Effects Of Simulated Nitrogen Deposition On Carbon And Nitrogen In Soil Of Xishan Artificial Forest,Beijing

Continuous nitrogen deposition significantly increased the nitrogen availability in soil,which causes great intervention in the carbon cycle and greenhouse gas emissions of ecosystem and thereby affecting the global warming potential.Previous studies focused on the short-term effects of adding single form of nitrogen,while the study reported that the response of greenhouse gas emissions from forest soil to the increase of atmospheric nitrogen deposition was nonlinear,which depended on the "nitrogen saturation" of forest ecosystem.In deposition,the mechanism of nitrogen deposition on soil carbon and nitrogen transformation is not clear.In view of this,it is necessary to carry out long-term multi-form and multi-level simulations of nitrogen deposition in the fields for the study of soil carbon and nitrogen transformation and microbial mechanism.In this paper,Xishan artificial forest soil was taken as the research object.From 2011 to 2019,the multi-level?0,50,150 kg N ha-1yr-1?nitrogen increasing control experiments were carried out to study the effects of nitrogen deposition on soil physical and chemical properties,greenhouse gas emissions,structural characteristics and stability of dissolved organic matter in soil,and carbon sequestration potential of temperate forest ecosystem.The response of microbial population abundance,community structure and soil enzyme activity to nitrogen deposition was compared and analyzed,and the microbial mechanism related to the main process of carbon and nitrogen transformation was explored.The results are as follows:For 8 years,nitrogen deposition significantly increased the available nitrogen content of soil,decreased the carbon nitrogen ratio of soil,and led to an average decrease of 0.85 units of soil p H,which resulted in obvious acidification,with ammonium nitrogen showing the strongest acidification effect.Therefore,the change of basic physical and chemical properties of soil will have a profound impact on soil biochemical process.The effect of nitrogen deposition on the exchange flux of greenhouse gases is not only controlled by nitrogen form and level,but also has a multi-stage process.Nitrogen deposition significantly promoted soil CO₂ and N2 O fluxes,and the effect peaked in the second year and the third year respectively,and then decreased year by year.CO₂ emissions were inhibited in 2019.The effect of nitrogen deposition on CH₄ annual absorption of soil faces a significant lag phenomenon.The effects of nitrogen deposition form and level on greenhouse gas exchange flux are: high level > low level,ammonium nitrogen > nitrate nitrogen.After 8 years of nitrogen deposition,the dissolved organicmatter?DOM?in soil is mainly UV humus,which has higher molecular weight,stronger aromaticity,and higher degree of substitution at oxygen-containing substituents on aromatic rings.This means that nitrogen deposition makes DOM more complex and difficult to mineralize and release.Long term nitrogen deposition may also promote the accumulation of soil carbon and achieve the effect of "nitrogen stimulating carbon sequestration".Temperate forest?Quercus liaotungensis?is an important carbon pool.Nitrogen application significantly increased biomass carbon storage?17.5%-48.6%?,and changed the distribution pattern of aboveground and underground biomass.The carbon sequestration efficiency of unit nitrogen deposition is 7.1-33.5 kg C kg-1 N.It is NH4+-N not NO3--N that plays a leading role in the increase of net ecosystem productivity?NEP?.The main reason for this phenomenon is that the increase of respiration is less than that of net primary productivity?NPP?.Under the condition of nitrogen application,the increase of gene copy number of fungi,bacteria and archaea,the growth of microbial community abundance and the activity of enzymes related to carbon transformation?catalase and polyphenoloxidase?can be used to explain why CO₂ emissions induced by nitrogen deposition in soil go up.After 8 years of nitrogen deposition,the microbial community structure changed significantly.While the decrease of p H promoted a dramatic increase of fungi,the promoting effect of enzyme activity related to carbon transformation decreased gradually.The fungal mycelium,a cement agent,could cement soil particles together,and the relatively stable recombination could increase the proportion of organic carbon,and contribute to the stability of soil organic carbon and carbon sink effect.The effect of nitrogen deposition on the function gene of nitrification?aobamoa gene?is greater than that of denitrification.It can be inferred that the increase of N2 O emissions mainly come from nitrification.The nitrogen deposition stimulated the activity of phosphatase rich in nitrogen,promoted the release of soil phosphorus to alleviate the limitation of phosphorus,and then regulated the nutrient balance of nitrogen and phosphorus.With the change of availability of one nutrient element,microorganisms will adapt to the change of other nutrient elements and respond to the global changes caused by human beings.In a word,the transformation of soil carbon and nitrogen depends on the form and dosage of nitrogen deposition.The annual change of greenhouse gas emissions and enzyme activity has nonlinear or time lag effect.Therefore,we emphasize the importance of long-term field ecological monitoring.This study can accurately evaluate the contribution of nitrogen deposition to carbon sequestration of temperate forest soil and its source sink function,and provide scientific basis for the government to formulate the corresponding development of reducing CO₂ emissions.