Effects Of Simulated Nitrogen Deposition On Soil Characteristics And Methane Emission In The Gahai Wetland

As a result of anthropogenic activities,such as agricultural fertilization and the burning of fossil fuels,nitrogen deposition in terrestrial ecosystems is continuously increasing.Nitrogen deposition destroys the original carbon and nitrogen balance of terrestrial ecosystems,and has a significant impact on the carbon cycle process of terrestrial ecosystems.However,in the context of complex global climate change,the dynamic impact of nitrogen deposition on CH₄sources/sinks of the wetland ecosystem in the Qinghai-Tibet Plateau is still unclear.Therefore,this study took the soil of Gahai wetland in the northeast margin of the Qinghai-Tibet Plateau as the research object,through the field in-situ test（artificial nitrogen addition simulates natural nitrogen deposition）and laboratory analysis method,and set up four nitrogen addition treatments(CK,no addition;N5,5 g m^-2a^-1;N10,10 g m^-2a^-1and N15,15g m^-2a^-1)to study the effects of nitrogen addition on plant characteristics,soil environmental factors,soil elemental stoichiometry,soil active carbon fractions and soil CH₄fluxes.The main findings are as follows:（1）N10 and N15 treatments significantly decreased the plant aboveground biomass of Gahai wetland（P<0.05）,compared with the CK treatment,the plant aboveground biomass under N10 and N15 treatments decreased by 38.63%and63.74%,respectively.Nitrogen addition increased the total nitrogen and total phosphorus contents of plants in the Gahai wetland.The total nitrogen contents of plants under N5,N10,and N15 treatments were 15.32%,15.13%,and 19.66%higher than those under CK treatment,respectively.The total phosphorus contents of plants under N5 and N15 treatments were significantly higher than those under CK treatment by 31.06%and 61.94%,respectively（P<0.05）.（2）Nitrogen addition significantly increased the soil ammonium nitrogen content in Gahai wetland（P<0.05）,and the soil ammonium nitrogen content gradually decreased with the increase of soil depth.N5 and N10 treatments significantly increased the soil nitrate nitrogen content（P<0.05）,and the soil nitrate nitrogen content first increased and then decreased with the increase of nitrogen addition.With the increase of nitrogen addition,the soil bulk density and soil density in the 0-10 cm layer of Gahai wetland gradually decreased,while soil capillary porosity and total porosity gradually increased,while non-capillary porosity and air permeability showed a trend of increased first and then decreased.（3）The average soil organic carbon and total nitrogen content of the 0-40 cm layer soil in Gahai Wetland showed a trend of increased first and then decreased with the increase of nitrogen addition,while the soil total phosphorus content gradually decreased with the increase of nitrogen addition.Nitrogen addition significantly increased the average soil C:N,C:P,and N:P of the 0-40 cm layer in the Gahai wetland,indicating that an appropriate amount of nitrogen addition is beneficial to the accumulation of soil nutrients,while excessive nitrogen addition would cause the transformation of the Gahai wetland soil system from nitrogen element restriction to phosphorus element restriction,thus increasing the risk of soil nutrient loss in the alpine wetland.（4）The average soil microbial biomass carbon（MBC）and soil dissolved organic carbon（DOC）content in the 0-40 cm soil layer in the Gahai wetland showed a trend of increased first and then decreased with the increase of nitrogen addition,while the soil particulate organic carbon（POC）content gradually increased with the increase of nitrogen addition.Under different nitrogen addition treatments,soil MBC,DOC,and POC contents showed significant seasonal variation trends,and the variation trends were not the same.The results showed that excessive nitrogen addition would reduce the content of soil active carbon fractions,and thus reduce the soil quality and carbon pool function of the Gahai wetland.（5）Nitrogen addition significantly increased CH₄emissions from Gahai wetland soil,the average soil CH₄emission in 2 years was 1.38 mg m^-2h^-1.There was a binomial relationship between soil CH₄emission flux and nitrogen addition level in the Gahai wetland,that was,there was a threshold value for the effect of nitrogen addition on soil CH₄flux（N10 treatment）,and the average CH₄emission of N10treatment is 79.93%,54.76%,and 25.35%higher than CK,N5,and N15 treatment,respectively.Under the four addition treatments,soil CH₄emissions showed a single peak pattern,with the peak occurring in the peak season of plant growth(1.72-3.58mg m^-2h^-1).PCA analysis showed that soil C:N,temperature,and water content were the main factors affecting CH₄emissions from Gahai wetland soil.This study indicates that although high levels of nitrogen addition in the future will reduce CH₄emissions from soil in Gahai wetland and slow down the occurrence of the greenhouse effect,it will reduce vegetation productivity,and the lower rate of soil active carbon conversion will reduce the soil quality and carbon pool function of Gahai wetland,resulting in the transformation of the Gahai wetland system from nitrogen restriction to phosphorus restriction,increasing the risk of wetland soil nutrient loss,it also exacerbates the occurrence of eutrophication in wetland water bodies.This study not only provides basic data for accurately assessing CH₄emission in the alpine wetlands under the background of continuous increase of nitrogen deposition in the future,but also provides theoretical basis for management decision on greenhouse gas emission reduction under the goals of carbon peaking and carbon neutrality in the upper reaches of the Yellow River.