Effects Of Simulated Nitrogen And Phosphorus Deposition On Soil Properties,Microbial Community Structure And Nitrogen Cycling In A Boreal Coniferous Forest

As a sensitive and fragile ecosystem,boreal forest is highly susceptible to global change.The increase of atmospheric nitrogen and phosphorus deposition caused by human activities may lead to changes in soil physical and chemical properties and microbial community structure,thus affecting soil nitrogen cycling.In this study,a typical boreal coniferous forest in Great Xing'an mountain region of China was selected as the research object.The annual changes of soil physical and chemical properties and key controlling factors were clarified by measuring soil moisture content,p H value and different forms of carbon,nitrogen and phosphorus concentrations of boreal coniferous forest under simulated nitrogen and phosphorus deposition conditions for two years.The soil microbial biomass carbon and nitrogen concentrations were determined by chloroform fumigation and extraction method,and the microbial community structure was determined by 16 Sr RNA high-throughput sequencing technology to reveal the effects of nitrogen and phosphorus deposition on soil microorganisms in boreal coniferous forest.Soil samples receiving simulated nitrogen and phosphorus deposition were collected for indoor culture experiments,and the relationship between soil nitrogen cycling process and microbial characteristics was explored in combination with soil physical and chemical properties and microbial data in field experiments.The main results are listed as follows:(1)The nitrogen deposition treatment reduced soil p H of boreal coniferous forest at the early stage,but with the extension of simulated deposition time,the effect of nitrogen deposition on soil p H gradually decreased,and the phosphorus deposition treatment and the nitrogen and phosphorus deposition treatment had no significant effect on soil p H in boreal coniferous forest.The nitrogen deposition treatment early promoted the accumulation of soil total carbon in boreal forest,but the phosphorus deposition treatment inhibited the accumulation of soil total carbon,and reduced the soil dissolved organic carbon concentration.The nitrogen and phosphorus deposition treatment inhibited the accumulation of soil total carbon at the early stage,but with the extension of simulated deposition time,the nitrogen and phosphorus deposition treatment promoted the accumulation of soil total carbon and increased the soil dissolved organic carbon concentration.The nitrogen deposition treatment and nitrogen and phosphorus deposition treatment promoted soil total nitrogen accumulation and increased concentrations of soil dissolved nitrogen and ammonium nitrogen in boreal coniferous forests,which indicates that nitrogen deposition is beneficial to improve soil nitrogen availability in boreal coniferous forests.The phosphorus deposition treatment significantly increased soil total and available phosphorus concentrations in the boreal forest soils.The nitrogen deposition treatment and the nitrogen and phosphorus deposition treatment had no significant effect on soil total and available phosphorus concentrations at the early stage,however,at the late stage,they significantly increased the soil available phosphorus concentration,which indicates that exogenous nitrogen input promotes the microbial decomposition of soil organic matter in the boreal forest limited by nitrogen availability,thus accelerating phosphate mineralization and return to the soil.(2)At the late stage of simulated deposition,both the phosphorus deposition treatment and the nitrogen and phosphorus deposition treatmentsignificantly increased soil microbial biomass carbon and nitrogen concentrations,while the nitrogen deposition treatment significantly increased soil microbial biomass nitrogen concentration.The nitrogen deposition treatment significantly increased soil bacterial richness and decreased soil fungal richness in boreal coniferous forest,while the nitrogen and phosphorus deposition treatment not only significantly increased soil bacterial richness but also increased soil fungal richness,which may be related to the different response of soil bacterial and fungal communities to nitrogen and phosphorus inputs.The nitrogen deposition treatment increased the abundance of Proteobacteria,while the phosphorus deposition treatment decreased the abundance of Proteobacteria,and the nitrogen and phosphorus deposition treatment increased the abundance of Acidobacteria.The concentrations of soil dissolved organic carbon,ammonium nitrogen and total phosphorus may be the main limiting factors causing the changes of soil bacterial community.The results showed that nitrogen and phosphorus deposition could affect the composition of soil bacterial community by changing soil nutrient concentrations in boreal coniferous forest.The nitrogen deposition treatment and the nitrogen and phosphorus deposition treatment significantly increased the abundance of Ascomycota,but significantly decreased the abundance of Basidiomycota.Concentrations of soil available phosphorus,microbial biomass nitrogen and ammonium nitrogen were the main limiting factors causing the changes of soil fungal community.(3)The incubation experiment showed that the nitrogen deposition treatment significantly increased the soil ammonium nitrogen and nitrate nitrogen concentrations in the boreal forest soil.Combined with the field data,the results showed that continuous nitrogen and phosphorus deposition significantly improved soil net nitrogen mineralization and nitrification rate,and phosphorus deposition significantly reduced soil net nitrogen mineralization rate and increased soil net nitrification rate,which can be explained by that phosphorus deposition intensifies soil nitrogen limitation in boreal coniferous forest and inhibits soil microbial activity,thus affecting soil organic matter decomposition.