Abstract: Rural non-point source pollution in the north of the Lake Erhai Region has been deemed as the main source of nitrogen and phosphorus load in Lake Erhai, which is now in the early stage of eutrophication. Through field investigation and sample analysis in laboratory, quantitative analysis was conducted of N influx through fertilization, precipitation and dustfall, irrigation, biological nitrogen fixation) and N outflux through harvest of crops and biological denitrogenation of farmlands under two different farming systems typical of the area, and calculation was one of the net discharge of nitrogen pollution load from farmland to water environment through surface runoff and underground flow. Results show that the farmland under the rice-horsebean (Vicia faba) farming system is environmentally friendly because it discharges little nitrogen pollution load to the water environment with the net discharge being -30.73 kg•hm^-2•a^-1. Its nitrogen influx comes mainly through biological nitrogen fixation (43.3%) and organic fertilizer (35.0%), while its nitrogen outflix goes principally through plant uptake (98.5%). However, under the rice-garlic farming system, the farmland can discharge nitrogen as much as 306.75 kg•hm^-2•a^-1 into the water environment, which could deteriorate water environment. In this system, fertilizer is the main nitrogen source, contributing 84.7%, of which chemical fertilizer accounts for 62.1%, and about 45.4% of the nitrogen outflux is discharged into the water environment, which suggests that it is essential to adopt reasonable fertilization and measures to improve of fertilizer utilization efficiency so as to prevent N non-point source pollution from the farmland under the rice-garlic farming system to the water envieronment.

Key words: farmland, nitrogen, pollution load, farming system, Lake Erhai