| Due to its huge amount,a small fluctuation in forest soil organic carbon（SOC）pool may have a significant impact on atmospheric carbon dioxide concentration,thereby affecting global climate.Therefore,promoting forest soil carbon sequestration is considered as an important measure based on natural climate solutions.SOC is mainly composed of plant residues and microbial residues,and organic components from different sources have differential effects on SOC maintenance or stability due to differences in their protective mechanisms.For example,except that a small portion exists as soluble small molecule compounds,most of plant residues exist as particulate organic matter and are not protected,making them prone to microbial decomposition and utilization.Since most microbial residues are small molecular weight organic compounds,they are easily combined with soil minerals to form organic mineral complexes and are protected,so that can be maintained in soil for a long time.Therefore,studying plant residue and microbial residue carbon and their influencing factors is of great significance for deepening the understanding of the regulatory mechanism of forest SOC dynamics.In order to explore the microbial mechanisms underlying the stimulatory effects of tree diversity on SOC accumulation,45 plots covering a tree diversity gradient,with Shannon index of plant species diversity ranging between 0.15 and 3.57,were established in a typical subtropical karst forest in northwest Guangxi,reflecting the.Firstly,phospholipid fatty acid method（PLFAs）was used to study the characteristics of soil microbial community changes with plant diversity.On this basis,lignin phenols and amino sugars were used as biomarkers for plant and microbial residues,respectively,to study the changes in soil plant and microbial residual carbon with plant diversity.Based on multiple soil biotic and abiotic factors,the mechanisms underlying the effects of tree diversity on microbial community,plant residues and microbial residues were explored,and the microbial mechanisms of tree diversity promoting SOC accumulation in the subtropical karst forest was further revealed.The main research results are as follows:（1）Based on the study of PLFAs as the microbial marker living microbial abundance and composition,The total contents of PLFAs ranged from 86.4 to 325.5nmol g-1,with an average of 183.2±66.1 nmol g-1.Tree diversity significantly increased microbial biomass and decreased microbial diversity.Hierarchical segmentation analysis showed that the soil matrix（including inorganic and organic nutrients）was the main determinants of microbial biomass.The results of structural equation model showed that the effects of tree diversity on soil microbial community biomass and diversity mainly depended on soil matrix and soil water content.In addition,soil microorganisms may metabolize carbon components that are relatively difficult to degrade under low tree diversity conditions,while soil microorganisms tend to utilize unstable carbon sources under high tree diversity conditions.Soil microbial community biomass and diversity varied significantly in different tree diversity gradients,which highlighted the key role of soil matrix availability in controlling microbial biomass.（2）Based on the study of lignin phenol as a marker of plant residues,The absolute contents of lignin phenols ranged from1.18 to 6.62 g kg-1soil with an average of 2.48±1.13 g kg-1soil across the 45 plots.Tree diversity significantly enhanced soil lignin accumulation via three mechanisms.First,Tree diversity benefited lignin accumulation by stimulating plant detritus inputs.Second,tree diversity directly and indirectly increased reactive minerals,so that enhanced mineral protection of lignin.Third,decrease in microbial C limitation due to increased soil C availability resulted in lowered peroxidase activity and subsequently lignin degradation,which in turn benefited lignin accumulation.Our study provides mechanisms underlying SOC accumulation in response to increased tree diversity,which may be integrated into Earth system models in order to better predict SOC dynamics under tree diversity alteration.（3）Based on the study using amino sugars as markers of microbial residues,the total carbon content of microbial residues ranged from 4.28 to 85.7 g kg-1,with an average of 36.3±17.6 g kg-1.The contribution of fungal residue carbon to SOC was greater than that of bacterial residue carbon.The increase of tree diversity significantly increased the SOC content of microbial residue,but did not affect the contribution of microbial residue carbon to soc.The increase of tree diversity increased soil total nitrogen（TN）,dissolved organic nitrogen（DON）and dissolved organic carbon（DOC）contents and p H values,and thus increased microbial carbon use efficiency（CUE）and microbial biomass,but there was no significant correlation between CUE and microbial residue accumulation.At the same time,tree diversity increased soil exchangeable calcium levels,which enhanced the protective effect of minerals on microbial carbon residues.This study reveals for the first time the microbial mechanism by which tree diversity promotes SOC accumulation in karst forests,i.e.,reducing lignin oxidation decomposition and promoting microbial residue carbon accumulation.This study revealed,for the first time,the microbial mechanisms underlying the positive effects of increasing tree diversity on the accumulation of SOC in a subtropical karst forest,namely,reducing the decomposition of lignin and promoting the accumulation of microbial residue C.Our research shows that in order to promote SOC sequestration following vegetation restoration in the karst region,it is necessary to increase the level of species diversity of afforestation.The results of this study have improved the understanding of SOC cycling of karst forests,provided a scientific basis for the assessment of SOC sequestration,and provided scientific and technological support for the in-depth implementation of vegetation restoration projects. |
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