The Unforeseen Anaerobic Contribution to Soil Carbon Cycling.

Soils contain arguably the largest dynamic stock (changing on decadal time scales) of carbon on the planet, having approximate twice the amount stored in the surface oceans and three-times that in the atmosphere.  Soil organic matter (SOM) storage, or residence time, is dominantly controlled by the mineralization (oxidation) rate, which is affected by climatic factors (particularly temperature and rainfall) influencing microbial metabolic rates as well as SOM chemistry, mineral-organic associations, and physical protection. Variation in anaerobic respiratory pathways can further, and dramatically, impact carbon oxidation rates.  The complex physical structure of soils and sediments results in a distribution of redox environments even within seemingly aerobic systems; in fact, the majority of the soil volume may persist in an anaerobic state within an upland, agriculturally productive system. Factors limiting oxygen diffusion and availability such as soil texture, soil moisture content, organic matter input, and aggregate size (soil structure) provide central controls on microbial carbon mineralization rates.  Here, we combine laboratory studies with manipulations of field samples and in-field measurements to illustrate how soil structure and carbon availability interact to impose anaerobic conditions and associated respiratory constraints on organic matter mineralization rates and thus storage within soils.