Contribution of soil microbial necromass to SOC stocks during vegetation recovery in a subtropical karst ecosystem

Carbon sequestration is a key soil function, and an increase in soil organic carbon (SOC) is an indicator of ecosystem recovery because it underpins other ecosystem services by acting as a substrate for the soil microbial community. The soil microbial community constitutes the active pool of SOC, and its necromass (microbial residue carbon, MRC) contributes strongly to the stable SOC pool. Therefore, we propose that the potential for restoration of degraded karst ecosystems lies in the abundance of soil microbial community and the persistence of its necromass, and may be measured by changes in its contribution to the active and stable SOC pools during recovery. We investigated changes in SOC stocks using an established space-for-time chronosequence along a perturbation gradient in the subtropical karst ecosystem: sloping cropland < abandoned cropland < shrubland < secondary forest < primary forest. Microbial biomarkers were extracted from soil profiles from surface to bedrock and used to measure the contributions of the soil microbial community composition (using phospholipid fatty acids, PLFAs) and MRC (using amino sugars) to SOC stocks at each recovery stage. The results showed that the SOC stocks ranged from 10.53 to 31.77 kg m^-2 and increased with recovery stage, with total MRC accounting for 17-28% of SOC. Increasing PLFAs and MRC abundances were positively correlated with improved soil structure (decreased bulk density) and organic carbon, nitrogen and phosphorus nutrient. Bacterial MRC contributes more to SOC stocks than fungal residue carbon during vegetation recovery. The PLFA analysis indicated that Gram positive bacteria were the largest microbial group and were relatively more abundant in deeper soils, and biomarkers for saprophytic and ectomycorrhizal fungi were more abundant in soils under woody vegetation. In conclusion, this study suggests that the soil microbial community in karst soils have the potential to adapt to changing soil conditions and contribute substantially to building SOC stocks after abandonment of agriculture in degraded karst landscapes.