Risk assessment of nitrate pollution in groundwater based on the environmental nitrogen-assimilation capacity of agricultural lands in Hokkaido

(pp. 17–24)
A trial calculation was performed of the environmental nitrogen-assimilation capacity and the amount of nitrogen input based on various statistical data, which were compiled from each city, town and village in Hokkaido prefecture. The relationship between the excess quantity of nitrogen, after nitrogen input, and the environmental nitrogen-assimilation capacity and the nitrate-nitrogen concentration of the groundwater was considered.
Environmental nitrogen-assimilation capacity = nitrogen output by the crops + acceptable level of residual nitrate in the soil profile.*
^*It is calculated by the amount of nitrate precipitation evapotranspiration ×10 mg L^?1.

• 1) 

  The average value of the environmental nitrogen-assimilation capacity in Hokkaido Prefecture was observed to be 183 kg ha^?1. The maximum and minimum values of the environmental nitrogen-assimilation capacity were 308 kg ha^?1 and 94 kg ha^?1, respectively. When the average value of the environmental nitrogen-assimilation capacity with respect to main agricultural land use was compared across municipalities, it was largely in the following order · grassland (218 kg ha^?1), upland (169 kg ha^?1), and paddy land (157 kg ha^?1).