| Nitrogen?N? is one of the causes of water eutrophication and acidification.Wutai Mountain is the headstream of many rivers.Under the background of increasing N deposition and climate change,the impact of subalpine meadow and forest ecosystem on water quality of the catchments should be paid attention to.This paper combines the project of National Natural Science Foundation of China"Research on Soil Nitrogen Dynamics of Alpine Vegetation Based on Hydrological Process and Freeze-thaw Cycle".The characteristics of hydrochemical changes at different spatial levels and at different times during wet deposition of typical subalpine vegetation communities in Wutai Mountains and their impacts on runoff water quality were preliminarily studied,it provides the basis for the protection and management of subalpine water conservation forest in Haihe Basin,and provides the basic data for the study of the impact of climate change on the hydrochemical process in subalpine forest watershed.Based on analysis of vegetation communities of Wutai Mountain,3 plots were set:subalpine meadow?CD?,Larix principis-rupprechtii?HL?,Picea meyeri×Larix principis-rupprechtii mixed forest?YH?.During 20152017,the concentrations of TN,NH4+and NO3-in rainfall,snowpack,throughfall,litter leachate,surface runoff and snowmelt runoff were monitored and analyzed.Firstly,through the analysis of rainfall,throughfall,litter leachate,surface runoff in growing season,through the analysis of snowpack,snowmelt runoff in seasonal freeze-thaw period,the interception effect of different spatial levels of typical vegetation community?canopy,litter layer,soil layer?on N and the causes of water quality change were revealed.Secondly,through the analysis of surface runoff and snowmelt runoff in different periods,the influence of temporal variations on runoff N concentration was revealed.Finally,the water quality of surface runoff and snowmelt runoff output from typical vegetation communities was evaluated comprehensively by single factor evaluation method,comprehensive index method and grey relational degree method.The results were summarized as follows:?1?During 20152017,the rainfall of Wutai Mountain was weak acidic with an average pH of 6.89,which is higher than the average pH value of rainfall in the north is 5.6,and the acid rain situation is not obvious.?2?The effects of different spatial levels of typical communities on water quality during the rainfall to runoff process were investigated.The results showed that:1)During the growing season,the pH of the water increased as it moved through HL and YH canopy,soil and CD soil.and soil had greater buffering effect on water pH,while the pH-buffer abilities of HL were better than YH,CD.During the seasonal freezing-thawing period,snow melting forms snow-melting runoff.The pH of CD,HL and YH snow-melting water increases,and the buffer effect of HL on water pH was still better than CD and YH;2)During the growing season,TN and NH4+in the rainfall were intercepted by HL canopy.The canopy of YH released TN,NH4+and NO3-into water.NO3-in throughfall was intercepted by HL litter,while NH4+was retained by YH litter.Both HL and YH soil intercepted TN,NH4+and NO3-.However,the concentrations of TN,NH4+and NO3-in throughfall,litter leachate and surface runoff of HL were always less than YH.Thus,the ability of intercepting N in water of HL was better than that of YH.After CD,the concentrations of TN,NH4+and NO3-in surface runoff is smaller than that of HL and YH.Therefore,CD has better effect on purifying water quality,and the main interception level is soil.In the seasonal freeze-thaw period,CD soil intercepts TN,NH4+and NO3-,while HL and YH release TN and NO3-to snowmelt runoff.Therefore,CD is still better than HL and YH in improving the water quality of snowmelt runoff.?3?Time variations of runoff water quality of typical communities were analyzed:1)The annual runoff pH of each vegetation community?CD:7.128.38,HL:7.028.06,YH:6.807.91?changed slightly in time,runoff pH was higher in growing season and lower in seasonal freeze-thaw period,and there was no obvious acidification;2)The concentrations of TN and NH4+in runoff of different vegetation communities were higher in growing season and lower in freezing-thawing season,the range of runoff TN concentrations?CD:1.274.11mg/L,HL:2.014.36mg/L,YH:1.335.34mg/L?,and the range of runoff NH4+concentrations?CD:0.043.38mg/L,HL:0.033.53mg/L,YH:0.063.39mg/L?.The NO3-concentrations in HL runoff were highest in seasonal freeze-thaw period and ranged from 1.47mg/L to 1.93mg/L during the year,and the NO3-concentrations in CD,YH runoff were highest in growing season,the range of runoff NO3-concentrations?CD:0.511.99mg/L,YH:0.193.43mg/L?.From the annual runoff N concentrations output situation,NH4+concentration output mainly concentrated in the growing season,while TN and NO3-concentrations output although the growing season occupies an important position,but the seasonal freeze-thaw period can not be ignored.?4?Comprehensive evaluation of surface runoff and snowmelt runoff shows that the water quality deviation of vegetation community runoff in this area may cause water pollution in a certain watershed,and the growing season is more vulnerable to the harm of N element,which makes the water eutrophication.However,the indexes TN,NH4+and NO3-selected in this evaluation are all necessary nutrients for plant growth.It also shows that the vegetation community in this area has sufficient nutrients,and its ability to release nutrients to supply plant growth is well,promoting the restoration of subalpine plantations and natural secondary forests in Wutai Mountain. |
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