Impact of Nitrogen Cycling Associated with Production and Consumption of Food on Nitrogen Pollution of Stream Water

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概要

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• We evaluated the impact of nitrogen (N) cycling on N pollution of stream water, with emphasis on N disposed of (hereafter referred to as "disposal N") from human and livestock excrement and the N surplus in cropland, compared to the total-N concentration of stream water, in seven zones of Asahikawa City characterized by various types of land use. In order to estimate N cycling, we used the Nitrogen Flow Model, composed of the N budgets of human, livestock, and cropland subsystems. The urban area with a population density of over 4,000 persons km^<-2> generated a very large amount of disposal N (about 2,700 kg N ha^<-1> cropland y^<-1>). Based on the amount of disposal N and the volume of domestic sewage water used, the N concentration estimated for the urban area was 34 mg N L^<-1>, which found in the effiuent from the sewage treatment facility (24-28 mg N L^<-1>), regardless of the season. Thus, it was indicated that most of the disposal N in the urban area was discharged directly to streams through the sewage treatment facilities, contributing to a point source of N pollution of stream water. In addition, the disposal N from livestock facilities was larger in pig and poultry farming areas than in other farming areas, contributing to some extent to a potential source of N pollution. As a result, the concentrations increased above 1 mg N L^<-1> in the urban and surrounding areas. On the other hand, the N surplus in cropland was practically determined by the N flows associated with chemical fertilizer, livestock excrement as manure, and crop uptake. The N surplus was similar among the seven zones, ranging from 69 t0 99 kg N ha^<-1> y^<-1>. The N concentration estimated from the amount of N surplus and 50% of mean annual precipitation as a discharge rate was 13.6-19.5mg N L^<-1>. Most of the surplus N was indicated to be leached out. However, the total-N concentration measured in the major streams flowing through Asahikawa City was mostly below 1 mg N L^<-1> except for the urban and surrounding areas. The surplus N in cropland may not reach the streams, even if N leaching occurs, probably due to N removal by plant uptake, denitrification, and sedimentation in the riparian zone and stream channels. Thus the effect of agricultural practices on N pollution of stream water was not appreciable.

• 社団法人日本土壌肥料学会の論文

著者

• NAGUMO Toshiyuki

  Laboratory of Soil Science, Graduate School of Agriculture, Hokkaido University

• Nagumo Toshiyuki

  Laboratory Of Soil Science Graduate School Of Agriculture Hokkaido University

• Hatano Ryusuke

  Laboratory Of Soil Science Faculty Of Agriculture Hokkaido University

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