Effects of agricultural land use on water chemistry of mire pools in the Ishikari Peatland, northern Japan

Concomitant with farmland development in the Ishikari Peatland of northern Japan, deterioration of water chemistry has become increasingly evident at mire pools there. Using spatial analysis methods with a geographic information system and a statistical methodology, this study examined the effects of agricultural land use on the pool water chemistry. A water chemistry survey of several pools showed that nitrogen, phosphorus, and suspended-solid concentrations were remarkably higher than those in pools that had been undisturbed or less-disturbed by land-use development. Principal component analysis (PCA) was conducted to summarize water chemistry variables. Correlation analysis was conducted between the principal component (PC) scores and land-use variables to clarify land-use effects. Results of the PCA showed that the water chemistry variances were accounted for by the first two PCs: PC1 included farmland subsurface drainage containing nitrogen, specifically nitrate, and mineral ions; PC2 included farmland surface drainage containing soil particles and ammonium nitrogen. These PC scores were also affected by the number of inflow ditches and the total area of farmland located within the drainage catchment of inflow ditches leading to the pools. Results suggest that farmland subsurface and surface drainage water flowing into the pools degraded the water chemistry. The extent of the effects of drainage water depended on agricultural practices such as fertilizer application and paddy-field puddling (i.e., mechanical homogenization of water-saturated soil). Accordingly, water chemistry of the study pools was affected by agricultural practices and by the amount of farmland in the drainage basin, engendering eutrophication, and the deterioration of water chemistry. Therefore, preservation of water chemistry necessitates changes in agricultural practices, such as improving fertilizer application methods, along with measures related to land-use planning such as changing the drainage ditches’ flow direction.