Relating Soil Carbon Mineralization Rates to Oxygen Availability - Insights from a Model System Approach.

The rate of soil organic matter (SOM) mineralization is known to be controlled by climatic factors as well as molecular structure, mineral-organic associations, and physical protection. A less frequently considered questions is how oxygen (O₂) limitations impact overall rates of microbial SOM mineralization (oxidation) in soils. Even within upland soils that are aerobic in bulk, factors limiting O₂ diffusion such as texture and soil moisture can result in an abundance of anaerobic microsites in the interior of soil aggregates. Variation in ensuing anaerobic respiration pathways can further impact SOM mineralization rates. Using a combination of (first) aggregate model systems and (second) manipulations of intact field samples, we show how limitations on diffusion and carbon bioavailability interact to impose anaerobic conditions and associated respiration constraints on SOM mineralization rates. In model soil aggregates, we examined how particle size (soil texture) and amount of dissolved organic carbon (bioavailable carbon) affect O₂ availability and distribution. Monitoring electron acceptor profiles (O₂, NO₃^-, Mn and Fe) and SOM transformations (dissolved, particulate, mineral-associated pools) across the redox gradients between aggregate exterior and interior, we then determined the distribution of operative microbial metabolisms and their cumulative impact on SOM mineralization rates. Our results show that anaerobic conditions decrease SOM mineralization rates overall, but also suggest that those are partially offset by the concurrent increases in SOM bioavailability due to transformations of protective mineral phases. In intact soil aggregates collected from soils varying in texture and organic matter content, we mapped the spatial distribution of anaerobic microsites. Optode imaging, microsensor profiling and 3D tomography revealed that soil texture regulates large-scale O₂ gradients between aggregate interior and exteriors, while particulate SOM and small biopores appears to control the fine-scale distribution of anaerobic microsites in the aggregate interior. Collectively, our results suggest that texture and particulate organic matter content are useful predictors for metabolic constraints imposed on SOM mineralization rates in anaerobic soil microenvironments.