Changes in the nitrogen balance in agricultural land in Japan and 12 other Asian Countries based on a nitrogen-flow model

Abstract

Gross and per unit agricultural land area nitrogen balance (NB _G and NB _A, respectively) in agricultural areas were estimated with a nitrogen-flow model for 13 Asian countries, for regions within a country, and for individual grid cells, from 1970 to 2005. Country- and regional-level estimates showed that NB _A is higher in Japan and South Korea than in other Asian countries, but has recently been stable or decreasing. The contribution of inorganic fertilizer to the nitrogen input is decreasing, whereas that of livestock manure is increasing in these countries. In many other countries, the primary nitrogen source is inorganic fertilizer and its input rate and NB _A have increased sharply since the 1980s. NB _A of some Chinese provinces and Indian states were as high as those of Japan and South Korea. The results suggest that regional-scale estimation is necessary because of the large spatial variability in nitrogen flows within a country. Based on the NB _G estimated for each 0.5° × 0.5° grid cell, nitrogen outflow from agriculture into major river basins was evaluated. About 20 % of the nitrogen balance flowed into China’s Changjiang River basin, versus 10 % into the Ganges River basin. Uncertainties in the basic data and estimation results, and the use of an alternative measure of a country’s environmental performance were discussed.