Controlling Factors Of Soil Erosion And Phosphorus Loss Process In A Karst Slope At Longfeng Trough Valley In Zhongliang Mountain,Chongqing,China

Phosphorus is a critical element affecting the growth of plants and eutrophication in aquatic ecosystems.It is necessary to understand the three dimensional soil erosion and phosphorus loss process in a karst slope,which affect groundwater management and scientific assessment of soil phosphorus level in slope land.This study based on karst trough valley slope vegetation-soil-epikarst observation platform.We took full advantage of high precision monitoring instruments and field sampling.The monitor-ing period was from April 2019 to October 2019.First of all,the three-dimensional soil erosion process of karst slope was monitored under different rainfall conditions and land use patterns.Based on this,the process of phosphorus loss in runoff and sediment during natural rainfall events was investigated.The difference of process of phospho-rus loss in runoff and sediment was compared and the controlling factors of phospho-rus concentration in different loss paths were analyzed.The main results are as fol-lows:In the runoff plots with different land use patterns,the total slope runoff including surface runoff,subsurface runoff and fissure flow decreased in an orderly way,with cultivated land being observed to have the greatest amount（1324.30L）,followed by orchard land（1252.49L）,and finally forestland（1172.3L）.The average runoff coeffi-cient of surface runoff,subsurface runoff and fissure flow were about 4%,1%and 1%respectively.Compared with the rainfall process,the runoff generation process had a certain lag time,with the lag time ranging from 30 min to more than 2 hours.Fissure flow generation was the earliest among three hydrological paths,while surface runoff reached the peak first.Compared with cultivated land and orchard land,forestland had the smallest yield and the latest response to rainfall process.In the runoff plots with different land use patterns,the surface sediment yield de-creased in an orderly way,with cultivated land being observed to have the greatest amount(11.91kg·hm^-2),followed by orchard land(7.22kg·hm^-2),and finally forestland(3.41kg·hm^-2).The soil erosion process mainly concentrated in heavy rain and rain-storm events.Deep percolation was not considered,phosphorus loss in karst slope was mainly caused by surface erosion,phosphorus loss carried by surface runoff accounted for73%of the total phosphorus loss,and phosphorus loss carried by surface sediment ac-counted for 9%of the total phosphorus loss.Subsurface runoff and fissure flow respec-tively accounted for 8%and 10%of the total phosphorus loss.Surface runoff was the most important path of phosphorus loss in karst slope.Controlling the process of run-off generation on sloping land is the key to control phosphorus loss.Under different rainfall conditions,the concentration of phosphorus in each hy-drological path was as follows:with the increase of rainfall,the concentration of phosphorus in surface runoff and fissure flow increased,while the concentration of phosphorus in subsurface runoff changed gently under different rainfall conditions.Under the same rainfall condition,the concentration of phosphorus forms in surface runoff was the highest compared with subsurface runoff and fissure flow.At the begin-ning of the rainy season,TDP was the main form of TP in surface runoff,and PP was the main form of TP with the increase of rainfall.Similarly,TP loss forms of subsur-face runoff and fissure flow were mainly PP.DOP was the dominated forms of TDP,compared with DIP,DOP was more likely to migrate downward.No matter what land use patterns were,the concentration of particulate phos-phorus in different hydrological paths was controlled by rain intensity,the concentra-tion of soluble phosphorus in different hydrological paths was controlled by different factors.The concentration of soluble phosphorus in surface runoff was greatly affected by rainfall intensity.After the buffer of soil layer,the concentration of soluble phos-phorus in subsurface runoff was little affected by rainfall intensity,but more affected by soil p H value.In fissure flow,the concentration of soluble phosphorus was not only affected by rainfall intensity,but also by weathering and dissolution of carbonate rocks.Compared with soil,the content of phosphorus in sediment increased.Soil erosion accelerated the mineralization process of OP and promoted the transformation of stable P to labile P.The process of phosphorus loss in sediment was little affected by the ex-ternal rainfall dynamic factors,while the internal soil properties had a profound effect on the content of phosphorus in sediment.The content of phosphorus in sediment of cultivated land,forest land and orchard land were controlled by p H,organic matter and clay,respectively.According to the analysis of water balance,the rainfall distribution of slope land during the monitoring period was as follows:evapotranspiration（59.30%）>deep seepage（28.94%）>soil and epikarst storage（5.71%）>surface runoff（4.01%）>sub-surface runoff（1.04%）>fissure flow（1%）.Deep seepage was the most important part of karst slope runoff.Combined with the average concentration of phosphorus in each hydrological path,it was estimated that the amount of phosphorus loss carried by deep seepage accounted for about 67%of the total phosphorus loss.