can be derived from the Michaelis–Menten equation.These parameters include maximum net ecosystem exchange(NEE_(max)),apparent quantum use efficiency(a)and daytime ecosystem respiration rate(R_(e)).However,little is known about the effects of land conversion between steppe and cropland on these parameters,especially in semi-arid regions.To understand how these parameters vary in responses to biotic and abiotic factors under land conversions,seasonal variation of light-response parameters were evaluated for a steppe and a cropland of Inner Mongolia,China,during three consecutive years(2006–08)with different precipitation amounts.Methods NEE was measured over a steppe and a cropland in Duolun,Inner Mongolia,China,using the eddy covariance technique,and NEE light-response parameters(NEE_(max),α and R_(e))were derived using the Michaelis–Menten model.Biophysical regulations of these parameters were evaluated using a stepwise regression analysis.Important Findings The maximum absolute values of NEE_(max) occurred in the meteorological regimes of 15℃<T_(a)<25℃,vapor pressure deficit(VPD)<1 KPa and 0.21 m^(3) m^(-3)<volumetric soil water content at 10 cm(SWC)<0.28 m^(3) m^(-3) for both the steppe and the cropland ecosystems.The variations of α and R_(e) showed no regular variation pattern in different T_(air),VPD and SWC regimes.Under the same regime of T_(air),VPDand SWC,the cropland had higher absolute values of NEE_(max) than the steppe.Canopy conductance and leaf area index(LAI)were dominant drivers for variations in NEE light-response parameters of the steppe and the cropland.The seasonal variation of NEE light-response parameters followed the variation of LAI for two ecosystems.The peak values of all light-response parameters for the steppe and the cropland occurred fromJuly to August.The values of NEE light-response parameters(NEE_(max),α and R_(e))were lower in the driest year(2007).Seasonally averaged NEE light-response parameters for the cropland surpassed those for the steppe.Land conversion from steppe to cropland enhanced NEE light-response parameters during the plant growing period.These results will have significant implications for improving the models on regional NEE variation under climate change and land-use change scenarios.展开更多
Anthropogenic disturbances may decrease as we take measures to control them.However,the patterns and mechanisms of postdisturbance ecosystem succession have rarely been studied.Here we reported that disturbance level ...Anthropogenic disturbances may decrease as we take measures to control them.However,the patterns and mechanisms of postdisturbance ecosystem succession have rarely been studied.Here we reported that disturbance level determined the importance of stochastic relative to deterministic changes in ecosystem components(plant community composition,soil microbial community composition,and soil physicochemical indices),and thus predefined the pattern of post-disturbance ecosystem succession.We proposed a theoretical framework with five disturbance levels corresponding to distinct succession patterns.We conducted a nitrogen addition experiment in a temperate steppe,monitored these ecosystem components during"disturbance"treatment(2010-2014)and post-treatment"succession"(2014-2018).The disturbance level experienced by each component in each treatment was inferred by fitting the observed succession patterns into the theoretical framework.The mean disturbance level of these components was found to increase quadratically with nitrogen addition rate.This was because increasing nitrogen addition reduced the importance of stochastic relative to deterministic changes in these components,and these changes had a quadratic relationship with disturbance level.Overall,our results suggested that by monitoring the importance of stochastic relative to deterministic changes in an ecosystem,we can estimate disturbance levels and predict succession patterns,as well as propose disturbance-level-dependent strategies for post-disturbance restoration.展开更多
植物凋落物分解是陆地生态系统碳平衡和养分周转的关键,对持续的人为氮输入增加十分敏感。氮素增加影响凋落物分解的研究大多依赖于短期实验,这可能掩盖氮素增加对凋落物分解的真实影响。因此,长期实验对于全面评价凋落物分解对氮素增...植物凋落物分解是陆地生态系统碳平衡和养分周转的关键,对持续的人为氮输入增加十分敏感。氮素增加影响凋落物分解的研究大多依赖于短期实验,这可能掩盖氮素增加对凋落物分解的真实影响。因此,长期实验对于全面评价凋落物分解对氮素增加的响应是非常必要的。本研究基于氮添加实验(0-50 g N m^(-2) yr^(-1)),对羊草(Leymus chinensis)凋落物分解进行了长达4年的研究,探讨了影响凋落物分解的非生物和生物因子。研究结果表明,随着氮添加速率的增加,凋落物分解速率呈持续下降趋势,为氮添加抑制凋落物分解提供了可靠的证据。氮添加条件下土壤环境(酸化和养分化学计量学)、微生物活性(微生物生物量和酶活性)、凋落物质量(残余木质素和养分含量)和植物群落(地上生产力和物种丰富度)的变化共同导致了凋落物分解速率的降低。在分解过程中,凋落物质量的变化,包括木质素的积累和养分的残留,主要受土壤环境和微生物活性的影响。该研究有助于阐明不同氮添加速率对凋落物分解的长期影响,并有助于理解生态系统氮素增加与碳循环之间的关系及相关机理。展开更多
