Long-term rice–crayfish coculture increases plant lignin but not microbial necromass contribution to soil organic carbon

Rice–crayfish coculture (RC) has emerged as a transformative agricultural practice in China, significantly influencing soil microorganisms and enhancing soil organic carbon (SOC) accumulation. However, the contribution of plant residues and microbial necromass to the increased SOC within RC systems remains uncertain. This study aimed to investigate phospholipid fatty acids (PLFAs), microbial necromass C (MNC, with amino sugars as biomarkers), plant-derived C (VSC, with lignin phenols as biomarkers) levels, along with soil properties across conventional rice monoculture (RM) and RC systems of 5-, 10-, and 15-yr durations (RC5, RC10, and RC15, respectively). The results showed that long-term RC fields exhibited stronger aggregation, higher soil nutrient levels, organically complexed Fe oxides (Fep), and lower bulk density and oxidation–reduction potential than those with RM. The SOC levels were significantly higher in RC10 and RC15 than in RM, by 31.8 % and 37.2 %, respectively. Moreover, RC significantly reduced the levels of bacterial (25.3–35.4 %) and fungal (19.5–34.7 %) PLFAs compared with RM, with RC10 exhibiting the lowest levels. With RC duration increasing to 10–15 years, MNC and VSC were respectively higher by 12.4–25.3 % and 48.8–72.4 % than those in RM. Specifically, fungal necromass C, as well as vanillyl- and syringyl-type phenols, showed the most pronounced enhancements. Concurrently, the contribution of VSC to SOC (12.4–25.3 %) significantly increased in the 10 −15-yr RC period compared with RM, whereas MNC decreased proportionally (17.5–18.5 %). SOC and Fep were the primary factors regulating the contribution of MNC to SOC, whereas the contribution of VSC to SOC was mainly influenced by soil aggregation. Thus, long-term RC improved soil C sequestration primarily by increasing the contribution of plant-derived C rather than that of microbe-derived C. However, the findings of this study indicated that long-term RC might limit microbial biomass, thereby raising concerns about the long-term sustainability of microbial communities in these systems.