Land use and topography as predictors of nitrogen levels in tropical catchments in Xishuangbanna, SW China

Abstract

The conversion of vast tropical forests to cash crops represents a widespread land-use change in tropical regions, and has consequences for soil and water biogeochemical cycles. We examined how the conversion of tropical forest to rubber plantations in Xishuangbanna, southwest China, influenced the nitrogen concentrations in stream water. Water samples were collected between 2008 and 2010 at 35 sites to search for spatial and temporal variations in nitrogen concentrations and for their relationships with environment variables. Streams associated with rubber plantations (“rubber streams”) had significantly higher total N (TN) and total dissolved N (TDN) concentrations than forest streams during all seasons. TN, TDN and nitrate concentrations were observed to be significantly higher in streams associated with mixed land use than in forest streams only in the middle rainy season. The highest TN, TDN and nitrate concentrations were observed in rubber streams, and the lowest were observed in forest streams, indicating that land-use change had a significant effect on N concentrations. Statistically significant seasonal differences in TN, TDN and nitrate concentrations, with the greatest values in the middle rainy season, were observed in rubber and mixed streams, but not in forest streams. Canonical correspondence analysis identified the proportions of rubber plantation and tropical forest land, as well as slope gradient, as primary predictors of nitrogen concentration. Our results indicate that controlling rubber plantation expansion on steep slopes and avoiding single or large areas of rubber plantation within a catchment can mitigate nutrient loss.