Aims Bacteria and fungi are two primary groups of soil microbes,and their stability determines the persistence of microbial functions in response to a changing environment.Recent studies reported higher fungal than ba...Aims Bacteria and fungi are two primary groups of soil microbes,and their stability determines the persistence of microbial functions in response to a changing environment.Recent studies reported higher fungal than bacterial stability under precipitation alteration,the underlying mechanisms,however,remain elusive.Methods A 3-year precipitation manipulation experiment in a semi-arid grassland was used to compare the bacterial and fungal diversities,including alpha diversity,beta diversity and microbial community composition turnover,in response to precipitation manipulations.A framework is proposed to understand the stability properties of bacteria and fungi under precipitation alteration.We conceived a diagrammatic valley to illustrate microbial stability with the depth representing resistance and the width ecological resilience.Important Findings We found that±60%in precipitation significantly reduced the richness and increased the evenness of bacteria but had trivial impacts on fungi.Precipitation alteration yielded stronger impacts on the variation in alpha diversity of bacteria than fungi,suggesting that the bacterial community is more sensitive to water stress than the fungal community.Moreover,fungi had wider composition turnover than that of bacteria,indicating higher composition variation of fungi than bacteria.The population turnover of fungi,reflected by composition variation,coefficient variation of diversity index and composition turnover,was larger than that of bacteria at both temporal and spatial scales,indicating the population turnover promotes fungal stability.The higher stability of fungal community in tolerating water stress is analogous to a ball in a wide valley that swing substantially but remain close to its steady state;while the lower stability of bacteria community is analogous to a ball that swings slightly but stay far away from its steady state.Our finding that the fungal community had higher stability than bacterial community in a semi-arid grassland might be applicable to other biomes.展开更多
基金supported by the National Natural Science Foundation of China(Nos.42130515 and 32301450)the Open Foundation of State Key Laboratory of Desert and Oasis Ecology,Xinjiang Institute of Ecology and Geography,Chinese Academy of Sciences.
文摘Increasing anthropogenic nitrogen(N)inputs has profoundly altered soil microbial necromass carbon(MNC),which serves as a key source of soil organic carbon(SOC).Yet,the response pattern of MNC and its contribution to SOC across a wide range of N addition rates,remain elusive.In a temperate grassland with six years'consecutive N addition spanning seven rates(0-50 g N/(m^(2)·year))in Inner Mongolia,China,we explored the responses of soil MNC and its contribution to SOC.The soil MNC showed a hump-shaped pattern to increasing N addition rates,with the N saturation threshold at 18.07 g N/(m^(2)·year).The soil MNC was driven by nematode abundance and the ratio of bacterial to fungal biomass below the N threshold,and by plant biomass allocation pattern and diversity above the N threshold.The contribution of soil MNC to SOC declined with increasing N addition rates,and was mainly regulated by the ratio of MNC to mineral-associated organic carbon and plant diversity and the ratio of bacterial to fungal biomass.In addition,the soil MNC and SOC differentially responded to N addition and were mediated by disparate biological and geochemical mechanisms,leading to the decoupled MNC production from SOC formation.Together,in this N-enriched temperate grassland,the soilmicrobial necro-mass production tends to be insufficient as a general explanation linking SOC formation.This study expands the mechanistic comprehension of the connections between external N input and soil carbon sequestration.
基金supported by a grant from the National Natural Science Foundation of China(grant no.32071603 and 32122055)the Strategic Priority Research Program of the Chinese Academy of Sciences(XDA26020101)。
文摘Tall clonal grasses commonly display competitive advantages with nitrogen(N)enrichment.However,it is currently unknown whether the height is derived from the vegetative or reproductive module.Moreover,it is unclear whether the height of the vegetative or reproductive system regulates the probability of extinction and colonization,and determines species diversity.In this study,the impacts on clonal grasses were studied in a field experiment employing two frequencies(twice a year vs.monthly)crossing with nine N addition rates in a temperate grassland,China.We found that the N addition decreased species frequency and increased extinction probability,but did not change the species colonization probability.A low frequency of N addition decreased species frequency and colonization probability,but increased extinction probability.Moreover,we found that species reproductive height was the best index to predict the extinction probability of clonal grasses in N-enriched conditions.The low frequency of N addition may overestimate the negative effect from N deposition on clonal grass diversity,suggesting that a higher frequency of N addition is more suitable in assessing the ecological effects of N deposition.Overall,this study illustrates that reproductive height was associated with the clonal species extinction probability under N-enriched environment.
