| As an important water and soil conservation area and water conservation area,the ecological environment of the Three Gorges Reservoir area is fragile.In recent years,affected by human activities,its ecological environment quality has been further damaged.In this study,the Lanlingxi small watershed in Zigui County,which is the demonstration site of science and technology of conversion of cropland to forest,was selected by the State Forestry Administration.Based on the long-term continuous monitoring data of the export section of the small watershed from 2015 to 2019 and the observation data of the catchment area and runoff area of the Three Gorges reservoir(Zigui)forest ecological positioning station of the Yangtze River,the monitoring methods of the watershed section,the filtering method of the basic flow segmentation and the spatial analysis of the geographic information system were used to determine The temporal distribution characteristics and variation range of soil nutrient N and P loss flux and load,the contribution of runoff and soil erosion to nutrient loss,the surface water quality differentiation of various land use structures in the watershed were analyzed,and the control and environment of nutrient loss in the watershed were evaluated based on the observation of nutrient loss characteristics of typical rainfall runoff process Environment effect.The main results are as follows:(1)Runoff and soil erosion.The annual average runoff depth of small watershed is 714.3mm,the maximum is 876.1 mm,and the minimum is 621.5 mm.Runoff coefficient is 0.58-0.76,runoff conversion rate is high.The basic flow of small watershed accounts for 9.7-14.7%of the total runoff,and the surface runoff accounts for 85.3-90.3%of the total runoff.The soil erosion in the small watershed was completely controlled.The annual average erosion modulus was260.24 t·km-2·a-1,which was 87.3%less than that before returning farmland to forest.The erosion intensity reached the micro erosion level.(2)Soil nutrient loss.The loss loads of total nitrogen and total phosphorus in small watershed were 8.748-14.524 t·km-2·a-1 and 0.193-0.391 t·km-2·a-1,respectively,in which nitrate accounted for 72.2-74.1%of the total nitrogen output.The loss rate of nitrogen fertilizer is 12.8-21.2%in small watershed,and the loss rate of phosphorus fertilizer is 2.8-5.7%.The loss rate of nitrogen fertilizer in small watershed is higher than the average level of reservoir area(9.0%),and the loss rate of phosphorus fertilizer is slightly lower than the average level of reservoir area(5.7%).(3)Runoff,erosion and soil nutrient export season.The runoff production of small watershed is mainly from May to October,and the peak usually occurs in July and August.Soil erosion is also concentrated in May to October.The five rainstorms with the largest rainfall contributed about 68.06%of the annual soil erosion.At the same time,may to October is the key period of nutrient loss in small watershed,contributing more than 80%of nitrogen and phosphorus nutrient loss load.(4)Impact of land use structure.The concentration of nitrogen and phosphorus in the main catchment area was significantly higher than that in the main catchment area and the main catchment area(P<0.05).The variation range of nitrogen and phosphorus concentration in the catchment area was significantly different in the process of rainfall runoff(P<0.05).The largest variation was found in the main catchment,and the response of nutrient concentration to rainfall was the fastest.The average concentration of total nitrogen(17.4 mg·L-1)in the garden area is4.4 times of that in the forest area,and 1.8 times of that in the forest area,which is the key area for nutrient loss control.(5)Contribution of soil nutrient output.The loss load of total phosphorus accounts for a large proportion(82.29%)in the erosion sediment,and the contribution of sediment to the loss load of total nitrogen is only about 1.1%.Runoff has the largest contribution to total nitrogen(11.397 t·km-2·a-1),about 98.9%of total nitrogen is lost through runoff,and runoff has a small contribution to total phosphorus(0.057 t·km-2·a-1),only accounting for about 18.7%of total phosphorus loss load.In runoff loss,surface runoff contributed 87.10%of total nitrogen runoff loss,89.18%of total phosphorus runoff loss,and base runoff contributed less to total nitrogen and phosphorus runoff loss,respectively,12.90%and 10.81%.(6)Compared with before returning farmland to forest,soil erosion in small watershed decreased by 87.3%.However,the runoff coefficient of small watershed is relatively large(0.58-0.76),and the average concentration of total nitrogen in runoff of small watershed exceeds the class V water quality standard,so the risk of runoff nutrient loss is high.In order to reduce the load of runoff nitrogen loss and the input risk of agricultural non-point source pollution nitrogen in eutrophication of water body,we should optimize the allocation of land use types and structures,increase the proportion of forest land,set up buffer forest belt or implement tea forest intercropping. |
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