Effect Of Simulated Nitrogen Deposition On The Stability Of Soil Organic Carbon In A Subalpine Coniferous Forest

Atmospheric nitrogen deposition has become one of the major global environmental problems,as human activities emit large amounts of nitrogen into the atmosphere,and atmospheric nitrogen deposition to the ground can affect food production,environmental quality and climate change on a global scale.The stability of SOC determines its decomposition and the ease of soil-atmosphere carbon exchange,but the changes in SOC stability under nitrogen deposition are unclear.As the second largest forest area in China,southwestern mountain forests are one of the important terrestrial ecosystem carbon sinks in China,of which subalpine coniferous forests are the mainstay of southwestern mountain forests.Therefore,in this study,we investigated the effects of multi-year simulated nitrogen deposition(0,8 and 40 kg·ha^-1·a^-1 NH₄NO₃)on SOC accumulation at different soil depths using a typical subalpine coniferous forest understory sample plot and combined biomarkers,chemical component classification,multivariate statistics and model fitting to analyse the drivers of organic carbon accumulation.The main results are as follows.The main results are as follows:（1）The results of the compositional analysis of the soils at different depths and with different N additions showed that the N additions increased the SOC concentration and storage per unit area,with the mineral layer 0-5 cm SOC being significantly affected at all three soil depths.This increased SOC was mainly derived from microbial residues（amino sugars）and plant residues（lignin phenols）.Nitrogen addition had a small effect on the content of organic carbon in the iron complexed and active mineral bound states,indicating that the organic carbon content protected by iron ions and minerals did not change significantly.The results of the random forest analysis showed that plant residues and microbial residues were the most important factors driving soil carbon accumulation in the organic and mineral layers,respectively.（2）Results from continuous 300-day indoor incubation experiments indicated that nitrogen addition increased the rate of SOC mineralisation,i.e.nitrogen addition reduced the biological stability of SOC,and this change correlated with an increase in microbial biomass（P<0.05）.However,the results of the two pool model fit indicated that nitrogen addition did not significantly change the relative size of the active and inert carbon pools（P>0.05）.There was no significant effect of N addition on the temperature sensitivity(Q₁₀)of SOC mineralisation（P<0.05）,but the response trend of Q₁₀ differed between soil depths,showing a decreasing trend of Q₁₀ in the organic layer with N addition and an increasing trend of Q₁₀ in the mineral layer with N addition.（3）The excitation effect of exogenous carbon input on SOC decomposition was simulated by adding ¹³C-labelled glucose,and the results showed a positive excitation effect from glucose addition,indicating that SOC accumulation in subalpine forests may be related to carbon limitation by microorganisms;N addition reduced the excitation effect,indicating that the stability of accumulated SOC in the context of N addition will not be reduced by exogenous carbon input.In summary,although nitrogen deposition promotes SOC accumulation in subalpine forests,the accumulated organic carbon is not protected by minerals.In the context of global warming,there is a tendency for the decomposition of mineral SOC to accelerate,thereby reducing the soil carbon sink potential.This study will help to accurately predict future changes in soil carbon sink capacity,which is important for achieving China’s"double carbon"target and mitigating global warming.