Effects Of Stimulated Nitrogen Deposition On Soil Carbon Budget In The Yellow River Delta Wetland

Global nitrogen deposition(N_dep)has increased significantly during last few decades,influencing the structure and function of terrestrial ecosystems,and global carbon(C)cycle.As a transition of sea and land,coastal wetlands not only provide remarkable services for creatures,but also play an important role in functioning of‘blue carbon',storing large amount of soil organic carbon(SOC).Under global change,impacts of N_dep on soil carbon pool and its variations in coastal wetlands and underlying mechanisms remain elusive yet.This research conducted N_dep simulation experiment and several incubation experiments under the platform in the coastal wetland of the Yellow River Delta Ecological Research Station of Coastal Wetlands,Chinese Academy of Sciences,exploring the responses of soil environment,vegetation growth,C input and output,and soil C composition to simulated N_dep.The conclusions are as follows:(1)Higher N availability decreased soil C/N ratio and electronic conductance(EC),but increased soil TC,SOC,DOC,and TN.Simulated N_dep also stimulated vegetation growth,which is highly related with soil environment,and enhanced the percentage of Phragmites australis(increased from 50% to 72%)and decreased the percentage of Suaeda salsa(decreased from 43%to 15%),further regulating vegetation structure and function in coastal wetlands.(2)Simulated N_dep overall enhanced the carbon uptake via photosynthesis,varying with the level and type of added N.Temperate NH₄NO₃ addition showed strongest enhancement of GPP.Simulated N_dep also increased the percentage of root in top layer soil(0-10cm),possibly due to the easy N absorption in topsoil.This study also illustrated that CO₂ flux might be associated with leaf area index and vegetation density,associated with leaf maximum photosynthesis rate(p<0.05).(3)Simulated N_dep with different levels and types of N overall stimulated soil RE.Simulated N_dep increased soil temperature sensitivity(Q₁₀),with the highest Q₁₀observed in temperate NH₄NO₃ treatments(41%increase).The rate of soil respiration is highly related to soil C,N contents in different forms.Soil total carbon and total nitrogen jointly increased soil respiration.(4)Incubation-based experiments revealed that the stimulation of SOC decomposition varied with types of N addition,among which NO₃^--N showed strongest stimulation.The rate of SOC decomposition also increased with levels of N addition.As for soil salinity,this study showed that increased soil salinity reduced soil(p<0.05)microorganisms and further slowed down SOC decomposition.In the middle of the incubation,soil DOC had significant stimulation on SOC decomposition.(5)Simulated N_dep obviously increased soil C(16%increase).The response of soil C content to N_dep varies along with soil depth.Highest increase of soil C was observed in topsoil(0-10 cm).As to different levels of N addition,high-level N showed strongest stimulation on soil total carbon.There is no statistically significant difference of soil total carbon among different forms of N addition.