Impacts Of Nitrogen Deposition On Forest Soil Respiration And Its Components On Urban And Rural Gradients

Forest soil respiration（Rs）is an important component of the carbon（C）cycle in terrestrial ecosystems and is also a key process regulating the C cycle in terrestrial ecosystems.The dramatic increase in nitrogen（N）deposition since the industrial revolution has led to the differentiation of forest Rs under its influence and the formation of regional environmental differentiation characterized by urban-rural gradients with increasing urbanization,with different degrees of negative impacts on ecosystems.However,the response of forest Rs to N deposition on the urban-rural gradient is currently unclear.Therefore,understanding the effects of N deposition on forest Rs and its components（soil heterotrophic respiration and autotrophic respiration）on urban-rural gradients is essential for predicting soil carbon dioxide（CO₂）emissions and assessing C stability and cycling processes in ecosystems.To investigate the effects of N deposition on forest Rs and its components on the urban-rural gradient,this study selected sawtooth oak（Quercus acutissima）forests with similar neutral ground conditions and vegetation growth in urban forest（Shushan National Forest Park）,suburban forest（Zipeng Mountain National Forest Park）,and rural forest（Wanfoshan National Forest Park）on the urban-rural environmental gradient as the research subjects and established experimental plots.In the fourth year of the simulated N deposition experiment,a one-year observation experiment was conducted(CK treatment:0kg N ha^-1·year^-1,LN treatment:50 kg N ha^-1·year^-1,and HN treatment:100 kg N ha^-1·year^-1).Meanwhile,Rs and its components,soil temperature,soil moisture and soil physicochemical properties were also measured to investigate the response mechanism of N deposition to forest Rs and its components on the urban-rural gradient.The results of this study indicate that:（1）The forest Rs and its components on the urban and rural gradient had obvious seasonal dynamic characteristics and showed a clear single-peaked curve,with both Rs and its components being highest in the Mid-growing season.Meanwhile,forest Rs and its components have obvious urban and rural variation characteristics,and both Rs and its components are higher in urban forest than in suburban and rural forests.In addition,the cumulative annual soil CO₂emission fluxes from urban forest were significantly higher than those from the other two forest stands.（2）The effect of N deposition on forest Rs and its components on the urban and rural gradient occurred mainly in the Mid-growing season.The seasonal dynamic characteristics of forest Rs and its components on the urban and rural gradient were not changed under different N addition treatments.Throughout the study period,the N addition experiment significantly promoted Rh and significantly inhibited Ra in urban forest and had no significant effect on Rs in urban forest;N addition had no significant effect on Rs and its components in both suburban and rural forests.（3）The Q₁₀values of forest Rs and its components on the urban and rural gradient gradually decreased with the urban and rural gradient,and the N addition treatment had a significant inhibitory effect on the Q₁₀values of Ra in urban forest and rural forest.Compared to the single factor fit model,the dual factors fit relationship of forest Rs and its components on the urban and rural gradient with ST and SW has a higher explanation rate.This indicates that the interaction of ST and SW can better explain the changes in Rs the urban-rural environmental gradient.（4）The correlation study in this thesis indicates that forest Rs and its components on the urban and rural gradient are subject to the combined effect of multiple soil physicochemical factors.The dominant factors of soil physicochemical factors on forest Rs and its components on the urban and rural gradient were screened separately by establishing a random forest model,in which ST,SW,soil DOC,DON,MBC,MBN,NH₄⁺-N and TN exerted significant effects on forest Rs and its components on the urban and rural gradient.Meanwhile,the results of structural equation modeling（SEM）indicated that under N addition treatment,soil DON,MBC and MBN directly affected the Rs components of urban forest,soil DOC,DON and MBC directly affected the Rs components of suburban forest,and soil DOC,MBC and MBN directly affected the Rs components of rural forest.In summary,the results of this study indicate that there are significant urban-rural differences in forest Rs and its components on urban and rural gradient,and that there are differences in the response of forest Rs and its components on urban and rural gradient to N deposition,which can help deepen the knowledge of N deposition affecting forest soil Rs,provide theoretical reference for the influence mechanism of Rs in subtropical forest ecosystems,and provide a correct assessment of the contribution of subtropical forests to global terrestrial C emissions.It also provides basic data and theoretical support for the correct assessment of the contribution of subtropical forests to global terrestrial C emissions.