Molecular Composition of Exogenous Dissolved Organic Matter Regulates Dissimilatory Iron Reduction and Carbon Emissions in Paddy Soil

Soil carbon (C) cycling under anoxic conditions is mechanistically linked to dissimilatory iron (Fe) reduction, potentially influenced by exogenous dissolved organic matter (DOM). However, the impact of complex exogenous DOM on soil microbial activity and C–Fe coupling in paddy soils remains underexplored. With a 100-day microcosm experiment, we found that biochar-DOM significantly promoted Fe reduction and accelerated CH₄ and CO₂ emissions, and manure-DOM increased soil CO₂ emissions. These effects may be caused by the following mechanisms: DOM molecules with high aromaticity and high double bond equivalence (DBE), including lignins-polyphenols, lignins-polycyclic aromatics, and condensed aromatics-polycyclic aromatics, promoted soil Fe reduction and CH₄ emissions with enrichment of soil Fe-reducing bacteria, r-strategists, and reduction of methanotrophs at the early stage of incubation. Conversely, DOM with low aromaticity, low DBE, and high H/C enhanced CO₂ emissions with the enhancement of recalcitrant C degradation and CH₄ oxidation at the late stage of incubation. In conclusion, our study highlights the importance of the molecular composition of organic amendment-derived DOM in regulating soil Fe reduction and greenhouse gas emissions. The findings offer novel insights into the effective utilization of agricultural resources and the potential mitigation of greenhouse gas production and emissions.