Effects Of Tree Mycorrhizal Types On Soil Organic Carbon And Microbial Characteristics In Subtropical Secondary Forests On The Western Slope Of Wuyi Mountain

Soil carbon pool is the largest organic carbon pools in the forest ecosystem,a slight change of soil carbon pool will have a profound impact on the global climate carbon cycle systems.Thus,the maintenance and management of forest soil carbon sink function have become one of the important ways to achieve the aim of the carbon neutrality in terrestrial ecosystems and mitigate global climate change.Arbuscular mycorrhiza（AM）and ectomycorrhiza（ECM）are the most ubiquitous fungal-plant symbionts.They play an important role in the regulation of soil carbon cycling in forest soils.Thus,tree mycorrhizal type is considered to be integrated indicator to explain and predict forest soil organic carbon dynamics.Although it has been widely recognized that tree mycorrhizal type plays an important role in regulating forest soil carbon cycle,there is still a lack of relevant experiments to confirm whether tree mycorrhizal type can effectively indicate and characterize the change pattern of soil organic carbon particularly in subtropical forests,which greatly limits the better understanding of soil carbon sequestration mediated by mycorrhizal in subtropical forests and its regulation mechanisms.Therefore,relied on the Matoushan Observation and Research Station of Forest Ecosystem of Jiangxi Province,we took the typical subtropical secondary forest on the western slope of Wuyishan Mountain as the research object,selected sampling sites Along the gradient of increasing ECM tree dominance（from 1%to 97%）,and compared the differences in tree species diversity,soil organic carbon pool,soil extractable organic carbon dynamics,soil organic carbon components and chemical molecular structure,soil organic carbon mineralization and enzyme activity,soil microbial residue carbon and soil microbial community composition in AM and ECM-dominated stands.The aims of this study were to systematically reveal the effects of tree mycorrhizal type on the distribution pattern,formation process and stabilization mechanisms of soil organic carbon in subtropical secondary forests and its regulatory factors.These results are help to accurately understand the long-term carbon sequestration ability in subtropical forests,and provide a scientific basis for the assessment and prediction of forestry carbon sinks in southern China.The main results are as follows:1)In this study,the AM-dominated plots were dominated mainly include tree species Taxodiaceae,Styracaceae,Poaceae,Hamamelidaceae,Elaeocarpaceae,Theaceae,Lauraceae and a few Fagaceae.The dominant tree species were Cunninghamia lanceolata,Alniphyllum fortunei,Phyllostachys heterocycla,Altingia gracilipes,Elaeocarpus chinensis,Schima superba,Litsea elongata var.faberi,Ternstroemia gymnanthera,Machilus grijsii,etc.The ECM-dominated plots mainly include tree species Fagaceae,Juglandaceae,Myrtaceae,Elaeocarpaceae,Lauraceae,Aquifoliaceae,and the dominant species were Castanopsis eyrei,C.carlesii,Lithocarpus glaber,Cyclobalanopsis gracilis,Engelhardtia fenzlii,Syzygium buxifolium,Elaeocarpus japonicus,L.elongata var.faberi,Ilex elmerrilliana,Altingia gracilipes,etc.In this subtropical forest,species richness,diversity and evenness index showed a trend of significant decrease with increasing ECM tree dominance.When the proportion of ECM tree species was close to 30%and that of AM species was close to 70%,the tree species diversity was the highest,and then the tree species diversity tended to decline with increasing ECM tree dominance.2)With increasing ECM tree dominance,the toal organic carbon（TOC）,total nitrogen（TN）and total phosphorus（TP）in the litter layer tended to increase,while C:N decreased.In each soil depth,the increase of ECM tree species dominance significantly increased SOC,TN,TP,available phosphorus（AP）,sand content and C:N ratio,but significantly decreased soil moisture,soil bulk density（BD）,p H,Mn content,clay content,silt content,δ¹³C value,C:P and N:P ratio.Therefore,the total SOC pools increased significantly with increasing ECM tree dominance.3)With the increase of ECM tree dominance,the soil extractable organic carbon（WEOC）and total nitrogen（WETN）concentrations increased significantly,while the soil extractable total phosphorus（WETP）concentrations did not change significantly.Furthermore,the WEOC:WETN ratio increased with increasing ECM tree dominance,but the WEOC aromaticity and molecular weight showed a decreased trend.WEOC concentration was positively correlated with SOC,TN,TP,AP and sand content,but negatively correlated with soil p H,clay and silt content.In addition,only in 10–30 cm depth,soil WEOC biodegradation 