Electron Acceptor Constraints on Soil Carbon Mineralization and Metal Cycling in Soils.

Oxidative carbon mineralization is well known to exceed anaerobic carbon mineralization in soils. Under anaerobic conditions, however, a combination of energetic and substrate availability control contributions to carbon mineralization by different alternative electron acceptors and affiliated microbial metabolisms. Microbial carbon mineralization corresponds to redox potential of the electron acceptor, where the reaction favorability generally decreases from oxygen to nitrate, Mn(IV), Fe(III), sulfate, and CO₂. The importance of spatial relationships and diffusion of alternative electron acceptors and dissolved organic carbon for soil carbon mineralization is largely unknown. We addressed this question by constructing flow-through reactors simulating a redox profile. Oxic water flows over an upland soil, allowing oxygen diffusion into the upper 0.5 cm of soil while anaerobic conditions prevail at depth. Two treatments included the addition of nitrate and sulfate into the groundwater influent. Reactors with added nitrate produced 1.2 and 1.4 times more dissolved inorganic carbon than the sulfate and control treatments, respectively. Added nitrate also increased dissolved organic carbon production 20% compared to the sulfate or control treatments. Anaerobic carbon mineralization increased with depth in all treatments, with negligible methane production and small nitrous oxide production within 1.5 cm of the soil surface. In agreement with the redox ladder, about the top 0.5 cm of soil were oxic for all treatments, beneath which signs of nitrate, Mn and Fe reduction were visible. The nitrate treatment significantly halted Fe reduction near the soil surface, but resulted in a great Fe(II) peak at depth. These results suggest that increased microbial activity from added electron acceptors fuels microbial metabolite production that promotes alternative electron accepting processes down-gradient. More importantly, anaerobic metabolisms contribute significantly to soil carbon mineralization and metal and nutrient cycling that may not be predicted based on the traditional redox ladder.