基金supported by the National Natural Science Foundation of China (No. 30830026,30870407)the Postdoctor Science Foundation of China (No.2011M500440)
文摘Nitrogen deposition has dramatically altered biodiversity and ecosystem functioning on the earth; however, its effects on soil bacterial community and the underlying mechanisms of these effects have not been thoroughly examined. Changes in ecosystems caused by nitrogen deposition have traditionally been attributed to increased nitrogen content. In fact, nitrogen deposition not only leads to increased soil total N content, but also changes in the NIL^-N content, NO3--N content and pH, as well as changes in the heterogeneity of the four indexes. The soil indexes for these four factors, their heterogeneity and even the plant community might be routes through which nitrogen deposition alters the bacterial community. Here, we describe a 6-year nitrogen addition experiment conducted in a typical steppe ecosystem to investigate the ecological mechanism by which nitrogen deposition alters bacterial abundance, diversity and composition. We found that various characteristics of the bacterial community were explained by different environmental factors. Nitrogen deposition decreased bacterial abundance that is positively related to soil pH value. In addition, nitrogen addition decreased bacterial diversity, which is negatively related to soil total N content and positively related to soil NOa--N heterogeneity. Finally, nitrogen.addition altered bacterial composition that is significantly related to soil NH4＋-N content. Although nitrogen deposition significantly altered plant biomass, diversity and composition, these characteristics of plant community did not have a significant impact on processes of nitrogen deposition that led to alterations in bacterial abundance, diversity and composition. Therefore, more sensitive molecular technologies should be adopted to detect the subtle shifts of microbial community structure induced by the changes of plant community upon nitrogen deposition.
基金Funding for this work came from the National Natural Science Foundation of China (30830026)the National Basic Research Program of China (2009CB825103)the Innovative Research Group Project of the National Natural Science Foundation of China (30821062)
文摘Ammonia （NH3） emission and redeposition play a major role in terrestrial nitrogen （N） cycles and can also cause environmental problems, such as changes in biodiversity, soil acidity, and eutrophication. Previous field grazing experiments showed inconsistent （positive, neutral, and negative） NH3 volatilization from soils in response to varying grazing intensities. However, it remains unclear whether, or to what extent, NH3 emissions from soil are affected by increasing grazing intensities in Inner Mongolian grasslands. Using a 5-year grazing experiment, we investigated the relationship between NH3 volatilization from soil and grazing pressure （0.0, 3.0, 6.0, and 9.0 sheep/hm2） from June to September of 2009 and 2010 via the vented-chamber method. The results show that soil NH3 volatilization was not significantly different at different grazing intensities in 2009, although it was higher at the highest stocking rate during 2010. There was no significant linear relationship between soil NH3 volatilization rates and soil NH4^-N, but soil NH3 volatilization rates were significantly related to soil water content and air temperature. Grazing intensities had no significant influence on soil NH3 volatilization. Soil NH3 emissions from June to Sep- tember （grazing period）, averaged over all grazing intensities, were 9.6±0.2 and 19.0±0.2 kg N/hm2 in 2009 and 2010, respectively. Moreover, linear equations describing monthly air temperature and precipitation showed a good fit to changes in soil NH3 emissions （r=0.506, P=0.014）. Overall, grazing intensities had less influence than that of climatic factors on soil NH3 emissions. Our findings provide new insights into the effects of grazing on NH3 volatili- zation from soil in Inner Mongolian grasslands, and have important implications for understanding N cycles in grassland ecosystems and for estimating soil NH3 emissions on a regional scale.
基金supported by the National Natural Science Foundation of China(32071547,U21A20188)the Top-Notch Young Talents Program(to Ximei Zhang)of Chinathe Agricultural Science and Technology Innovation Program(to Ximei Zhang)。
文摘Anthropogenic environmental changes may affect community assembly through mediating both deterministic(e.g.,competitive exclusion and environmental filtering)and stochastic processes(e.g.,birth/death and dispersal/colonization).It is traditionally thought that environmental changes have a larger mediation effect on stochastic processes in structuring soil microbial community than aboveground plant community;however,this hypothesis remains largely untested.Here we report an unexpected pattern that nitrogen(N)deposition has a larger mediation effect on stochastic processes in structuring plant community than soil microbial community(those<2 mm in diameter,including archaea,bacteria,fungi,and protists)in the Eurasian steppe.We performed a ten-year nitrogen deposition experiment in a semiarid grassland ecosystem in Inner Mongolia,manipulating nine rates(0–50 g N m^(-2)per year)at two frequencies(nitrogen added twice or 12 times per year)under two grassland management strategies(fencing or mowing).We separated the compositional variation of plant and soil microbial communities caused by each treatment into the deterministic and stochastic components with a recently-developed method.As nitrogen addition rate increased,the relative importance of stochastic component of plant community first increased and then decreased,while that of soil microbial community first decreased and then increased.On the whole,the relative importance of stochastic component was significantly larger in plant community(0.552±0.035;mean±standard error)than in microbial community(0.427±0.035).Consistently,the proportion of compositional variation explained by the deterministic soil and community indices was smaller for plant community(0.172–0.186)than microbial community(0.240–0.767).Meanwhile,as nitrogen addition rate increased,the linkage between plant and microbial community composition first became weaker and then became stronger.The larger stochasticity in plant community relative to microbial community assembly suggested that more stochastic strategies(e.g.,seeds addition)should be adopted to maintain above-than below-ground biodiversity under the pressure of nitrogen deposition.
