Linking Rice Agriculture to Nutrient Chemical Composition, Concentration and Mass Flux in Catchment Streams in Subtropical Central China.

Abstract

The widespread agricultural nutrient pollution, especially nitrogen (N) and phosphorus (P), has challenged ecologists to understand how agricultural land uses affect water quality. One forest and five rice agriculture catchments in a typical hilly red soil catchment in subtropical central China were investigated during the period of 2010-2012, to investigate and quantify the connections between rice agriculture and nutrients in catchment streams. The results suggested that there was a moderate nutrient pollution in all the 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 total N (TN) and total P (TP), respectively. The proportions of ammonium-N (NH₄⁺-N) in TN and DP in TP increased with the increasing area proportions of rice agriculture in the catchments, suggesting an alteration of rice agriculture to the nutrient chemical composition in stream water. The monthly mass flux of NH₄⁺-N, NO₃^--N, TN, DP and TP averaged to be 0.21 kg ha^-1 mon^-1, 0.59 kg ha^-1 mon^-1, 0.93 kg ha^-1 mon^-1, 0.03 kg ha^-1 mon^-1 and 0.06 kg ha^-1 mon^-1. The average concentration and monthly mass flux of NH₄⁺-N, NO₃^--N, TN, DP and TP in stream water were positively correlated to the area proportions of rice agriculture in the catchments, mainly due to the great fertilizer application rate, as well as the atmospheric deposition in the rice agriculture ecosystem. Such a finding indicated that rice agriculture has a potential to degrade stream water quality. The non-linear fitting analysis using a Boltzmann sigmoid function suggested that influence of rice agriculture on the NH₄⁺-N, NO₃^--N, TN, DP and TP concentrations and mass fluxes in stream water only can be detected when its area proportion in a catchment was higher than the thresholds of 13-30%. Therefore, a reasonable land use planning for rice agriculture could be an important control for not only the water quality but also the nutrient export from the catchments.