[Land Use Structure Change and Its Control Effect of Nitrogen Output in a Small Watershed of Three Gorges Reservoir Area: A Case Study of Lanlingxi Watershed]

Abstract

The nitrogen (N) output in Lanlingxi watershed of Three Gorges Reservoir Area in 2015 was monitored and the current land use map was investigated. Cluster analysis and correlation analysis were used to identify the major sources of pollutants and to discriminate the source and the sink land use types. The effects of land use on N exports were quantitatively analyzed by stepwise regression analysis. The results showed that: ① After the returning farmland to forest project the land use structure of this area changed dramatically. The area proportions of forest land and garden plot increased to 76.85% and 13.87% respectively, and the proportion of cultivated land dropped to 1.16%. Cultivated lands were stellate distributed and garden plots in some catchments were flake distributed. ② The content of TN in some monitoring points surpassed the Class V standard of the national groundwater environmental quality. The ammonium-N (NH₄⁺-N) concentrations ranged 0.089-0.214 mg·L^-1, 2.925-13.203 mg·L^-1 for nitrate-N (NO₃^--N) and 3.561-14.572 mg·L^-1 for total-N (TN). And NO₃^--N accounted for more than 80% of TN. ③ There were significant positive correlations between garden plot, residential land and N loss, and negative correlations between forest land, unutilized land and N export, which indicated that the former were N sources and the latter were sinks. ④ Forest land area should be increased and garden plot should be controlled, and the area ratio of residential land should be kept below 5% when adjusting land use structure of this watershed. Moreover, forest-tea and forest-fruits modes should be applied to change single land use type of garden plot in some catchments.

Keywords: Three Gorges Reservoir Area; land use; nitrogen output; non-point pollution; returning farmland to forest.