基金financially supported by Hebei Natural Science Foundation,China (C2022201042)
文摘Background Nitrogen(N)addition profoundly alters soil phosphatase activities and induces widespread microbial phosphorus(P)limitation in grassland ecosystems.However,the long-term effects of different rates and chemical forms of N addition on soil acid phosphatase(ACP)activity and P availability remain unclear.Here,based on a 10-year N addition experiment,we investigated the effects of a range of N addition rates(0–50 g N m^(-2)yr^(-1))applied as either(NH_(4))_(2)SO_(4)or urea on ACP activity in a northern China meadow steppe,with and without mowing management.Results We found that ACP activity remained unaffected by N addition until certain thresholds were reached(<10 g N m^(-2)yr^(-1)for[(NH_(4))_(2)SO_(4)]and<20 g N m^(-2)yr^(-1)for urea),beyond which it declined sharply.(NH_(4))_(2)SO_(4)exerted a stronger inhibitory effect on ACP activity compared to urea,with the former primarily acting through soil acidification and the latter through a decrease in bacterial diversity.In contrast,mowing regimes did not significantly alter ACP activity under either N form.Conclusions Our findings underscore the necessity of accounting for both N addition rates and N forms when assessing atmospheric N deposition impacts on soil phosphatase activity.These results inform the development of management strategies to mitigate declines in soil enzyme activity and enhance the long-term sustainability of grassland ecosystems.
基金supported by the Strategic Priority Research Program of Chinese Academy of Sciences(XDB15010300)the National Science Foundation of China(U1602234,41807316,41471218,41501282,and 31870467)+1 种基金the CAS 135 project(2017XTBG-F01)the National Key Research and Development Program(2016YFC0500702)。
文摘Terrestrial species are predicted to migrate northward under global warming conditions,yet little is known about the direction and magnitude of change in microbial distribution patterns.In this continental-scale study with more than 1600 forest soil samples,we verify the existence of core microbiota and lump them into a manageable number of eco-clusters based on microbial habitat preferences.By projecting the abundance differences of eco-clusters between future and current climatic conditions,we observed the potential warming-driven migration of the core microbiota under warming,partially verified by a field warming experiment at Southwest China.Specifically,the species that favor low p H are potentially expanding and moving northward to medium-latitudes(25°–45°N),potentially implying that warm temperate forest would be under threat of soil acidification with warming.The eco-cluster of high-p H with high-annual mean temperature(AMT)experienced significant abundance increases at middle-(35°–45°N)to high-latitudes(>45°N),especially under Representative Concentration Pathway(RCP)8.5,likely resulting in northward expansion.Furthermore,the eco-cluster that favors low-soil organic carbon(SOC)was projected to increase under warming scenarios at low-latitudes(<25°N),potentially an indicator of SOC storage accumulation in warmer areas.Meanwhile,at high-latitudes(>45°N)the changes in relative abundance of this eco-cluster is inversely related with the temperature variation trends,suggesting microbes-mediated soil organic carbon changes are more responsive to temperature variation in colder areas.These results have vital implications for the migration direction of microbial communities and its potential ecological consequences in future warming scenarios.
基金supported by the National Natural Science Foundation of China(32071547U21A20188)+1 种基金the Top-Notch Young Talents Program(to Ximei Zhang)of Chinathe Agricultural Science and Technology Innovation Program(to Ximei Zhang)。
文摘Because of the aggravated agricultural fertilization and combustion of fossil fuels,the nitrogen(N)deposition rate has increased from the pre-industrial levels of about 0.1–0.3 to as high as 10 g N m^(-2)year^(-1)in some developed countries,and it is predicted to increase similarly over the next 50 years in many developing countries(Galloway et al.,2004).
基金funded by the National Natural Science Foundation of China(grant numbers 31971464 and 32371639)basic research business fees of central universities of China(grant number 0919-140124).
