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.