Effects Of Warming On Soil Greenhouse Gas Fluxes At Different Soil Depths In A Subtropical Evergreen Broad-leaved Forest

Carbon dioxide(CO₂),methane(CH₄)and nitrous oxide(N₂O)are the main greenhouse gases that lead to the rise of global temperature.Forest soil is an important carbon and nitrogen pool in terrestrial ecosystem,and the slight changes of CO₂,CH₄and N₂O in forest soil will have a great impact on the global climate.Temperature is the key factor regulating soil carbon and nitrogen cycle in terrestrial biogeochemical cycle,and it is one of the main factors affecting soil greenhouse gases.It directly or indirectly affects soil production or consumption of CO₂,CH₄ and N₂O by regulating soil microbial and root activities,soil organic matter mineralization and gas diffusion rate.The study of soil greenhouse gas fluxes at different depths and their response to warming is of great significance for predicting climate change in the future.The soil structure and physical and chemical properties of deep soil are different from those of surface soil.However,so far,few studies have paid attention to the dynamic changes of greenhouse gas fluxes in forest soil profiles and their response to warming.Based on this,this study relied on the subtropical evergreen broad-leaved forest of Sanming Forest ecosystem and Global change Research Station in Fujian Province.Two treatments of control(CT)and warming(W)were set up to study greenhouse gas fluxes at different soil depths(0-10 cm,10-20 cm,20-40 cm,40-60 cm).Soil gas samplers were installed in the field,and soil greenhouse gases at 0,10,20,40 and 60 cm were collected regularly for long-term observation in the field.During the observation period,the soil solution was extracted regularly,and its concentration and structure were analyzed.The aim was to reveal the effects of soil warming on soil greenhouse gas fluxes and environmental factors at different depths in a subtropical evergreen broad-leaved forest.The main results were as follows:(1)The soil CO₂ concentration increased exponentially with the increase of soil depth,and the CO₂concentration in the bottom soil was higher than that in the surface soil.Compared with the control treatment,soil CO₂ concentration at different soil depths(0,10,20,40,60 cm)increased by 4.18%,30.93%,36.01%,14.84%and 13.23%,respectively.During the study period,except for 20-40 cm soil layer,warming increased the average CO₂ flux of each soil layer significantly,and the average CO₂ flux in 0-10,10-20,20-40 and 40-60 cm layers increased by 52.02%,54.82%,5.49%and30.77%,respectively.This showed that warming promoted the emission of CO₂ from each soil layer.(2)The concentration of CH₄ decreased exponentially with the increase of soil depth,and the concentration of CH₄ in the bottom soil was lower than that in the surface soil.After increasing temperature,the change of CH₄ concentration was more complicated.After increasing temperature,the average concentration of CH₄ remained basically unchanged at 0 cm and increased by 1.92%at10 cm,but decreased by 2.74%,8.2%and 9.43%at 20,40 and 60 cm,respectively.Except for 0-10cm soil layer,the average soil methane flux decreased by 0.11%,while the average methane flux in other soil layers increased by 24.03%,22.79%and 44%,respectively.This showed that warming promoted the absorption of soil CH₄ as a whole.(3)The soil N₂O concentration increased with the increase of soil depth,but the average N₂O concentration of each soil layer increased slightly or even unchanged,and the effect of warming on it was not significant.The warming treatment reduced the average N₂O emission flux in 0-10 cm soil layer by 7.04%,and significantly increased the N₂O emission flux in 10-20 and 20-40cm soil layers by 12.92%and 6.02%,respectively,and significantly increased by 47.45%in 40-60 cm soil layer.This showed that warming had little effect on N₂O emission from surface soil,and it increased N₂O emission from deep soil.(4)The annual average soil temperature of CT and W treatments were 19.82,19.86,20.03,19.88?and 19.82,19.86,20.03,19.88?respectively.The annual average soil moisture content in each soil layer was 22.97%,23.88%,26.38%and 26.81%for CT and 22.49%,23.36%,25.81%and 25.99%for W treatment,respectively.Through the analysis,it is found that the fluxes of CO₂,CH₄ and N₂O had good correlations with temperature and water content.(5)With the increase of temperature,the concentrations of p H,ammonium nitrogen and nitrate nitrogen in different soil layers decreased,while the concentrations of dissolved organic carbon(DOC),dissolved organic nitrogen(DON)and dissolved total carbon(DTN)increased.Through the analysis of ultraviolet spectrum,infrared spectrum and fluorescence spectrum,it was found that the structure and composition of soil solution became simpler,the molecular weight decreased,the hydrophobic substance decreased and the aromatization index decreased.Through the fluorescence analysis of dissolved organic matter(DOM)solution,it was found that the metabolites of dissolved microorganisms increased with the increase of temperature.The results showed that the concentration of soil DOM solution increased with the increase of temperature,and the structure of soil DOM solution became simpler.(6)Correlation analysis and redundancy analysis showed that soil moisture content and soil solution DOC concentration were the key factors driving the change of greenhouse gas flux in 0-10cm soil layer.Soil temperature was the key factor driving the change of greenhouse gas flux in 10-20cm soil layer,and soil moisture content and DOC/DON ratio were the key factors driving the change of greenhouse gas flux in 20-40 cm soil layer.Soil moisture content and temperature were the key factors driving the change of greenhouse gas fluxes in 40-60 cm soil layer.The DOM structure of soil solution(DOM structure became simpler under increasing temperature)was also one of the important influencing factors.