| As a main issue of global change,nitrogen(N)deposition has become a hotspot of science and society.With the rapid increasing of fossil fuels burning,the production and use of chemical fertilizers,and animal husbandry and other human activities from the 1950 s,the increase of the active N compounds release to the atmosphere makes the atmospheric N deposition increase rapidly and continues to increase worldwide,which would seriously affect the structure and function of terrestrial ecosystems.Soil microbes,as the main participants and regulators of soil biochemical processes and material circulation,are one of the most active components of terrestrial ecosystems.They are great power in the source and sink of nutrient flow in soil ecosystems,and they play vital role in the degradation of plant litter,nutrient cycling and balance,and improving soil physical and chemical properties.N deposition directly or indirectly affects the growth,reproduction and activity ability of soil microbes,and the species,quantity,species diversity,community structure and function of soil microorganisms would change accordingly,thus affecting the material transformation in soil.Although large number of studies did research on the effects of N deposition on soil microbes in different test sites and different ecosystems at home and abroad,these studies have not achieved the consistent results.There have also been some qualitative reviews on the response of soil microbial characteristics and litter decomposition to N deposition under the background of global warming,but there have been few quantitative evaluations of existing N deposition experiments.The quantitative evaluation of response of soil microbial characteristics and litter decomposition to N deposition is of vital significance to the in-depth understanding of soil carbon and nitrogen cycling and its response to climate change.Meta-analysis method is applied in this dissertation.From different groups of soil microbial biomass,microbial community structure and function of soil microbes and litter decomposition,under the background of global climate change,it is of great theoretical and practical significance to study the response of soil microbial characteristics and litter decomposition to N deposition and its mechanism,and to seek more general and universal laws on a global scale for soil material transformation and recycling process.The specific research contents are as follows: through the collection,collation and analysis of a large number of published literature data,the effects of N deposition on soil microbial biomass,composition and function are discussed.To explore the effects of ecosystem types on soil microbial characteristics and the response of litter decomposition to N deposition under different climatic conditions.The effects of different N application levels,N application time and even different experimental methods on soil microbial characteristics and the response of litter decomposition to N deposition are investigated.Ecological forecasts has become an emerging imperative as global change is threatening the Earth.Terrestrial ecosystems play a vital role in both the global carbon(C)cycle and the regulation of climate change.Therefore,improving model projections is of a high priority in terrestrial C cycle models.To understand the essence of model projections and disentangling the sources of uncertainty,it is significant to measure relative contributions of model,parameters,forcing,and data to projections.We applied an Ecological Platform for Assimilating Data(EcoPAD,v1.0)into Models,which is a web-based software system automating data transfer from sensor networks,data assimilation,and ecological forecasting,to the Spruce and Peatland Responses Under Climatic and Environmental change(SPRUCE)experiment at North Minnesota.We used data collected from 2011 to 2015 at SPRUCE site to constrain model parameters in the Terrestrial Ecosystem(TECO)Model and forecast the responses of C cycle variables to elevated CO2 and five levels of warming in the future time once a week from 2016 to 2024.We analyzed the weekly forecast results with data in 2016-2018 to measure relative contributions of external forcing variables and model parameters using information theory.The main conclusions of this study are as follows:1.Globally,the negative effects of N deposition on soil microbial growth,composition and function is widespread in terrestrial ecosystems,including tundra,grassland,cropland,forest,and wetland.N deposition reduces microbial biomass(total microbial biomass,bacteria biomass and fungal biomass reduce 13.2%,16.6%,19.2% respectively),alters the composition of microbial community structure(such as fungi bacteria ratio decrease 7.9%,gram-positive bacterium and gram-negative bacteria ratio increase(p=0.075),etc.),decreases microbial respiration by 8.1%.Besides,there is significant correlation between the response of microbial respiration and biomass to N deposition.2.The results of the structural equation model show that the inhibition of microbial biomass by N deposition leads to the inhibition of microbial respiration and the increase of N deposition rate and time will further inhibit microbial activity,while the higher temperature weakens these effects to some extent.3.N application rate and time influence the response of litter decomposition and nutrient release to N additions.Climate change and precipitation also interactively modulate these responses.Importantly,the decomposition of litters reduced 3.45% in plantations,and promoted 2.00% in secondary forests,which is possibly due to differences in litter quality and plant diversity.4.External forcing variables contributed more information to forecasting C cycle dynamics than parameters.That is,the accuracy of forecasting carbon dynamics is relatively high given forcing.However,yearly and decadal forcing,especially soil temperature are highly uncertain in driving ecological forecasting.Soil microorganisms,outweighing all other organisms on earth combined,play a critical role in the biogeochemical cycle of the earth,as they are responsible for the cycling of soil carbon and nutrients in ecosystems.The adverse effects of N deposition on soil microorganisms cannot be addressed without significant social changes to reduce atmospheric N deposition and the more conservative use of N fertilizers.Our results suggest a high confidence in forecasting carbon dynamics within a bound of the stochasticity of external forcing variables. |