文摘Background Caloric value is an important indicator of grassland ecosystem function,but the response of caloric value to nitrogen(N)addition and mowing is still unclear.We explored the adaptive changes of plant caloric value and energy standing crop along a N addition gradient after six-year NH_(4)NO_(3) addition and mowing treatments in an Inner Mongolian temperate meadow steppe in northern China.Results We found that the response of plant caloric value to N addition at different organizational levels was diverse.The caloric value of legumes increased linearly with N addition rates.The caloric value of grasses exhibited a non-linear response trend,initially increasing followed by saturation or decrease,with a N response threshold present.Due to the dominance of grass species,the caloric value at the community level followed a similar pattern to that of the grasses along the N addition gradient.Under mowing,the caloric value of plants at each organizational level increased and usually mowing enhanced the N response threshold.Amongst these,the N response threshold of Leymus chinensis increased from 3.302 to 5.443 g N m^(−2) yr^(−1),grasses increased from 4.414 to 5.746 g N m^(−2) yr^(−1),and community increased from 5.373 to 9.216 g N m^(−2) yr^(−1).Under non-mowing treatment,the N response thresholds of the most dominant species,Leymus chinensis,and community energy standing crop were 10.001 and 15.119 g N m^(−2) yr^(−1),respectively.Under mowing,the energy standing crops showed a linear increasing trend.Conclusions N response thresholds of plant caloric value and energy standing crop vary at different organizational levels(community>functional group>species).The results reveal varying regulatory capabilities of plants on the ecological environment at different organizational levels.These findings enhance our understanding of plant-environment interactions in grassland ecosystems under N deposition from an energy perspective,which is of great significance to clarify the response mechanism of grassland ecosystem structure and function to N deposition.
基金National Natural Science Foundation of China(grants 30521002 and 30821062)The 948 program of the State Forestry Administration of China(grant 2006-4-02)+1 种基金NASANEWS NN-H-04-Z-YS-005-N programUSCCC program.
文摘Estimation of gross primary production(GPP)from remote sensing data is an important approach to study regional or global carbon cycle.However,for a given algorithm,it usually has its limitation on applications to a wide range of vegetation types and/or under diverse environmental conditions.This study was conducted to compare the performance of two remote sensing GPP algorithms,the MODIS GPP and the vegetation photosynthesis model(VPM),in a semiarid temperate grassland ecosystem.Methods The study was conducted at a typical grassland site in Ujimuqin of Inner Mongolia,North China,over 2 years in 2006 and 2007.Environmental controls on GPP measured by the eddy covariance(EC)technique at the study site were first investigated with path analysis of meteorological and soil moisture data at a daily and 8-day time steps.The estimates of GPP derived from the MODIS GPP and the VPM with site-specific inputs were then compared with the values of EC measurements as ground truthing at the site.Site-specific emax(a)was estimated by using rectangular hyperbola function based on the 7-day flux data at 30-min intervals over the peak period of the growing season(May to September).Important Findings Between the two remote sensing GPP algorithms and various estimates of the fraction of absorbed photosynthetic active radiation(FPAR),the VPM based on FPAR derived from the enhanced vegetation index(EVI)works the best in predicting GPP against the ground truthing of EC GPP.A path analysis indicates that the EC GPP in this semiarid temperate grassland ecosystem is controlled predominantly by both soil water and temperature.The site water condition is slightly better simulated by the moisture multiplier in the VPM than in the MODIS GPP algorithm,which is a most probable explanation for a better performance of the VPM than MODIS GPP algorithm in this semiarid grassland ecosystem.
基金National Basic Research Program of China(973 program,2014CB954400)the National Natural Science Foundation of China(31522010)State Key Laboratory of Forest and Soil Ecology(LFSE2013-13 and LFSE2015-18).
文摘Aims We aimed to quantify the variation of leafδ^(13)C along an arid and semi-arid grassland transect in northern China.We also evaluated the effects of environmental factors(i.e.precipitation,temperature and altitude)on the spatial variation of leafδ^(13)C in northern grasslands and Tibetan Plateau,China.Method We sampled leaves of plant species belonging to three herb genera(Stipa spp.,Leymus spp.and Cleistogenes spp.)and three shrub genera(Caragana spp.,Reaumuria spp.and Nitraria spp.)for carbon isotope analysis from 50 locations along a 3200-km arid and semiarid grassland transect in northern China.Leafδ^(13)C data in Tibetan Plateau and northern grasslands in China were also compiled from studies in literature.Important Findings Along the transect,leafδ^(13)C for C_(3)plants ranged from−28.0‰to−23.3‰,and from−16.3‰to−13.8‰for C_(4)plant Cleistogenes spp..The change in leafδ^(13)C ranged from−0.26‰to−3.51‰with every 100 mm increase of annual precipitation,and leafδ^(13)C of shrubs(Nitraria spp.,Reaumuria spp.and Caragana spp.)responded more markedly to climatic factors(precipitation and temperature)than that of herbs(Stipa spp.,Leymus spp.and Cleistogenes spp.),indicating higher sensitivity of shrubδ^(13)C to climatic changes.The most important factor regulating spatial variations of leafδ^(13)C in Tibetan Plateau was altitude,while it was precipitation in northern grasslands.Our results suggested that shrubs are more adapted to increasing drought in arid and semi-arid grassland.Controls of environmental factors on leafδ^(13)C depended on the most limiting factors in arid grassland(precipitation)and Tibetan grasslands(atmospheric CO_(2)concentration).
