摘要
By means of ^15N tracer technique the fate of N in ammonium sulfate,milk vetch,sesbania and azolla,and the availability of their residual N were studied in a microplot experiment.Results showed that a) at the end of the first crop of early rice,both plant recovery and loss of N from ammonium sulfate were the highest whereas those from azolla were the lowest with those from milk vetch and sesbania in between;the sequence was reversed in terms of recovery of N in soil;the net residual N from ammonium sulfate was very low,about 1/7-1/4 of that from green manures,indicating that chemical N fertilizer contributes little to the soil N reserve;b) plant recovery of the residual N was low and it did not always decrease with time;the total plant recovery (from the second to the fifth crops) of the residual N from various test fertilizers was only 8-11% of the total N originally applied;c) the plant recovery of the residual N from ammonium sulfate was the highest,followed by those from milk vetch and sesbania,and that from azolla was the lowest,no matter in which cropping season (from the second to the fifth);N availability ratio showed the same trend,indicating that chemical N fertilizer helps renovate soil organic N,maintain and increase availability of soil N.
By means of 15N tracer technique the fate of N in ammonium sulfate, milk vetch, sesbania and azolla, and the availability of their residual N were studied in a microplot experiment. Results showed that a) at the end of the first crop of early rice, both plant recovery and loss of N from ammonium sulfate were the highest whereas those from azolla were the lowest with those from milk vetch and sesbania in between; the sequence was reversed in terms of recovery of N in soil; the net residual N from ammonium sulfate was very low, about 1 / 7-1 / 4 of that from green manures, indicating that chemical N fertilizer contributes little to the soil N reserve; b) plant recovery of the residual N was low and it did not always decrease with time; the total plant recovery (from the second to the fifth crops) of the residual N from various test fertilizers was only 8-11 % of the total N originally applied; c) the plant recovery of the residual N from ammonium sulfate was the highest, followed by those from milk vetch and sesbania, and that from azolla was the lowest, no matter in which cropping season (from the second to the fifth); N availability ratio showed the same trend, indicating that chemical N fertilizer helps renovate soil organic N, maintain and increase availability of soil N.
