Substrate Carbon Controls Regional Divergence of Wetland Biofilm Carbon Sinks under Climate Warming

Microbial biofilms at soil-water interfaces mediate carbon exchange between soils and the atmosphere, yet whether substrate state alters their climate sensitivity remains poorly understood. Here, we analyzed 1080 rice-paddy sites across 26 regions of China to test whether soil organic carbon (SOC) regulates periphytic biofilm organic carbon (PB-TOC) responses to warming. SOC emerged as the dominant predictor of PB-TOC and showed a statistically supported threshold at ∼20 g kg-1. Below this threshold, PB-TOC increased with SOC and was positively associated with mean annual temperature and precipitation, consistent with a substrate-limited compensatory response. Above the threshold, PB-TOC approached saturation, and climate associations became negative, indicating greater vulnerability of carbon-rich interfaces. CMIP6-based projections suggest that under SSP5-8.5, China's paddy-interface biofilm carbon pool could decline by about 0.70 Tg C by 2100, driven mainly by 4.5-6.8% losses in high-SOC regions that are only partly offset by modest gains in low-SOC regions (0.8-1.5%). These findings indicate that wetland climate responses are state-dependent and that substrate thresholds should be incorporated into carbon-climate assessments.