Evaluating impact of land use and N budgets on stream water quality in Hokkaido, Japan

This study was conducted to evaluate the impact of land use system and N loadings to the environment estimated from N budgets on quality of stream water in Hokkaido, Japan. A case study was carried out in three towns of southern Hokkaido, which are Shiraoi, Yakumo, and Shizunai, characterized by intensive poultry farming (IPF), dairy cattle farming (DCF), and race horse farming (RHF), respectively. The estimation of N budgets using an N flow model indicated that the highest disposal N (880 Mg N yr(-1)) was generated in the IPF town and it resulted in 250 kg ha(-1)yr(-1) surplus N in croplands. The disposal N was much lower in the DCF and the RHF town (102 and 71 Mg N yr(-1), respectively) than that of the IPF town. Cropland surplus N in DCF town was 31 kg N ha(-1)yr(-1), whereas RHF town had negative N balance. The linear regression analysis indicated that NO3-N concentration in stream water was significantly correlated with the proportion of upland field in drainage basins. The regression slopes varied among the towns, and it was the highest for IPF (0.040), intermediate for DCF (0.023) and the lowest for RHF town (0.006). The multiple regression analysis showed that regression slopes were significantly correlated (R-2 = 0.77 at 5% level) with livestock disposal N and cropland surplus N. Therefore, we assumed that these regression lines were the baselines for non-point source pollution, and the regression slopes were determined to act as impact factors of stream water quality. However, two sampling sites in the IPF area were scattered above the baseline. This fact strongly suggests that the area was affected by point source pollution.