Linking rice agriculture to nutrient chemical composition, concentration and mass flux in catchment streams in subtropical central China

Abstract

Widespread agricultural nutrient pollution, particularly nitrogen (N) and phosphorus (P), has challenged ecologists to determine how agricultural land use affects water quality. One forest and five rice agricultural catchments in a typical hilly red soil catchment in subtropical central China were investigated from 2010 to 2012 to evaluate and quantify the connections between rice agriculture and nutrients in catchment streams. The results indicated moderate nutrient pollution in all studied catchments. Nitrate-N (NO3−-N) and dissolved P (DP) were the main species of N and P in stream water, contributing 63.4% and 58.1% to the total N (TN) and total P (TP), respectively. The proportions of ammonium-N (NH4+-N) in the TN and of DP in the TP increased with increasing areal proportion of rice agriculture in the catchments, suggesting an association between rice agriculture and the nutrient chemical composition of stream water. The average monthly mass fluxes of NH4+-N, NO3−-N, TN, DP, and TP were 0.21kgha−1mon−1, 0.59kgha−1mon−1, 0.93kgha−1mon−1, 0.03kgha−1mon−1, and 0.06kgha−1mon−1, respectively. The average concentrations and monthly mass fluxes of NH4+-N, NO3−-N, TN, DP, and TP in stream water were positively correlated with the areal proportions of rice agriculture in the catchments, mainly due to the high fertilizer application rate. This finding indicates that rice agriculture has a potential to degrade stream water quality. A non-linear fitting analysis using a Boltzmann sigmoid function suggested that the influence of rice agriculture on the NH4+-N, NO3−-N, TN, DP, and TP concentrations and mass fluxes in stream water can only be detected when the areal proportions of rice agriculture in the catchments are greater than 12–29%. Therefore, reasonable land-use planning for rice agriculture could be important for managing stream water quality as well as nutrient export from catchments.