Recent Trend in Residual Nitrogen on National and Regional Scales in Japan and Its Relation with Groundwater Quality

We calculated the residual nitrogen (N) from agricultural production on national and regional scales in Japan for 1985, 1990, 1995, 2000, and 2005 and tested the suitability of the N concentrations applied in agricultural production systems using residual N and excess water as indicators of negative effects on groundwater. Chemical fertilizer application declined consistently during the period from 1985 to 2005, while the application of livestock manure peaked in 1990 and declined thereafter. Crop production remained relatively constant, and surplus N on farmland (total input minus total output of N) declined during this period. Although the disposal of excreta N increased, the residual N from agricultural production (surplus N plus disposal of excreta N) declined consistently during this period. However, this trend was not consistent at the regional level because residual N was largely affected by the movement of chemical fertilizer and livestock excreta, which varied with each farm and region. A comparison of residual N per farmland area in 1985 and 2005 indicated that regions were becoming bipolarized, i.e., certain regions exhibited reduced residual N while others exhibited the opposite trend, primarily due to an intensification of livestock production. A significant correlation was observed between residual N in excess water (precipitation minus potential evapotranspiration within the regions), and the percentage of observation wells exceeded >10 ppm of N from 2000 to 2005. Although this correlation had a large dispersion, residual N levels in excess water indicated the risk of water contamination by N.