Effects Of Drought On Decomposition Of Coarse Woody Debris And Its Underlying Mechanisms In A Subtropical Forest

Coarse woody debris（CWD）is an important component of forest carbon cycle,which accounts for 3%-6%of the topsoil carbon in forest ecosystems.The decomposition of CWD releases approximately 10%of the total CO₂ efflux from the forests.However,climate change is altering the global hydrological cycle,resulting in significant changes in precipitation regimes that have profound impacts on carbon cycle in terrestrial ecosystems.Understanding the responses of CWD decomposition to changes in precipitation is necessary for assessing the forest carbon cycle under future climate change.In this study,we conducted a 3-year field throughfall exclusion experiment from2016 to 2018 to examine the drought effects on CWD decomposition in three angiosperm and three gymnosperm tree species at a subtropical broadleaf forest in China.The treatments consisted of control（no rainfall reduction）,mild drought（35%rainfall reduction）,and heavy drought（75%rainfall reduction）.The CO₂ efflux,environment factors,wood traits,and microbial community were continuously measured during the experimental period.Our aims were to explore how the CWD decomposition respond to drought and its underlying mechanisms.The main results were as follows:（1）Drought significantly inhibited the decomposition of CWD in the six tree species,and the suppression effects increased with the drought intensity.The responses of CWD decomposition to drought were different among tree taxa（i.e.,angiosperm vs.gymnosperm）.On average,the CO₂ flux of CWD from angiosperm and gymnosperm trees decreased by 35.3%and 24.6%,respectively,under mild drought（35%rainfall reduction）,and by 44.8%and 39.4%,respectively under heavy drought（75%rainfall reduction）.Overall,the temperature sensitivity of CWD decompositionin angiosperms(Q₁₀=1.75)was lower than that in gymnosperms（1.93,p<0.05）,which was probably due to the differential characteristics between angiosperms and gymnosperms.（2）The mechanisms underlying of CWD decomposition were different under different drought intensities.In the control,the CWD decomposition was mainly regulated by the bacterial community（the path coefficientρ=0.59）,which in turn was determined by the wood moisture.Under the drought treatments,the CWD decomposition was primarily regulated by the fungal community,which was affected by nitrogen density of CWD.The effect of fungi on CWD decomposition was larger in heavy（ρ=0.81）than mild drought treatments（ρ=0.54）.These results indicated that the main driver of CWD decomposition shifted from bacteria to fungi community when the drought stresses became more severe.In conclusion,the effects of drought on the decomposition of CWD in subtropical forests were different between gymnosperms and angiosperms.There are different regulatory mechanisms under different drought intensities.The importance of fungi to CWD decomposition increased gradually along the increasing drought intensity.Our results might deepen the understanding of microbial mechanism regulating CWD decomposition,which could help to improve the accuracy of ecosystem models for predicting the forest carbon cycle in future climate change.