Reforestation accelerates soil organic carbon accumulation: Evidence from microbial biomarkers

Soils store more carbon (C) belowground than plants and the atmosphere combined, providing a critical ecosystem service. While previous research has shown that sustainable forest management practices can increase soil C storage by enhancing plant productivity, the role of soil microbes remains elusive. We analyzed changes in plant litter, soil C, and microbial parameters across a reforestation chronosequence-with average stand ages of similar to 20, 80, 120, 200 and >= 300 years-to evaluate how microbial communities mediate soil C transformation and sequestration. We observed generally consistent increases in microbial biomass (lipid biomarkers), microbial necromass (amino sugar biomarkers), and soil organic C with forest age, highlighting microbial regulation of soil C accumulation. Specifically, increases in microbial biomass preceded gains in soil C, suggesting microbial lipids are an early and sensitive indicator of ecosystem restoration. We also observed a rapid increase in microbial necromass relative to bulk soil C in forests restored for 80-200 years, likely due to accelerated microbial turnover rates. These patterns suggest high plant productivity (low litter C: N ratios) during the early and middle stages of reforestation facilitates efficient microbial growth and necromass accrual in SOC stocks. As forests age, the contribution of microbial necromass to the SOC pool declines toward background levels. Our results suggest reforestation offers a positive feedback solution that mitigates climate change by efficiently sequestering soil C belowground.