Effect Of Forest Conversion On Soil Microbial Residual Carbon And Soil Respiration

Forest soils are an important carbon reservoir for mitigating global climate change,and the size of forest carbon pool is regulated by both the root carbon input and microbial respiration carbon output processes.Therefore,revealing the mechanism of root carbon input on microbial respiration and soil organic carbon content in forest soils is an important theoretical basis for predicting changes in the source and sink of forest soil carbon pool in future using Earth system models.However,it is still difficult to understand comprehensively the effects of root carbon input on forest soil microbial respiration and soil organic carbon content from existing studies due to several reasons.First,most previous experiments manipulating root carbon input have been limited to controlling input from fine roots only,with minimal consideration of mycorrhizal roots and mycelium.Secondly,there is a lack of comprehensive comparison of multiple interrelated variables such as soil organic carbon content,microbial respiration,microbial community composition,and microbial residual carbon content under different root input pathways,and most studies only focus on one or a few of these variables.Thirdly,most of the previous studies have focused on a single forest type,and there is a lack of comparison between different types of forests from the same origin.To address the above research deficiencies,in this study,we selected a typical subtropical evergreen broad-leaved Castanopsis carlesii natural secondary forest（SF）,and Castanopsis carlesii assisted natural regeneration forest（ANR）,Castanopsis carlesii plantation（YCP）and Cunninghamia lcunninghami plantation（YCF）that were harvested and reforested from secondary forests.Root-mycorrhiza exclusion treatments were nested in each forest including three treatments:control（preserved root+mycorrhiza,CT）,root exclusion（retained mycorrhiza,NR）,and mycorrhizal exclusion（No mycorrhiza,NM）.Soil physicochemical properties,microbial community composition,microbial residual carbon and soil respiration and their components were analyzed.This study focuses on answering the following scientific questions:（1）How does forest conversion input pathways affect the accumulation of soil microbial residual carbon?（2）How does forest conversion affect soil respiration and its components?The main results were following:1)Compared with SF,the contents of SOC,total nitrogen（TN）,dissolved organic carbon（DOC）and mineral nitrogen（MN）were significantly lower in ANR,YCP and YCF.The contents of soil SOC and DOC in ANR were significantly higher than those of YCP and YCF.Compared with CT,the NR and NM treatments in all forest types reduced soil SOC,TN and MN contents,with the NR treatment was significantly lower than the CT and NM treatments.2)The conversion of SF to ANR,YCP and YCF did not change soil fungalα-diversity,but significantly changed the composition of soil fungal community.Compared with the SF,there was no significant change in soil fungal community composition in the ANR and YCP,while there was a significant decrease in the relative abundance of Basidiomycota and an increase in the relative abundance of Ascomycota in YCF.The redundancy analysis showed that this was mainly driven by soil MN and SOC.The conversion of SF in ANR,YCP and YCF did not change soil bacterialα-diversity and bacterial community composition.The soil microbial community structure differed significantly between exclusion treatments under the same forest type.For example,NR and NM treatments reduced soil fungalα-diversity in all forests compared to the CT.Compared with the CT,NR treatments significantly altered the soil fungal community structure in SF,but had no significant effect on the other three forest types.Compared with the CT,NR and NM treatments of the four forests had no significant effect on soil bacterialα-diversity,and NR and NM treatments significantly changed the soil bacterial community structure in SF and ANR,and had no significant effect on YCP and YCF.3)Compared with the SF,there was no significant change in microbial residual carbon（MRC）in ANR.The MRC content was significant lower in YCP and YCF,with mainly decreased in fungal residual carbon（FRC）.The MRC was significantly greater in ANR than that in YCP and YCF.Compared with the SF,there was not significant effects on bacterial residual carbon（BRC）in the ANR,YCP and YCF,indicating that assisted natural regeneration afforestation was more beneficial to the accumulation of microbial-derived organic carbon than the plantation.The contribution of MRC to SOC（MRC/SOC）was significantly increased after the conversion of SF to ANR,YCP and YCF,indicating that the proportion of microbial-derived organic carbon was increased and the proportion of plant-derived organic carbon was decreased after forest conversion.The results of redundancy analysis showed that soil MN and DOC were significantly positively correlated with MRC,FRC and BRC and negatively correlated with soil MRC/SOC in CT of all forests,indicating that soil dissolved carbon and nitrogen were the main factors affecting soil MRC accumulation.Compared with the CT,both NR and NM treatments reduced MRC in all four forests,which reached significant levels in NR treatments,and the portion of MRC reduction was mainly from FRC.In addition,NR treatments were significantly higher in MRC than NM treatments,indicating that roots input are an important pathway affecting soil MRC.4)The conversion of SF to ANR,YCP and YCF significantly reduced the annual mean fluxes of total soil respiration（Rs）and mineral soil respiration(R_NM),and the results of redundancy analysis indicated that soil fungal biomass and N availability were the main factors explaining the changes in Rs,while soil fungal biomass and bacterial community structure were the main factors explaining the changes in R_NM.Compared with the SF,the average annual flux of Ra was significantly higher in the ANR,no significant change in the YCP,and significantly lower in the YCF.The results of the random forest machine learning analysis indicated that this was mainly attributed to the differences in root biomass,soil fungal community structure and MN in different forest types.Compared with NM treatment,the presence of mycorrhizal fungi significantly inhibited soil respiration(R_NR)in SF,ANR and YCP with NR treatment and significantly promoted R_NR in YCF,indicating that soil respiration may be influenced by mycorrhizal type.In summary,although forest conversion reduced MRC,the annual mean fluxes of Rs and R_NM,the assisted natural regeneration afforestation is more beneficial to nutrient maintenance,soil MRC accumulation,and soil organic carbon storage than artificial afforestation.In addition,root pathway is an important source of soil organic carbon.This study has elucidated the effects of forest conversion on soil MRC and Rs,and its key regulatory mechanisms,which can provide a reference for the formulation of forest management measures,help improve the accuracy of Earth system model predictions,and provide a theoretical basis and technical support for achieving the strategic goal of"increasing sinks and reducing emissions".