Microbial life-history strategies mediate differential effects of straw and biochar amendments on soil POC/MAOC dynamics and SOC sequestration

Enhancing soil organic carbon (SOC) sequestration in paddy soils is a critical strategy for climate change mitigation. However, the mechanistic underpinnings of how substrate quality modulates microbial life-history strategies to regulate the formation and stabilization of distinct SOC fractions—particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)—remain poorly understood. We conducted a 65-day incubation experiment using 13C-labeled rice straw and straw-derived biochar to disentangle the relationships among energy inputs, microbial strategies, and SOC stabilization pathways. Both straw and biochar amendments increased SOC content, with biochar inducing a 103% increase compared to only 38.7% from straw. Straw improved nutrient availability (e.g., dissolved organic carbon and microbial biomass carbon) and stimulated the activities of β-glucosidase, β-1,4-N-acetylglucosaminidase, leucine aminopeptidase, and acid phosphatase, thereby enriching r-strategist microbes (e.g., Mortierellomycota and Firmicutes). This promoted fungal-mediated POC formation and MAOC accumulation derived from bacterial necromass. However, straw induced a positive priming effect, accelerating the mineralization of native SOC and resulting in a carbon sequestration efficiency of only 22.8% by day 65. In contrast, biochar alleviated microbial nitrogen demand, redirected microbial activity toward the decomposition of recalcitrant carbon, and enriched K-strategist microbes (Actinobacteriota and Chloroflexi). These shifts further facilitated MAOC accumulation via bacterial necromass formation, while inducing a negative priming effect that minimized native carbon loss, achieving a carbon sequestration efficiency of 99.7% at the end of the incubation. Our findings reveal that straw and biochar enhance SOC sequestration through distinct microbial pathways: straw drives rapid but less efficient carbon accumulation via r-strategist microbial activity, whereas biochar promotes stable and highly efficient sequestration through K-strategist-mediated processes. These results highlight the importance of substrate quality in shaping microbial community dynamics and SOC sequestration outcomes, providing a mechanistic basis for optimizing organic amendment strategies in paddy agroecosystems.