Microorganism-mediated Divergence of Particulate Organic Matter and Mineral-Associated Organic Matter in Karst Soils along a Vegetation Restoration Chronosequence

Soil organic matter (SOM) can be physically divided into particulate organic matter (POM) and mineral-associated organic matter (MAOM) that would be affected by microbial communities. Moreover, vegetation restoration affects belowground SOM, soil microbial communities and microbial extracellular enzymes. Understanding their inter-relationships is essential to decipher soil organic carbon sequestration and capacity. Therefore, microbial communities, six typical carbon-degrading enzyme activities and physicochemical properties of POM and MAOM with near-neutral pH along vegetation restoration chronosequences were investigated in subtropical karst area of southwest China. It was found that POM- and MAOM-carbon showed an increasing trend along the overall vegetation restoration though their organic carbon contents achieved the highest mean values of 78.10 and 65.62 g·kg^-1 respectively at secondary forest stage. Moreover, MAOM-carbon pool was unsaturated and POM-carbon pool had the huge potential for carbon storage, which both had the significant correlations with exchangeable Mg (P<0.05). The microbial community structures, assemblies and functions of POM and MAOM along vegetation restoration differed. Among the most frequent microorganisms, only Pyrinomonadaceae, Anaerolineaceae and Nectriaceae were simultaneously recognized as the dominant/keystone taxa. Moreover, keystone fungi had the significantly direct effects on POM- and MAOM-carbon. Consequently, these microorganisms play the decisive roles in POM and MAOM formation and stabilization. Besides that, the geometric mean of enzyme activity of POM was higher than that of MAOM, as well as aromatic compound degradation and chitinolysis were predominant in MAOM. Compared to traditional opinions about Ca-mediated SOM stabilization, the role of Ca²⁺ in SOM persistence can be replaced by Mg²⁺, when the protection of Ca²⁺ is weak at near-neutral environment. Then, SOM stabilization depends on the protective carbon by avoiding microbe attack rather than inherently stable carbon, and the organic carbon results of POM and MAOM may underestimate the true carbon storage potential at karst area. Despite that this study is the primary work to decipher the relationships among soil microbes, carbon-degrading enzyme activities and physicochemical properties of POM and MAOM, but our finding provides powerful information that can bring some perspectives to better understand the Microbial Efficiency-Matrix Stabilization framework along karst vegetation restoration chronosequences.