Iron-driven fast decomposition of soil carbon under anoxia

Soil organic carbon (SOC) decomposition underpins soil-atmosphere carbon exchange and is regulated by climate change-mediated variation in soil redox conditions. Soil anoxia, commonly occurring following precipitation, soil flooding and erosion events, is assumed to preserve SOC. Yet, water saturation may also increase SOC decomposition relative to unsaturated conditions, and contradictory findings among previous studies remain unexplained. Here, using incubation experiments on 20 soils collected across a 24° latitude gradient in China, we show that 70% of the soils showed higher or similar anoxic decomposition rate of SOC compared to oxic treatment after 2–3 weeks, suggesting fast SOC loss under anoxia. Variation in alternative terminal electron acceptors shows that fast anoxic decomposition was primarily driven by iron (Fe) reduction, which accounted for up to 90% of anoxic CO₂ production. Meanwhile, positive relationships among water-extractable organic carbon (OC), ferrous Fe, and SOC decomposition rate suggest release of readily metabolized substrates following Fe reduction, providing substrates for anoxic metabolism and potentially leading to the loss of OC protected by Fe (Fe-bound OC; a slow-cycling OC pool under oxic conditions). Mass balance calculation confirms that Fe-bound OC loss was similar to elevated anoxic SOC decomposition in magnitude, and random forest modeling indicates that soils rich in reducible Fe and SOC most likely experience elevated SOC decomposition under anoxia. Overall, our findings demonstrate that fast anoxic decomposition of SOC is a potentially important pathway that may stimulate SOC loss under climate change-mediated intense hydrologic regimes, particularly for soils rich in reducible Fe and SOC.