Nitrogen cycling in agro-ecosystems of East Asia

Nitrate (NO3-N) runoff from the pedosphere to the aquasphere causes marine eutrophication and groundwater pollution. Nitrous oxide (N2O) emission from the pedosphere to the atmosphere causes global warming and ozone layer depletion. The current net nitrogen input (NNI), which is defined as the difference between total N input to land area (chemical fertilizer N application, import of N in food and feed, biological N2 fixation and atmospheric N deposition) and total N output from land area (export of N in food and feed), was estimated to be 52.4 Mt N yr-1 in East Asia, in which chemical fertilizer N application account for 60%. The results of simple regression analysis using reported data on stream NO3-N runoff from the total land area of East Asia was estimated to be 8.61 Mt N yr-1, which accounted for 27% of the total applied chemical fertilizer N in East Asia. Regression analysis using reported data on N2O and CO2 emission from soil and chemical fertilizer N pplication showed that log-transformed soil N2O emission was significantly correlated with soil C2O emission and applied chemical fertilizer N. Dramatically large soil N2O emission was found in tropical peatland cultivated with vegetables. It reached 259 kg N ha-1 at maximum, which accounted for 39% of applied chemical fertilizer N. This indicates that soil N2O emission depends on organic matter decomposition as well as chemical fertilizer N application. Soil CO2 emission devided by soil CN ratio can be an indicator of gross mineralized N. Much better correlation for soil N2O emission was obtained using chemical N fertilizer application plus gross mineralized N (P less than 0.001). Reported values of CO2 emission from the soil of cropland, grassland and forest were significantly correlated with soil organic carbon (C) content. Soil N2O emission in each country was estimated by substituting the mean soil organic C content of ecological zones into the regression equation. Total N2O emission from the soil of cropland and grassland of East Asia was estimated to be 0.766 Mt N yr-1 including indirect soil N2O emission associated with stream NO3-N runoff. The proportion of soil N2O emission induced by chemical fertilizer N application and organic matter decomposition were 35 and 62%, respectively. These values were similar to those reported by FAO (2001). Organic matter decomposition is an important factor inducing soil N2O emission and its increase may intensify stream NO-3-N runoff. Chemical fertilizer N application stimulates soil organic matter decomposition, which was significant in soil with high organic matter content, such as peat soil. To improve the accuracy of estimation, high-quality geographical information including land cover area, soil organic C content, chemical fertilizer N application and manure N application as well as better parameterization for soil N2O emission and stream NO3-N runoff must be prepared by intensive monitoring.