Changes In Agricultural Production Activity And Its Impacts On The Soil Erosion And Non-point Source Pollution In The Three Gorges Reservoir Area

Soil erosion and agricultural non-point source pollution are one of the common eco-environmental degradation problems in the world,as well as important ecological and environmental issues that affect regional sustainable development.Their formation and development are the result of the joint action of natural factors and human activities.In the short term,human activities have a more significant impact on soil erosion and agricultural nonpoint source pollution than natural factors.Notably,agricultural production activity,as human activities,directly related to the intensity of land use,play a decisive role in changes in the eco-environment.In recent years,with the rapid development of urbanization and rural labor transfer,major adjustments have been made to agricultural production activity in the Three Gorges Reservoir Area,which have led to significant changes in the regional ecoenvironment.This main purpose of the study was to solve the problem about relationship between agricultural production activity and soil loss and agricultural nonpoint source pollution in the Three Gorges Reservoir Area.Therefore,based on household survey,remote sensing surveys,statistical data analysis and field long-term monitoring,we analysis the changes of agricultural production activity and its impacts on runoff,sediment yield,and nitrogen and phosphorus losses load in Wuqiaohe watershed,selected as a typical region affected by urbanization.The main findings are shown as follows:(1)In the past 20 years,the rapid development of urbanization in the Three Gorges Reservoir has accelerated changes in agricultural production activity.The planting area of grain crops per household has been decreasing at the household scale,and the planting area of economic fruit trees per household has increased.The adjustment of crop planting structure has led to changes in cropping patterns and multiple cropping index.The representative cropping pattern in arable sloping land has changed from wheat-maize-sweet potato triple cropping system to the maize-sweet potato double cropping intercropping system and single planting of maize or sweet potato,resulting in a continuous decrease in multiple cropping index.Meanwhile,the intensity of chemical fertilizer input has been reduced by 26.34%,but it is still higher than the national ecological county construction arable land chemical fertilizer input standard.The loss of rural labor has accelerated the abandonment of cultivated farmland based on the household survey data and UAV remote sensing data of watershed.The average cultivated abandonment area at households scale accounts for 30.93% of the farmland area of households,and the proportion of farmland abandonment at the watershed scale is as high as 22.21%.As the land circulation advances,the moderately large-scale operations with citrus as the mainstay account for 51.40% of the transferred land.The changes in planting structure,multiple cropping patterns,the abandonment of arable land,and large-scale operations caused by urbanization have significantly changed land use structure and intensity.(2)The effects of changes in cropping patterns at each stage of urbanization on runoff and sediment yield and nitrogen and phosphorus loss on slopes are the result of the combined effects of the temporal and spatial distribution of crops,surface soil disturbances,and fertilizer inputs.The high fertilization input and high frequency surface physical disturbances in wheat-maize-sweet potato triple cropping system are the important reasons for the higher runoff coefficient,sediment yield,surface runoff nitrogen and phosphorus concentration and losses load than the maize-sweet potato intercropping and maize single planting structure.The change of cropping pattern in the urbanization stage has a stronger impact on the loss of total nitrogen on slopes than on the loss of total phosphorus.Representative grain crops show different runoff and sediment yield during the rainy season.Maize has lower runoff and sediment yield during the high mulch period.Sweet potato have lower runoff and sediment yield during the same period of maize harvest.The sediment yield capacity of wheat is higher than that of maize;the runoff and sediment yield capacity of wheat in its mature period is lower than that of maize in the same period,and the runoff and sediment yield capacity in harvest period is higher than that of maize in the same period.(3)Cultivated land abandonment can be regarded as a land management measure for land fallow.Short term abandonment of cultivated land can promote the rapid restoration of vegetation,and is beneficial to the accumulation of soil organic matter,soil total nitrogen and soil total phosphorus.Abandonment can increase the threshold rainfall on the purple soil slope and significantly reduce the slope runoff.Especially in spring and summer,the single rainfall runoff coefficient is only 27.25% and 34.72% of the cultivated land in the same season.Abandonment can significantly reduce the sediment yield capacity of the slope.And with the increase of the abandonment years,the sediment yield coefficient of the slope continues to decrease.In addition,abandonment reduces the total nitrogen concentration during the sowing and fertilization periods of crops,but has no significant effect on the total phosphorus concentration.Abandonment increases the enrichment rate of total nitrogen and total phosphorus in eroded sediment,and its enrichment rate is 2.59 and 1.20 of that of cultivated land.Compared with cultivated land,the nitrogen losses load of abandoned farmland is reduced by 59.6%,and the phosphorus loss load is reduced by 79.8%.The interception and reduction of phosphorus by abandonment is better than that of nitrogen.After abandonment,the nitrogen was loss mainly via surface flow,and the phosphorus via sediment.Abandoned farmland has achieved slope nitrogen and phosphorus loss controlling by reducing surface runoff and sediment,in which the interception and reduction of nitrogen and phosphorus in sediment is the main mechanism for reducing nitrogen and phosphorus losses load.The error between the verified value and the measured value of the nitrogen and phosphorus losses load estimation model constructed is within 5%,which has high estimation accuracy.(4)Land operation management types has a significant impact on runoff and sediment yield and nitrogen and phosphorus losses.The annual surface runoff coefficients of large-scale citrus orchard operation are 3.32 times and 3.12 times that of traditional orchards and cultivated farmland,but soil erosion modulus is only 52.72% and 29.67% of traditional orchards and cultivated land.The large-scale citrus orchard operation significantly increased the concentration of nitrogen and phosphorus in surface runoff,which far exceeded the class V level water quality standard(GB3838-2002 Environmental quality standard for surface water).In spring,the concentration of nitrogen in surface runoff of large-scale citrus orchard operation was 14.31 4.74,4.77 times that of abandoned farmland,traditional orchard and cultivated farmland,and the concentration of phosphorus was 39.08,1.94,3.84 times that of abandoned farmland,traditional orchard and cultivated farmland.Large-scale citrus orchard operation have significantly increased the load of nitrogen and phosphorus loss.Among them,special attention needs to be paid to fertilization management in large-scale citrus orchard operation.The study found that the first two heavy rains after spring fertilization contributed 70.4% total nitrogen(TN),72.1% soluble nitrogen(DN),68.9% nitrate nitrogen(NN),94.1% ammonium nitrogen(AN),67.1% total phosphorus(TP),64.1% soluble phosphorus(DP)and 73.0% of particulate phosphorus(PP)loss,and mainly NN and DP losses.Meanwhile,Large-scale citrus orchard operation increased the first flush effect of pollutants.The initial 20% surface runoff contributed 58.0% of TN,57.0% of DN,58.5% of NN,79.0% of AN,and 62.0% of TP.63.5% of DP and 60.0% of PP.It can be seen that although large-scale land operation has little impact on slope runoff and soil erosion,but it significantly increases the nitrogen and phosphorus concentration and losses load,and speeds up the rapid losses of slope surface nitrogen and phosphorus in the early stage of runoff.Overall,The changes in planting patterns and farmland abandonment have reduced soil erosion and agricultural nonpoint source pollutant during the rapid development of urbanization,but large-scale land operations have exacerbated nitrogen and phosphorus load.Its adverse impact on non-point source pollutant is greater than that of the change in planting patterns and farmland abandonment.Since agricultural nonpoint source pollution from large-scale land operation,the main way of land management in the Three Gorges Reservoir Area in the future,needs special attention.