Soil microbial biomass carbon as an indicator of ecological restoration success along a chronosequence

Carbon inputs of the free light fraction (FLL) are supposedly fostered by ecological restoration, modulate microbial metabolic efficiency and biomass accumulation, which is consistent with strengthening the microbial pathway of necromass formation, a central soil carbon stabilization mechanism. We evaluated how the progress of ecological succession modulates the labile carbon inputs of the FLL and the metabolic efficiency of the microbial community in areas undergoing forest restoration. The study was conducted in 10 (PL10) and 20 (PL20) year-old native tree restoration plantations, in addition to a mature forest (FM) and a grassland (PA) as reference sites, located in lowland areas in the Brazilian Atlantic Forest. Our findings revealed that the microbial biomass (MBC) increased as succession progressed, reaching the maximum value in the 20-year area, while the grassland and the 10-year planting showed the lowest accumulations. In contrast, basal respiration (RBS) and metabolic quotient (qCO2) were significantly higher in PA and PL10, indicating greater carbon loss and lower metabolic efficiency in these systems. Although the levels of light free fraction (FLL) were statistically equal among all areas, this fraction showed a strong and positive correlation with RBS (r= 0.99), confirming itself as the main energy substrate for the microbiota. In the isotopic compartment, all areas maintained a sign of δ¹³C characteristic of C3 plants. In pasture, this pattern evidences the occurrence of negative priming, in which the microbial community preferentially uses the fresh input of grasses and preserves the old carbon stock, limiting the renewal of soil organic matter.