Accumulation of soil microbial extracellular and cellular residues during forest rewilding: Implications for soil carbon stabilization in older plantations

Soil microbial residues are a critical component of stable soil organic carbon pools in terrestrial ecosystems. Plantations are anticipated to increase soil stable carbon stocks and contribute toward addressing global climate change. How rewilding, as a strategy to restore the multiple ecological functions of plantations, affects soil microbial residues remains to be elucidated. Here, we divided microbial residues into extracellular residues (using extracellular polymeric substances, i.e., EPS, as a proxy) and cellular residues (estimated by amino sugars) and investigated the changes in their accumulation over four decades (6-45 years) of Metasequoia glyptostroboides plantation development. It was found that forest rewilding resulted in a linear accumulation of soil microbial residues and enhanced their contributions to the soil organic carbon. Specifically, EPS-polysaccharide increased by 126.5%, while the total cellular residue carbon, fungal cellular residue carbon, and bacterial cellular residue carbon increased by 73.4%, 77.2%, and 54.3%, respectively. Correspondingly, the contributions of EPS-polysaccharide and total cellular residue carbon to the soil organic carbon increased by 66.1% and 32.1%, respectively. The main drivers behind extracellular and cellular residues differed, with fine root biomass being the main driver of extracellular residues, while soil nitrogen and organic carbon content were the main drivers of cellular residues. Our study demonstrated that through rewilding, microbial extracellular and cellular residues could continuously accumulate in soils and contribute significantly to the sequestration of atmospheric carbon, highlighting the need to incorporate both microbial residues into soil carbon models to address global climate change. The results suggested that plantations should be allowed to grow older rather than being prematurely harvested, as they promote soil carbon sequestration and stabilization, which has important implications for the establishment of stable soil carbon pools in plantations and can be an invaluable asset in the fight against climate change.