River water qualities and types of agricultural production -a comparison between paddy farming and intensive livestock production areas

摘要

The nitrogen (N) flows in the Omoigawa and Nakagawa river basins in Tochigi prefecture, central Japan, in 2000 have been estimated. Omoigawa was characterized as a paddy rice - upland field area and Nakagawa as an intensive livestock farming area. Residual N caused by agricultural production in Omoigawa was caused mainly by chemical fertilizer application and that in Nakagawa by livestock excreta. Residual N from agricultural production per farmland area was the same (c. 110 kg N ha~(-1)), and occupation of farmland was also the same (c. 20%). Because Omoigawa is a smaller river basin than Nakagawa, total residual N in Omoigawa was 2,093 Mg and that in Nakagawa was 2,469 Mg. Nitrogen flow in river water was divided into sewage-derived N and non-point-source-derived N, mainly from agricultural production. Non-point-source N in Omoigawa was 2,467 Mg and that in Nakagawa was 1,426 Mg, in spite of the smaller residual N in Omoigawa. This difference might be caused by differences in sources of residual N in each basin, and chemical-fertilizer-derived N might be more easily leached to the water environment than livestock-excreta-derived N. This difference should be considered in vulnerability assessments of water environments. However, livestock excreta N will eventually leach out too, so intensive livestock farming cannot be said to have a lower effect on water environments and be more sustainable than chemical-fertilizer-dependent agriculture. Increasing sustainability and reducing impacts will help achieve complete N cycling in river basins with minimal N input from outside.