Mulching increases soil organic carbon via plant inputs and its microbial transformation

Mulching with plant residues is one of the most effective means to increase soil organic C (SOC) in croplands. The mechanisms through which microorganisms transform plant-derived C into SOC under mulching conditions remain unclear. Soil properties (contents of SOC, dissolved organic matter (DOM), and microbial necromass C (MNC)) without mulching were compared with those of soils after 12 years of mulching with perennial ryegrass (Ryegr) and crop straw (Straw). Mulching with Straw increased SOC content by 83% and MNC by 129% compared with the Control, due to the high input of organic matter. The content of stable DOM compounds increased, indicating that straw mulching decreased the availability of organic matter for microorganisms. Straw mulching increased the abundance of copiotrophic bacteria and Ascomycota compared with the Control. Over 12 years, Ryegr mulching increased SOC by 14% and MNC by 29% compared with the Control. The bioavailable DOM compounds under Ryegr mulching increased compared with the Control, but the bacterial community composition remained unchanged. As microorganisms preferentially decomposed and utilized C derived from straw residues, Straw mulching increased SOC content by raising biologically refractory DOM compounds and formation of stable microbial necromass. Ryegr mulching stimulated microbial in vivo turnover by increasing bioavailable DOM compounds, which in turn increased microbial necromass C and SOC accumulation. This was reflected by an increase in microbial functions related to the metabolism of carbohydrates and amino acids. © 2025 Elsevier B.V., All rights reserved.