基金National Natural Science Foundation of China(NSFC31570469)+2 种基金China Postdoctoral Science Foundation(2015T80153)to Y.Z.,National Key R&D program of China(2016YFC0500202)N.H.,NSFC(41573063)C.W.and National Key R&D program of China(2016YFC0500700)and NSFC(31430016)to X.H.
文摘Aims Nitrogen(N)enrichment caused by human activities threatens bio-diversity and alters plant community composition and structure.It has been found that heavy and infrequent N inputs may over-estimate species extinction,but it remains unclear whether plant community structure will equally respond to frequent reactive N enriched conditions.Methods We independently manipulated the rates and the frequencies of N addition in a temperate steppe,northern China,between 2008 and 2013.Important Findings We found that plant community structure changes,measured by‘Euclidean distance’involving species richness,composition and productivity,were significantly positively related to increasing N enrichment rates rather than frequencies.Changes in aboveground net primary productivity(ANPP),plant species richness and shifts in dominant species were observed.Community ANPP increased with N enrichment,whereas species richness reduced.The frequency of N enrichment increased species richness but had no impacts on community ANPP and the relative ANPP of the two dominant spe-cies,C3 perennial bunchgrass Stipa grandis and C3 perennial rhi-zome grass Leymus chinensis.The ANPP and relative ANPP of the two dominant species were significantly negatively correlated with each other.Moreover,changes in the relative ANPP of S.grandis was negatively associated with the changes in community structure.After 5 years’treatment,direct influence of the frequency of N en-richment on plant community structure was not observed,but the effects of the rate of N enrichment were apparent.Our results sug-gested that further study in various ecosystems and with long-term and well-controlled comparisons the frequency vs.the rate of N enrichment may still be needed.
基金National Natural Science Foundation(31300431)State Key Laboratory of Forest and Soil Ecology(LFSE2013-15)of ChinaChina Scholarship Council(CSC NO.201404910179).
文摘Aims One major goal of modern community ecology is to understand how deterministic and stochastic processes combine to drive community assembly.However,little empirical knowledge is known about how their relative importance varies between common and rare species.Methods We exploited two 30-year data sets of plant communities in a temperate steppe using two different methods.One is a null model method,and the other is a recently developed direct-calculation method.Important Findings We found that stochastic processes tended to be more important in influencing rare than common species.This finding suggests that stochastic forces may play a more important role in structuring communities with more rare species,providing a possible solution to the debate on the varied importance of deterministic and stochastic processes among different communities.
基金supported by the National Natural Science Foundation of China(42130515 and31770506)the Open Foundation of the State Key Laboratory of Urban and Regional Ecology of Chinathe Open Foundation of the State Key Laboratory of Grassland Agro-ecosystems of China。
文摘Soil microbial biomass is critical for biogeochemical cycling and serves as precursor for carbon(C)sequestration.The anthropogenic nitrogen(N)input has profoundly changed the pool of soil microbial biomass.However,traditional N deposition simulation experiments have been exclusively conducted through infrequent N addition,which may have caused biased effects on soil microbial biomass compared with those under the natural and continuous N deposition.Convincing data are still scarce about how the different N addition frequencies affect soil microbial biomass.By independently manipulating the frequencies(2 times vs.12 times N addition yr^(–1))and the rates(0–50 g N m^(−2) yr^(−1))of N addition,our study aimed to examine the response of soil microbial biomass C(MBC)to different N addition frequencies with increasing N addition rates.Soil MBC gradually decreased with increasing N addition rates under both N addition frequencies,while the soil MBC decreased more at low frequency of N addition,suggesting that traditional studies have possibly overestimated the effects of N deposition on soil microbial biomass.The greater soil microbial biomass loss with low N frequency resulted from the intensifed soil acidifcation,higher soil inorganic N,stronger soil C and N imbalance,less net primary production allocated to belowground and lower fungi to bacteria ratio.To reliably predict the effects of atmospheric N deposition on soil microbial functioning and C cycling of grassland ecosystems in future studies,it is necessary to employ both the dosage and the frequency of N addition.
