Chemical Characterization Of Rainwater And Spatial-temporal Variation Of Nitrogen Deposition In A Small Karst Catchment

The substance circulation is the most significant process to maintain and promote the development of natural ecosystem,the atmospheric dry and wet deposition are the significant part of it.Dry and wet deposition accelerates the loss of earth surface substance,and also brings the material to the earth surface ecosystem from the atmosphere.Therefore,the dry and wet deposition process is of great significance to the interfacial substance exchange of the earth surface circle-layer,which changes the system function of earth surface and impacts the water,soil and atmosphere environment continuously.In this study,we study on the karst agricultural watershed in Puding county,GuiZhou province,southwest China.The dry and wet deposition samples in the basin were collected for the study of the chemical characteristics of precipitation and the variation of nitrogen deposition.Combined with isotope technology and model method,the major sources of rainwater substance and their contributions was distinguished,the spatial-temporal variations of nitrogen wet deposition was analyzed,the source contribution of nitrate wet deposition and the flux of nitrogen deposition was estimated in this study.Here we provide scientific basis for the research of acid rain pollution,nitrogen source pollution and environmental management in karst region,and provide the theoretical support of biogeochemical cycle of dry and wet deposition in karst region.The main results as follows:（1）The pH of the precipition ranges from 4.3 to 6.8,more than 70%of the rainwater is non-acidic precipitation.98.5%of the acidity of precipitation during the sampling period was neutralized by alkaline substances.NH₄⁺and Ca²⁺are the primary and secondary neutralizing ions,respectively.The order of the weighted average equivalent concentration of anion is:SO₄^2->NO₃^->Cl^->F^-,and in order with Ca²⁺>NH₄⁺>Na⁺>Mg²⁺>K⁺for cation.The variation trend of the total concentration of inorganic ions in precipitation is low in winter but high in summer,and the peaks of the ions concentration are present in winter.（2）In this study,there may be mainly（NH₄）₂SO₄,CaSO₄,NH₄ NO₃,Ca（NO₃）₂,NaCl and other substances in the precipition.The anthropogenic emission is main source of anion,and contributed 99.41%of SO₄^2-and 96.14%of NO₃^-.The main cation Ca²⁺（98.92%）and Mg²⁺（64.25%）are derived from rock/soil weathering.8.49%of K⁺is derived from sea source,and 91.51%is from terrestrial source。（3）NH₄⁺-N is the primary nitrogen species of wet deposition samples in this study,the average proportion was 57.4%⁶4.4%.The concentration of major nitrogen forms（NH₄⁺-N and NO₃^--N）showed the trend of high in winter and spring but low in summer and autumn.The spatial variation of nitrogen deposition presented as HZR>MZR>CQR due to the impact of the surrounding environment of the sampling sites.There is a negative natural logarithm relationship between rainfall and the concentration of major nitrogen species.（4）Theδ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-value of wet deposition ranges from-6‰to-1.4‰and+55‰to+68‰in this study,respectively.Theδ¹⁵N-NO₃^-andδ¹⁸O-NO₃^-value present obvious seasonal variation as positive in autumn and winter but negative in spring and summer at three sample sites.The isotopic composition of nitrate in wet deposition shows no obvious regularity on spatial scale.The SIAR model source analysis shows that the main contribution rate of nitrate in rainwater is:microbial N cycle（31.8%）>vehicle exhausts（26.9%）>biomass burning（22.5%）>coal combustion（18.8%）.Moreover,the contribution rate of the four sources showed strong monthly variation.（5）The study of wet deposition in depression shows that local airflow and the release of organic matter by plant growth will present a significant vertical variation in the concentration of nitrogen wet deposition.The vertical variation may have a certain effect on the estimation of nitrogen deposition.The flux of nitrogen deposition（wet and dry deposition）is 22.3 kg·hm^-2·a^-1in the basin,the amount of nitrogen deposition is relatively low.