ABSTRACT
Redox chemistry involving the quinone/phenol cycling of natural organic matter (NOM) is known to modulate microbial respiration. Complexation with metals or minerals can also affect NOM solubilization and stability. Inspired by these natural phenomena, a new soil amendment approach was suggested to effectively decrease methane emissions in flooded rice paddies. Structurally stable forms of NOM such as lignin and humic acids (HAs) were shown to decrease methane gas emissions in a vial experiment using different soil types and rice straw as a methanogenic substrate, and this inhibitory behavior was likely enhanced by ferric ion-NOM complexation. A mechanistic study using HAs revealed that complexation facilitated the slow release of the humic components. Interestingly, borohydride-based reduction, which transformed quinone moieties into phenols, caused the HAs to lose their inhibitory capacity, suggesting that the electron-accepting ability of HAs is vital for their inhibitory effect. In rice field tests, the humic-metal complexes were shown to successfully mitigate methane generation, while carbon dioxide emissions were relatively unchanged. Microbial community analysis of the rice fields by season revealed a decrease in specific cellulose-metabolizing and methanogenic genera associated with methane emissions. In contrast, the relative abundance of Thaumarchaeota and Actinomycetota, which are associated with NOM and recalcitrant organics, was higher in the presence of Fe-stabilized HAs. These microbial dynamics suggest that the slow release of humic components is effective in modulating the anoxic soil microbiome, possibly due to their electron-accepting ability. Given the simplicity, cost-effectiveness, and soil-friendly nature of complexation processes, Fe-stabilized NOM represents a promising approach for the mitigation of methane emissions from flooded rice paddies.