Coarse Root Enhancement Increases Soil Organic Carbon While Decreasing Its Stability in a Wetland

Wetland soil organic carbon (SOC) sequestration effectively reduces atmospheric CO2. Fractionating SOC into labile particulate organic carbon (POC) and stable mineral-associated organic carbon (MAOC) is crucial for predicting its stability. Precipitation-induced variations in coarse (>2 mm) and fine (<2 mm) roots are key regulators of SOC stability, but their specific contributions remain unclear. Here, we conducted a 10-year field simulation of precipitation changes (−60%, −40%, 0%, + 40%, + 60%) in a wetland. We found that increased precipitation enhanced Gramineae abundance, thereby increasing coarse-root biomass, while fine roots changed little. Compared to CK, coarse roots directly boosted topsoil POC (+37.4%) and enhanced MAOC (+31.5%) via microbial stimulation. Furthermore, increased coarse-root biomass was associated with greater vertical transport of DOC and nitrate (NO3–), potentially contributing to higher subsoil POC (+218.7%) and MAOC (+17.2%). With increasing precipitation, the MAOC/POC ratio decreased from 1.65 to 1.11 in the topsoil and from 8.73 to 2.66 in the subsoil, suggesting that coarse roots favored POC over MAOC, increasing SOC but reducing its stability. These results provide mechanistic insights into wetland carbon-climate feedbacks under altered precipitation regimes.