||Soil plays a critical role in global carbon (C) cycling, having one of the largest dynamic stocks of C on earth—3300 Pg of C are stored in soils, which is three-times the amount stored in the atmosphere. An important control on soil organic matter (SOM) quantities is the rate of carbon utilization by microorganisms (SOM mineralization). The rate and extent of SOM mineralization is affected by climatic factors influencing microbial metabolic rates in combination with SOM chemistry, mineral-organic matter stabilization, and physical protection. What remains elusive is to what extent constraints on microbial metabolism induced by the respiratory pathway, and specifically the electron acceptor in respiration, control overall rates of carbon mineralization in soils. The complex physical structure of soils and sediments result limited oxygen ingress, resulting in anaerobic environments even within seemingly aerobic systems. The overarching goal of this study is to determine if variations in microbial metabolic rates induced by anaerobic microsites in soils are a major control on SOM mineralization rates and thus carbon storage. A combination of laboratory experiments and field investigations will be performed to fulfill our research goal. Model, laboratory studies will be performed to examine fundamental factors of respiratory constraints (i.e., electron acceptor) on organic matter mineralization rates. We will ground our laboratory studies with both manipulation of field samples and in-field measurements. Moreover, we will use reactive transport modeling to integrate our micro-scale measurements to deduce the field-scale (macroscopic) observable redox induced metabolic controls on carbon mineralization. A major outcome of our research will be the ability to quantitatively place the importance of cm-scale variation in microbial metabolisms on the rate of carbon mineralization in soils. Further, we will provide the ability to upscale our results into ecosystem models that can couple with global carbon models using well mapped or remotely sensed inputs such as soil texture, soil moisture, and plant ecosystem.