Abstract
Field level nitrogen balances within Japanese prefectures were estimated through the calculation of nitrogen concentrations in hypothetical drainage water, assuming that surplus nitrogen in the soil will readily dissolve in water percolating through the soil. 1) Average nitrogen concentrations were 7.8, 8.8 and 2.9 mg N L^<-1>, for nationally, nationally excluding Hokkaido, and Hokkaido, respectively. A wide variation in the concentration was observed, ranging from below zero to >30 mg N L^<-1>. 2) Many of the prefectures with high N concentrations coincided with high animal numbers and, hence, high manure N loading. The high concentrations could not be explained by the rate of chemical fertilizer application alone. 3) A reduction of 30% in chemical fertilizer N use resulted in substantial reduction in N concentrations : 7.8 → 5.4 mg N L^<-1> (-31%), and 8.8 → 6.3 mg N L^<-1> (-38%) in the national average and national average except Hokkaido, respectively. 4) The effect of 'discharging' the nitrogen in the manures, which can be treated by sewage treatment systems, was not clear. It might, however, be worthwhile considering the amount of manure that could be treated by such systems. 5) Utilizing all of the fallow fields by cropping decreased N concentrations remarkably : 7.8 → 5.9 mg N L^<-1> (-24%), and 8.8 → 6.6 mg N L^<-1> (-25%) for the national average and national average except for Hokkaido, respectively. More significant reduction of the N concentration was estimated for combining the chemical fertilizer reduction with fallow field utilization. 6) The calculation procedure involves, however, a number of potential problems including 1) treating whole prefectures as the unit of calculation, 2) the use of mean N concentrations and 3) the reduction of N loading as a result of ammonia emission. These are important factors likely to contribute to a reduction in the accuracy of estimates and should be improved in the near future.
