Iron-driven fast decomposition of soil carbon under periodic anoxia

Soil organic carbon (SOC) decomposition underpins soil-atmosphere carbon
exchange and is regulated by climate change-mediated variations in soil
redox conditions. Periodic 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, indicating fast SOC loss under relatively short anoxia.
Methane production was far lower than CO2 due to the presence of
alternative terminal electron acceptors (TEAs). Variation in alternative
TEAs and microbial community 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), hydrochloric acid-extractable
ferrous Fe, relative abundance of Fe-reducing prokaryotes, and SOC
decomposition rate suggest the release of readily metabolized substrates
following Fe reduction. This release provided 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 mostly similar to elevated
anoxic SOC decomposition in magnitude, and random forest modeling
indicates that soils rich in reducible Fe, SOC, and Fe-reducing
prokaryotes most likely experience elevated SOC decomposition under
periodic 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.