Iron reduction promotes carbon mineralization and nutrient release of iron-associated organic matter in anoxic environments

The association of dissolved organic matter (DOM) with minerals is critical for the protection and stabilization of organic matter in peatlands, yet the stabilization of mineral-associated DOM and its role in nutrient supply remain poorly understood. Here we conducted incubation experiments to investigate the stability of synthesized ferrihydrite-associated DOM (Fh-DOM) under oxic and anoxic conditions. The DOM from Fh-DOM and inoculated microbial consortia were extracted from fen and bog peats that have different redox conditions. Our results showed that carbon (C) mineralization of Fh-DOM under anoxic conditions was approximately 5-fold higher than that under oxic conditions, because of iron reduction under anoxic conditions. Surprisingly, the bog incubations exhibited a higher proportion of C mineralization from Fh-DOM (14.1 ± 0.1 %) than that of the fen incubations (4.1 ± 0.3 %), despite having less active iron reduction in the bog incubations. This difference was attributed to a higher proportion of protein-like fluorophores (17.7 %) in Fh-DOM of the bog incubations, which act as an effective nutrient to promote bacterial growth. In addition, the enhancement of potential C metabolic functions in the bog incubations under anoxic conditions was significantly higher than that in the fen incubations. The effect of iron reduction on C mineralization mostly comes indirectly through microbial respiration of disassociated Fh-DOM (over 70 %), rather than directly from redox reaction itself. Furthermore, Fh-DOM addition under anoxic conditions enhanced bacterial community stability in the both fen and bog incubations by increasing the number and abundance of key species in the microbial networks, which are primarily linked to nutrient-mining functions. Our study highlights the prevalent but overlooked role of mineral-associated DOM as a nutrient source for microbes under anoxic conditions, providing new insights into stabilization and destabilization mechanisms of organic matter in peatlands and other anoxic environments.