Assessment of the long-term variation in the nitrogen pollution load potential from farmland to groundwater in the Tedori River Basin, Japan

The long-term nitrogen pollution load potential (NPLPg) to groundwater from farmlands was examined in the Tedori River Basin. The NPLPg was estimated using the difference between N in the fertilizer application rate and N outputs in crop yield at 5-yearly intervals from 1960 to 2005. The total yearly NPLPg of 1,085 t (103 kg) in the 1960s decreased to 774 t by 1975. Thereafter, the NPLPg gradually increased to 976 t in the 1990s, but decreased again to 369 t in 2005. The NPLPg decreased by 23% for rice and by 37% for horticultural crops from 1960 to 2005 with an overall decrease of 34%. The NPLPg per unit area was relatively stable over time for rice, soybean, barley, and horticultural crops, but there were significant differences among them. The NPLPg for rice ranged from 39 to 85 kg ha⁻¹ year⁻¹ with an average of 65 kg ha⁻¹ year⁻¹ and that of the horticultural crops ranged from 273 to 357 kg ha⁻¹ year⁻¹ with an average of 302 kg ha⁻¹ year⁻¹ The significant long-term changes in the NPLPg suggest that evaluation at a specific point in time is insufficient for an integrated assessment of groundwater pollution.