The Effects And Regulation Mechanisms Of Throughfall Reduction On Soil Respiration In A Warm-temperate Oak Forest

Climate change models predicted most regions around the world are experiencing drought,with increase in intensity and duration.Prolonged drought would have a considerable impact on carbon cycling of terrestrial ecosystems,particular to soil respiration（SR）.However,the effects of long-term drought on SR and the mechanisms underlying this effect remain unclear,especially in forest ecosystems.In the present study,we have conducted manipulated throughfall reduction（TFR）experiment platform in Henan Baotianman Forest Ecosystem Research Station,and we explored the effects of long-term TFR（2019-2020）on SR and its components:autotrophic respiration（AR）and heterotrophic respiration（HR）in an oak forest,basing on the first four years of studies（2013-2016）.We also determined fine root growth and physiology properties,soil microbial communities,root exudation and rhizosphere microbiome to reveal the mechanisms in regulating soil respiration under long-term TFR.The important results were as follows:（1）The long-term consecutive TFR treatment reduced soil moisture in each layer（5 cm,20 cm and 40 cm）,but not soil temperature.SR and its components had no significant interannual variability during 2013-2020,except for AR under the TFR,however,SR and its components showed significant seasonal variation.During the first four years of TFR,SR and its components,showed no significant changes.Compared with the short-term TFR,long-term TFR increased AR by 92%and its temperature sensitivity(Q₁₀ value),but decreased HR by23%and its Q₁₀value,leading to the relative contribution of HR to SR decreased from 79%to60%.The increased AR and decreased HR seemed to counteract each other,resulting in the unchanged SR under the long-term TFR treatment.Therefore,AR and HR could shift with the duration of TFR and had the opposite direction in response to long-term TFR,with consequence for the response of SR.The different responses of soil respiration components should be considered when predicting CO₂ emissions under long-term drought conditions.（2）The consecutive TFR treatment had no significant effects on soil microbial biomass carbon（MBC）and nitrogen（MBN）during 2013-2020.The long-term TFR treatment had little effects on the measured enzyme activities,such asβ-glucosidase,amylase and peroxidase,and only slightly increased the polyphenolase activity during 2019-2020.Long-term TFR did not change the mass fractions of soil particulate（POM）and mineral-associated（MAOM）organic matter,but slightly increased the recalcitrant carbon of POM in 2020,which reduced the microbial degradability of SOM and hence may also lead to a decrease in HR.We analyzed the soil microbial community through molecular techniques during 2019-2020,and found long-term TFR decreased soil bacterial richness,and was positively correlated with HR,but not fungal richness.Our random forest regression and linear regression further showed the abundance of Novosphingobium,norank＿o＿＿11-24 and norank＿o＿＿Vicinamibacterale were positively correlated with HR.While the abundance of 1959-1 and norank＿c＿＿Acidimicrobiiaand fungal Leptobacillium were negatively correlated with HR.Our results indicate that bacterial diversity and microbial phylotypes can predict the response of HR to long-term TFR,which is of great significance for improving the prediction of terrestrial ecosystem carbon emissions under future prolonged drought.（3）The short-term TFR had little influence on fine root biomass during 2013-2016,which was accounted for the unchanged AR.Compared with the short-term TFR,long-term TFR increased the fine root biomass and production during 2019-2020,and correlated positively with AR.The results of fine root physiological properties showed that long-term had little effects on fine root carbon,nitrogen and starch,while increased the soluble sugar content during 2019-2020,which may also lead to the increased AR.In addition,long-term TFR increased root exudation amount by 30%in 2020,leading to the enrichment of Proteobacteria with higher carbon turnover in the rhizosphere,which may lead to the increase in AR（rhizosphere respiration）.In turn,we found long-term TFR increased the bacterial genes（e.g.,nitrilase and pyrroloquinoline-quinone synthase）related to plant growth,and correlated positively with fine root production.Our results suggest that the fine root growth and physiology,rhizosphere bacteria and their interaction together regulated the process of AR.Altogether,our study revealed that the autotrophic and heterotrophic respiration will change with the duration of TFR and had opposite direction in response to TFR,thus determining the response of SR to long-term TFR.Soil microorganisms and fine roots account for the respective mechanisms underlying the divergent responses of HR and AR to long-term TFR.Under long-term TFR,the abundance of some drought-sensitive copiotrophic taxa（e.g.,Proteobacteria）,while some oligotrophic and drought-tolerant taxa（e.g.,Actinobacteria）,which together led to the decrease of HR.In response to the chronic water deficit,plant allocated more carbon into the underground parts,including increased root biomass and production,which eventually led to increased AR.It is important to note that long-term TFR increased root exudation amount and leading to the enrichment of Proteobacteria with PGP traits,in turn,Proteobacteria may facilitate the plant host growth.As Proteobacteria contained characteristics of copiotrophs,the enrichment of Proteobacteria in rhizosphere would result in a higher rhizosphere respiration rate,which is also an important reason for the increase of AR in this study.Therefore,in the context of future climate change,we should not only pay attention to the study on roots and microorganisms,respectively,but also the interaction between the roots and microorganisms,such as the rhizosphere carbon process,so as to fully understand the soil carbon cycle process.