Abstract: Agricultural non-point source pollution is the main factor of the deterioration of water quality of Erhai Lake in recent years. We studied the inorganic nitrogen isotope characteristics in the surface runoff of forest land, cultivated land and residential area in a typical small watershed of Erhai Lake. Source analysis of inorganic nitrogen showed that 8.53% and 91.47% of nitrate nitrogen in forest runoff came from atmospheric rainfall and soil, respectively; 19.78% and 80.22% of ammonia nitrogen in forest runoff were from atmospheric precipitation and soil. For cultivated land, 5.90%, 68.76% and 25.34% of nitrate nitrogen came from atmospheric rainfall, soil, and fertilizer, respectively, while ammonia nitrogen mainly came from soil. In residential runoff, 3.32% of nitrate nitrogen was coming from atmospheric rainfall, 55.42% from domestic sewage, and 41.26% from animal feces, while ammonia nitrogen was mainly coming from domestic sewage and animal feces.

Key words: Erhai lake, stable isotope, source appointment, agricultural non-point source pollution, inorganic nitrogen