基金The National Natural Science Foundation of China(41371076,31370009)the National Key Basic Research Program of China(2011CB403204)the State Key Laboratory of Forest and Soil Ecology(LFSE2013-01).
文摘Aims We aimed to improve the understanding of the carbon and nutri-ent physiological responses and adaptation of semi-arid grassland plants to environmental changes.Methods We investigated plant leaf non-structural carbohydrate(NSC=solu-ble sugars+starch),nitrogen(N)and phosphorus(P)levels in an Inner Mongolian semi-arid grassland community treated with water,N and P additions for 8 years.Two dominant grasses(Agropyron cris-tatum(L.)Gaertn.,Stipa krylovii Roshev.)and two forbs(Artemisia frigida Willd.,Potentilla bifurca L.)were analyzed.Important Findings Water addition decreased plant leaf N and P concentrations,whereas N and P addition increased them,indicating that the semi-arid grassland studied suffers from a shortage of N and P sup-ply.Both N and P addition decreased the levels of soluble sugars,starch and thus also NSC in plant leaves,which may be attributed to(i)increased carbohydrate consumption associated with a higher growth rate,and(ii)a dilution effect of greater plant size under N and P addition.Water addition tended to increase the leaf NSC levels both in the grasses(+9.2%)and forbs(+0.6%only),which may be a result of increased photosynthesis of plants with increased water availability.Under conditions of ambient and increased water supply in the present study,N addition resulted in an N/P ratio of>16 in the grasses but a significantly lower N/P ratio of<11 in the forb species.This finding implies that growth of the two grass spe-cies will be limited mainly by P availability but the forbs will still be mainly limited by N supply if N deposition,alone or in combination with summer precipitation,continues to increase as predicted in Inner Mongolia.
基金National Basic Research Program of China(973 program)(2010CB833501)the National Natural Science Foundation of China(30800141)+1 种基金the LCLUC Program of the National Aeronautics and Space Administration(NN-H-04-Z-YS-005-N)the US-China Carbon Consortium,which promotes collaborative research among institutions in the USA and China.
文摘Aims Ecosystem carbon models often require accurate net ecosystem exchange of CO_(2)(NEE)light-response parameters,which can be derived from the Michaelis–Menten equation.These parameters include maximum net ecosystem exchange(NEE_(max)),apparent quantum use efficiency(a)and daytime ecosystem respiration rate(R_(e)).However,little is known about the effects of land conversion between steppe and cropland on these parameters,especially in semi-arid regions.To understand how these parameters vary in responses to biotic and abiotic factors under land conversions,seasonal variation of light-response parameters were evaluated for a steppe and a cropland of Inner Mongolia,China,during three consecutive years(2006–08)with different precipitation amounts.Methods NEE was measured over a steppe and a cropland in Duolun,Inner Mongolia,China,using the eddy covariance technique,and NEE light-response parameters(NEE_(max),α and R_(e))were derived using the Michaelis–Menten model.Biophysical regulations of these parameters were evaluated using a stepwise regression analysis.Important Findings The maximum absolute values of NEE_(max) occurred in the meteorological regimes of 15℃<T_(a)<25℃,vapor pressure deficit(VPD)<1 KPa and 0.21 m^(3) m^(-3)<volumetric soil water content at 10 cm(SWC)<0.28 m^(3) m^(-3) for both the steppe and the cropland ecosystems.The variations of α and R_(e) showed no regular variation pattern in different T_(air),VPD and SWC regimes.Under the same regime of T_(air),VPDand SWC,the cropland had higher absolute values of NEE_(max) than the steppe.Canopy conductance and leaf area index(LAI)were dominant drivers for variations in NEE light-response parameters of the steppe and the cropland.The seasonal variation of NEE light-response parameters followed the variation of LAI for two ecosystems.The peak values of all light-response parameters for the steppe and the cropland occurred fromJuly to August.The values of NEE light-response parameters(NEE_(max),α and R_(e))were lower in the driest year(2007).Seasonally averaged NEE light-response parameters for the cropland surpassed those for the steppe.Land conversion from steppe to cropland enhanced NEE light-response parameters during the plant growing period.These results will have significant implications for improving the models on regional NEE variation under climate change and land-use change scenarios.