0–10decreased with increasing ECM tree dominance.Correlation analysis suggested that Soil WEOC biodegradation was negatively related to WEOC:WETP and WETN:WETP ratios,but was positively related to WEOC aromaticity and molecular weight.4)With the increase of ECM tree dominance,soil particulate organic carbon（POC）,and mineral-associated organic carbon（MAOC）fractions significantly increased,whileδ¹³C_POC andδ¹³C_MAOC showed a significant decrease.The relative abundance of Hydroxyketone,Aliphatic,and Aromatic of SOC and POC all increased with increasing ECM tree dominance,while the relative abundance of Polysaccharide gradually decreased.For the MAOC fractions,the relative abundance of Hydroxyketone and Aromatic increased with increasing ECM tree dominance,while the relative abundance of Aliphatic did not change significantly,and the relative abundance of Polysaccharide decreased gradually.Therefore,with increasing ECM tree dominance,SOC degration degree（aromatic:aliphatic）showed a decreasing trend,while SOC stability（aliphatic+aromatic:polysaccharide）showed an increasing trend.5)Based on a 361-day aerobic incubation experiment,the mean CO₂ emissions rate and cumulative of CO₂ emissions ECM-dominant plots were significantly higher than AM-dominant plots,but C mineralization was faster in AM-dominant plots in 0–10 cm soil depth.In addition,with the increase of ECM tree dominance,the enzyme activities ofβ-1,4-glucosidase（βG）,hemicellulase（CB）,β-1,4-N-acetyl-glucosaminidase（NAG）,leucine aminopeptidase（LAP）,ln（βG）:ln（AP）,and ln（NAG+LAP）:ln（AP）at each soil depth showed an increasing trend.However,the enzyme activities of acid phosphatase（AP）and ln（βG）:ln（NAG+LAP）did not change significantly.Correlation analysis showed that the effects of tree mycorrhizal type on SOC mineralization were mediated by C,N and P hydrolase activities and their ecological stoichiometric ratios.6)With the increase of ECM tree dominance,the muramic acid,glucosamine and soil total amino sugar increased significantly in the 0–10 cm soil depth,while only glucosamine and soil total amino sugar increased in the 10–30 cm soil depth.At each soil depth,bacterial necromass carbon（BNC）,fungal necromass carbon（FNC）,microbial necromass carbon（MNC）and F:B ratio increased with the increasing ECM tree dominance,but BNC:SOC,FNC:SOC and MNC:SOC decreased.Finally,in AM-dominant plots,the contribution of MNC to SOC was 35.6-34.5%,but in ECM-dominated plots,the contribution of MNC to SOC was 27-24.5%.This indicates that with increasing ECM tree dominance,the contribution of soil microbial necromass carbon to soil total organic carbon showed a decreased trend.7)Increasing ECM tree dominance significantly increased the 16S gene copy number of soil bacteria and ITS gene copy number of soil fungi,but decreased soil fungi diversity rather than soil bacteria diversity.Along the gradient of increasing ECM tree dominance,there was no significant change in soil bacterial community composition,which was dominated by Proteobacteria,Acidobacteria,Actinobacteria and Chloroflexi.However,with increasing ECM tree dominance,soil fungal community composition changed significantly,the relative abundance of Basidiomycota increased and the relative abundance of Ascomycota and Mortierellomycota decreased.At the OTU level,PCo A analysis showed that soil microbial community,especially fungal community composition was aggregated and significantly separated between AM and ECM-dominated stands,and RDA analysis showed that the effects of the tree mycorrhizal type on soil fungal community structure was closely related to soil organic carbon in this subtropical forest.Summarily,the increase of ECM tree dominance can alter the supply of limiting nutrients in mineral soil by influence of the composition of tree species and the litter quality,and control the soil organic carbon sequestration and stability by mediating the variation of soil microbial community composition in different mycorrhizal forests in the subtropical regions.Compared with AM-dominated forest,the ECM-dominant dominated forest has higher SOC content,more complex chemical molecular structure of SOC,lower C biodegradability and C mineralization rate,and thus more stable SOC.By contrary,the AM-dominant dominated forest showed faster soil carbon cycling and lower carbon sequestration.Moreover,microbial carbon sources have a nonnegligible role in the soil total organic carbon pool.These results may help to better understand the soil organic carbon cycle process,and then extend the applicability of Mycorrhizal-associated nutrient economy theory in subtropical region.This is of great theoretical and practical significance for accurately predicting and evaluating the function of soil carbon sink in subtropical forests.