| Water quality degradation and aquatic ecosystem destruction caused by agriculture are important environmental problems in the worldwide. Soil and water erosion not only result in non-point source pollution, but also transport the pollutants into rivers. Therefore,identification of soil and water erosion characteristic, the sources of diffuse pollution and influence factors will improve agricultural non-point source control, the restoration of aquatic environment and water resource management in basin. This study was conducted in a small watershed, hilly region of Northeast China. Coupling with plot and field experiments,characteristics of diffuse pollution were determined in hillslope and small watershed scale.The main sources of pollutants and their contribution rate were analyzed. The BMPs for agricultural diffuse pollution were evaluated. The main results were shown as follows:(1) The erosive rainfalls mainly occured in June and August. The soil and water loss was significantly positive correlated to precipitiaon and rainfall intensity. Slop gradient also had great effects on soil loss and diffuse pollution.(2) Both TN and TP concentration values in slope farmland runoff exceeded class ?of the surface water environmental quality standard. Nitrogen was considered as the main pollutant. The annual output amount of TN accounted for 55%- 69% of the total pollutants output in 2013 and 2014.(3) NH4+-N concentration was the highest in surface runoff from sloping croplands.However, NO3--N, TN and TP concentration were highest in village runoff. Under different landuse types, the total output load of four pollutants followed the order of cropland > forest >village, with contribution to the watershed of 68.0%- 70.7%, 16.0%- 21.3% and 10.7%-13.3% respectively. Moreover, N was regarded as the primary pollutant of agricultural non-point pollution in the study area, and the export load of NO3--N was larger than NH4+-N.(4) The precipitation, soil and fertilizer were three major sources of NO3--N and NH4+-N in runoff generated from sloping land. Two forms of N pollutant in forest runoff was mainly from precipitation and soil. However, the major sources of N pollutants from village runoff were precipitation, sewage and excreta.Soil source has the largest contributions to NO3--N in cropland and forest runoff between2013 and 2014, which were 51.38%- 54.13% and 52.82%- 69.11%, respectively. For runoff from village, NO3--N from sewage and excreta had the greatest contribution than the other sources in 2013(51.30%) and 2014(50.64%).(5) Micro-basin tillage had significant effects on controlling soil and water erosion as well as agricultural non-point pollution. Runoff and sediment in treatments of micro-basin tillage were reduced by 54%- 68% and 91%-97% during rainy season(May – September),and the grain yield was increased by 14.96%- 25.67%. Treatments of 75 cm block interval had a better effect on soil and water control than those of 65 and 85 cm treatments.Moreover,the optimal block interval treatments had the best effect on non-point pollutants reduction.The mitigation rate of annual exported pollutants including total four forms of nutrients was80.60%- 93.47% in the treatments of micro-basin tillage.(6) Grass strip had positive effect on postponing runoff-yield time, significantly cutting the amount of soil loads of non-point pollutants. Reduction rate of runoff and sediment in Alfalfa grass strip in May and September was 55.36%- 100% and 91.47%- 98.03%, and load of four pollutants was reduced by 68.85%- 90.23% as well. However, compared with Alfalfa grass strip, treatments of natural vegetation decreased runoff and sediment by 6.66%- 100%and 76.11%- 89.63% respectively, and pollutant output load was decreased by 15.82%-73.90%. Therefore, Alfalfa grass strip had a better prevention effect on agricultural non-point pollution.Overall, N, O stable isotope technique could be better applied in source identification of agricultural non-point pollution, providing the scientific basis to agricultural non-point source pollution control and prevention. Both micro-basin tillage and grass strip can be considered as effective prevention measures for soil and water loss control on sloping and agricultural non-point source pollution prevention at the drinking water sources area in the black soil region of northeat China. Thus, loads of non-point pollutants could be significantly reduced,and then negative influence on aquatic environment could be greatly decreased as well. This two typical control practices were proposed to be applied in the future work of soil and water conservation, restoration of ecological environment of the black soil region of northeast China,especially in the drinkying water catchments. |
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