Distributio Characteristics Of Global Snow Physicochemical Factors And Microbial Nitrogen Cycle In Snow Water

Nitrogen is a major inorganic pollutant in atmospheric rainfall.However,the importance and contribution of nitrogen compounds and the related microbial nitrogen cycling processes in fresh snowfall,which contributes to over 90%of wet deposition in winter,are not well understood.We collected 27 fresh snow samples from 21 cities that 80%are from China during 2016 and 2017,and investigated the majorpollutionfactorsandmicrobialnitrogencycleprocessesusing physicochemical analysis,molecular biology,and ¹⁵N isotope tracer technique combined with wet settlement mechanism analysis and statistical analysis.?1?Principal component analysis showed that SO₄^2-,Ca²⁺,Mg²⁺,K⁺,particulate matter were the first principal component,and N-compounds?NH₄⁺,NO₂^-,NO₃^-?were the second;according to the principal component score,the sampling points are divided into three categories,The higher the score,the heavier the pollution,the air quality index?AQI?of the sampling points also conforms to this rule.?2?Furthermore,the results show that the snow water collected in this study is unpolluted atmospheric snowfall,which conforms to the ion balance,the main pollutant ions SO₄^2-and NO₃^-were from anthropogenic sources,the main cations Ca²⁺,Mg²⁺,K⁺are the sources of soil,which were confirmed by the neutralization factor,enrichment factor,and water?¹⁸O and?D isotope analysis.Consistent with the principal component analysis,SO₄^2-contributed?61%?more to the pollution load than NO₃^-?29%?;Similar water isotope ratios in coastal and inland areas at the same latitude prove that the contribution of oceans to snowfall pollution is very small.?3?Molecular assays of key functional genes in various nitrogen conversion processes,including both oxidation?aerobic ammonium oxidation,anaerobic ammonium oxidation,nitrite oxidation?and reduction processes?denitrification,dissimilatory nitrate reduction to ammonium?DNRA??,showed a high number of archaeal?AOA?and bacterial?AOB?amoA genes(above 10⁷ copies g^-1),and AOB abundance was always greater than AOA.However,the gene abundance levels in other N-processes were all below the detection limit.?4?The determination of the microbial transformation rate using the¹⁵N-isotope tracer technique showed that the potential rate of above five processes was very low,which is basically consistent with the results from molecular biology studies.Taken together,our results illustrated that microbial nitrogen cycle processes are not the primary biological processes causing the pollution in China fresh snow.