Influence of Agricultural Activity on Nitrogen Budget in Chinese and Japanese Watersheds

To analyze the effect of agricultural activity on nitrogen (N) budget at the watershed scale, a comparative study was conducted at two Japanese watersheds, the Shibetsu River watershed (SRW) and Upper-Naka River watershed (UNRW), and one Chinese watershed, the Jurong Reservoir watershed (JRW). The total area and the proportion of agricultural area (in parentheses) of the watersheds were 685 (51%), 1 299 (21%), and 46 km2 (55%) for SRW, UNRW, and JRW, respectively. The main agricultural land use in SRW was forage grassland, while paddy fields occupied the highest proportion of cropland in UNRW (11% of total area) and JRW (31% of total area). The farmland surplus N was 61, 48, and 205 kg N ha−1 year−1 for SRW, UNRW, and JRW, respectively. The total input and output for the whole watershed were 89 and 76, 83 and 61, and 353 and 176 kg N ha−1 year−1 for SRW, UNRW, and JRW, respectively. The proportion of discharged N to net anthropogenic N input was 31%, 37%, and 1.7% for SRW, UNRW, and JRW, respectively. The two watersheds in Japan showed similar proportions of discharged N to those of previous reports, while the watershed in China (JRW) showed a totally different characteristic compared to previous studies. The high N input in JRW did not increase the amount of discharged N at the outlet of the watershed due to high proportions of paddy fields and water bodies, which was an underestimated N sink at the landscape scale.