Organic fertilizer substitution increased soil organic carbon through the association of microbial necromass C with iron oxides

Organic fertilizer was widely used to enhance the buildup of soil organic carbon (SOC) and microbial necromass C. Iron and aluminum (Fe/Al) oxides serve as critical factors influencing SOC by controlling microbial necromass C. Nevertheless, the alterations and dynamics of microbial necromass C alongside Fe/Al oxides in the presence of organic fertilizer remain poorly elucidated. To evaluate the effect of organic fertilizer substitution for chemical fertilizer on Fe/Al oxides and its relationship to microbial necromass C, a site experiment was initiated in 2010 including three treatments: chemical fertilizer (CF), 50 %CF+ 50 % organic fertilizer (50 % OF), and 100 % organic fertilizer (100 %OF). The data were collected after 4, 8, and 13 years of experiments in 2014, 2018, and 2023, respectively. The results showed that organic fertilizer substitution decreased C loss from microbial mineralization and increased microbial necromass C, and thus contributed to SOC accumulation. With experiment duration, SOC content did not increase from 2018 to 2023 under 100 %OF may be due to C saturation, while microbial necromass still had an increasing trend. In 2023, bacterial and fungal necromass C was increased by 157.4 % and 178.5 % under 50 %OF, and by 230.7 % and 337.8 % under 100 %OF compared with CF, respectively. This suggests that prolonged use of organic fertilizer can enhance the stable SOC. Organic fertilizer increased microbial necromass C mainly through promoting the formation of Fe/Al oxides, and Fe oxides had a more important effect than Al oxides. Overall, we concluded that organic fertilizer substitution increased stable SOC sequestration through the association of microbial necromass C with iron oxides.