Risk Assessment On Non-point Source Pollution At The North Piedmont Of Qinling Mountains

The seriously deficient water resources and aquatic environmental pollution have been one of the global issues. Much greater than 50% of total nitrogen(TN) or total phosphorus(TP) of polluted watershed was from non-point source pollution, which, as a main factor, restricts sustainable development of agriculture and ecological environment. Furthermore, non-point source pollution is hard to prevention and governance due to its quite complex occurrence mechanism. At present, the research on non-point source pollution is mainly made in both hilly and mountainous region of south China, while rarely investigated at the north piedmont of Qingling Mountains. Therefore, we studied topography, the change of land use, spatial variability of soil nutrients, soil erosion and nutrients loss, the distribution of nitrate-nitrogen(NO3--N) and Olsen phosphorus(Olsen-P) on soil profile, Yujia River’s TN and TP concentration during different rainfall intensity in Yujia Catchment at the north piedmont of Qingling Mountains, which could lay scientific basis for prevention and control of non-point source pollution. The main results are:1. 80.77% of the total land area distributed on 0°~15° slope land, and 64.34% of the total land area distributed on land 0~20 m near the river. The terrain of Yujia Catchment is quite gentle. Arable land and orchards were the two main land use types, accounted for 28.46% and 38.80% of the total land area, and kiwi-fruit orchards accounted for 83.82% of the orchards. Influenced by the distribution of water, the area of orchards decreased with the rising of relative elevation due to less water of higher relative elevation, while the area of cultivated land increased.2. The distribution of soil organic matter(SOM), TN, Olsen-P and available potassium(AP) in topsoil were consistent with the distribution of land use types, which dominantly influenced the spatial distribution of soil nutrients. The soil nutrients of arable land were in medium or low level, especially the slope land transforming into terrace in recent two years. A large area of kiwi-fruit orchards’ soil SOM and TN were in optimal levels, while a small area in medium or low level. More than 80% area of kiwi-fruit orchards’ Olsen-P and AP content were both in high lever. It is urgent to investigate soil nutrients regulation techniques of different land use types.3. The 57 years’ sedimentation amount of closing reservoir at the end of Yujia Catchment was near the soil erosion amount, estimated by RUSLE, which was proved to be feasibly used in this area. The estimated result indicated the annual soil erosion modulus and amount of 1957~1989 were 2.46 times that of 1990~2013’s, which were dramatically decreased mainly due to cultivated land converted to orchard and transforming slope into terrace. To some extent, the change of land use led to less water and soil loss, however, the kiwi-fruit orchards’ excessive fertilizer input outclassed that of arable land, leading to more soil nutrients loss, which increased the risk of non-point source pollution.4. A part of kiwi-fruit orchards’ accumulation of NO3--N in 0~200 cm soil was up to 1200 kg/hm2, far above that of other land use types, and NO3--N content in 100~200 cm soil was greater than that in 0~100 cm soil. 14.82% area of kiwi-fruit orchards’ Olsen-P content(>80 mg/kg) was approximately twice of the change-point of phosphorus leaching(>40.11 mg/kg), which increased the risk of phosphorus leaching and contributed to aquatic environment pollution. In Yujia River, the content of TN and TP increased as the increase of rainfall intensity. The loss rate of particles of nitrogen and phosphorus followed by runoff, which was indispensable, was a high proportion of the total loss of nitrogen and phosphorus.5. The key point to prevent and control non-point source pollution is source control, which requires kiwi-fruit orchards’ rational fertilization, meanwhile, it should be combined with the measures, such as transforming slope into terrace, contour planting, controlled release fertilizer application, integrated management of water and fertilizer and control of irrigation, to decrease the loss of runoff, sediment and soil nutrients.