Periphytic Biofilms Modulate Priming Effect of Soil Organic Matter Mineralization in Paddy Soil

The priming effect (PE) of soil organic matter (SOM) mineralization plays a crucial role in regulating soil carbon (C) dynamics. However, in flooded ecosystems like rice paddies, where the water‐soil interface is a critical hotspot for carbon turnover, the influence of periphytic biofilms (PB) on SOM mineralization and PE remains poorly understood. Here, we employed 13 C‐labeled PB and glucose to investigate PB‐mediated effects on SOM mineralization and PE in paddy soils. PB amendments significantly increased dissolved organic carbon (DOC), redox potential (Eh), and the microbial biomass carbon‐to‐nitrogen ratio (MBC:MBN), while reducing MBN and pH. These shifts stimulated CO 2 emissions but suppressed CH 4 emissions. In the early stage (day 7), CO 2 PE was negative and inversely correlated with PB‐derived CO 2 emissions, suggesting preferential utilization of labile PB‐C. As labile C was depleted, CO 2 PE shifted to positive values under higher biomass inputs, with negative correlations to total nitrogen, implicating nitrogen mining. In contrast, CH 4 PE remained consistently negative, indicating sustained suppression of SOM‐derived methanogenesis when PB served as an alternative substrate. Over 60 days, cumulative PE increased linearly with PB biomass ( R 2 = 0.98, p < 0.05). In glucose‐amended soils, PB presence also lowered MBN and pH, reduced glucose‐induced PE, and enhanced net glucose‐C retention. These findings reveal PB's regulatory role by directly influencing SOM mineralization through stoichiometric constraints and indirectly modulating PE by restructuring soil biogeochemistry, offering mechanistic insights into C sequestration and greenhouse gas mitigation in paddy ecosystems.