Study On Nutrient Element Loss Process In A Small Watershed Of Qianxi Rocky Desertification Area

The loss of nitrogen,phosphorus and potassium in karst environment inevitably leads to the decrease of land productivity.The migration of soil nutrient with soil erosion will directly affect water ecological environment.Therefore,regulars of nitrogen and phosphorus and other elements outputting in the karst rocky desertification region are particularly important.Most of the research on rainfall and soil erosion in the karst area,belongs to the simulation study.The observation is relatively few,at the same time,the research on nutrient loss during rainfall is more scarce.In this study,the characteristics of nutrient element which includes nitrogen and phosphorus were analyzed in different scales of space and time,and the characteristics of nutrient elements from water and small watershed were analyzed too.This paper studies mainly output characteristics of nutrient elements those are from surface and underground two-dimensional structure in karst small watershed during natural rainfall,discusses the dynamic migration,output law of nutrient,and provides theoretical support for element management of karst watershed.The research on the source of spring is helpful to learn more about karst hydrological system and sustainable utilization of water resources.The results are as follows:(1)By analyzing stable isotopes value of ?18O-?D from local precipitation,it is determined that the precipitation is the main source of karst spring and small watershed streams,in addition that precipitation in the wet season plays a dominant role in aquifer recharge.(2)The results from correlation analysis between different rainfall types and rainfall erosivity found that the greater rainfall is,the more obvious rainfall erosivity is,and the best season for controlling soil erosion is summer.(3)When river runoff is large,nitrogen and phosphorus output is also large.The nitrogen output forms is dominated by NO3--N.The output concentration of NO3--N was higher in winter and spring.The change trends of TN and NH4+-N output were similar,and the winter was higher than other three seasons.During soil and water loss processes,nitrogen loss is mainly in the form of dissolved state and also associated with phosphorus loss.Beginning of the rainfall is the key period for nitrogen and phosphorus loss.Different states of nitrogen loss have significantly positive correlation with rainfall.(4)The rainfall will promote the loss of Ca2+ and Mg2+.Ca2+ and Mg2+ loss concentration showed positive correlation.With Mg2+ concentration increased,Ca2+ output increased too.