基金supported by the National Key Research and Development Program(2016YFC0500702)the National Natural Science Foundation of China(32071547)+1 种基金the Top-Notch Young Talents Program of Chinathe Science and Technology Innovation Project of Chinese Academy of Agricultural Sciences。
文摘Anthropogenic disturbances may decrease as we take measures to control them.However,the patterns and mechanisms of postdisturbance ecosystem succession have rarely been studied.Here we reported that disturbance level determined the importance of stochastic relative to deterministic changes in ecosystem components(plant community composition,soil microbial community composition,and soil physicochemical indices),and thus predefined the pattern of post-disturbance ecosystem succession.We proposed a theoretical framework with five disturbance levels corresponding to distinct succession patterns.We conducted a nitrogen addition experiment in a temperate steppe,monitored these ecosystem components during"disturbance"treatment(2010-2014)and post-treatment"succession"(2014-2018).The disturbance level experienced by each component in each treatment was inferred by fitting the observed succession patterns into the theoretical framework.The mean disturbance level of these components was found to increase quadratically with nitrogen addition rate.This was because increasing nitrogen addition reduced the importance of stochastic relative to deterministic changes in these components,and these changes had a quadratic relationship with disturbance level.Overall,our results suggested that by monitoring the importance of stochastic relative to deterministic changes in an ecosystem,we can estimate disturbance levels and predict succession patterns,as well as propose disturbance-level-dependent strategies for post-disturbance restoration.
基金supported by the National Natural Science Foundation of China(42130515,31770506)the Open Foundation of the State Key Laboratory of Urban and Regional Ecology of China and the Open Foundation of the State Key Laboratory of Herbage Improvement and Grassland Agro-ecosystems.
文摘植物凋落物分解是陆地生态系统碳平衡和养分周转的关键,对持续的人为氮输入增加十分敏感。氮素增加影响凋落物分解的研究大多依赖于短期实验,这可能掩盖氮素增加对凋落物分解的真实影响。因此,长期实验对于全面评价凋落物分解对氮素增加的响应是非常必要的。本研究基于氮添加实验(0-50 g N m^(-2) yr^(-1)),对羊草(Leymus chinensis)凋落物分解进行了长达4年的研究,探讨了影响凋落物分解的非生物和生物因子。研究结果表明,随着氮添加速率的增加,凋落物分解速率呈持续下降趋势,为氮添加抑制凋落物分解提供了可靠的证据。氮添加条件下土壤环境(酸化和养分化学计量学)、微生物活性(微生物生物量和酶活性)、凋落物质量(残余木质素和养分含量)和植物群落(地上生产力和物种丰富度)的变化共同导致了凋落物分解速率的降低。在分解过程中,凋落物质量的变化,包括木质素的积累和养分的残留,主要受土壤环境和微生物活性的影响。该研究有助于阐明不同氮添加速率对凋落物分解的长期影响,并有助于理解生态系统氮素增加与碳循环之间的关系及相关机理。
基金supported by the Chinese National Key Development Program for Basic Research(grant no.2017YFA0604802 and 2016YFC0500703)National Natural Science Foundation of China(grant no.31770526 and 41573063)+1 种基金Strategic Priority Research Program on Soil and Microbes of the Chinese Academy of Sciences(grant no.XDB15010401)Key Laboratory of Vegetation Ecology,Ministry of Education.X.X.acknowledges the financial support from the San Diego State University and the CSU Program for Education&Research in Biotechnology.
文摘Aims Bacteria and fungi are two primary groups of soil microbes,and their stability determines the persistence of microbial functions in response to a changing environment.Recent studies reported higher fungal than bacterial stability under precipitation alteration,the underlying mechanisms,however,remain elusive.Methods A 3-year precipitation manipulation experiment in a semi-arid grassland was used to compare the bacterial and fungal diversities,including alpha diversity,beta diversity and microbial community composition turnover,in response to precipitation manipulations.A framework is proposed to understand the stability properties of bacteria and fungi under precipitation alteration.We conceived a diagrammatic valley to illustrate microbial stability with the depth representing resistance and the width ecological resilience.Important Findings We found that±60%in precipitation significantly reduced the richness and increased the evenness of bacteria but had trivial impacts on fungi.Precipitation alteration yielded stronger impacts on the variation in alpha diversity of bacteria than fungi,suggesting that the bacterial community is more sensitive to water stress than the fungal community.Moreover,fungi had wider composition turnover than that of bacteria,indicating higher composition variation of fungi than bacteria.The population turnover of fungi,reflected by composition variation,coefficient variation of diversity index and composition turnover,was larger than that of bacteria at both temporal and spatial scales,indicating the population turnover promotes fungal stability.The higher stability of fungal community in tolerating water stress is analogous to a ball in a wide valley that swing substantially but remain close to its steady state;while the lower stability of bacteria community is analogous to a ball that swings slightly but stay far away from its steady state.Our finding that the fungal community had higher stability than bacterial community in a semi-arid grassland might be applicable to other biomes.
