Soil organic matter (SOM) comprises an array of carbon compounds, some of which are abundant (e.g., cellulose), whereas others exist in low-concentration (e.g., starch). Due to resource allocation constraints, labile low-concentration SOM substrates (like starch) can degrade at significantly lower rates than abundant substrates, thereby increasing the residence time of low-concentration SOM constituents. These observations were made under ambient temperature and moisture conditions in boreal forest soil. However, climate change is predicted to cause warming and drying in many boreal forest habitats, and drying can constrain microbial decomposition. We hypothesized that drying would exacerbate the “low-concentration constraint” on decomposition. To test this, we used closed-top greenhouses that warmed the soil by 0.8°C, but significantly reduced soil moisture content (3% in greenhouses vs 30% in control plots). To test the low-concentration constraint, we constructed soil cores that contained combusted (SOM-free) soil from the field site, to which we added two organic substrates: 13C-labeled starch at levels of 0%, 0.01%, 0.1%, 0.5%, 1%, 5%, and 10% of total SOM, and the difference was composed of unlabeled cellulose. The soil cores were incubated in greenhouse and control plots (n=5 of each) in interior Alaska for the entire four-month growing season in 2010.
The drier conditions in the greenhouses caused a significant decrease in soil respiration rates (53.2 ± 14.8 mg C m-2 h-1) in comparison to the control plots (153.7 ± 45.4 mg C m-2 h-1). Despite the significantly drier conditions, the greenhouse-incubated soil cores displayed significantly higher (2-10X higher) activities of starch- and cellulose-degrading enzymes than in the control cores, potentially reflecting a temperature effect on enzyme activities. However, in congruence with the decreased respiration rates, the greenhouse-incubated cores lost significantly less organic matter (6.0 ± 1.7%) than the control-incubated cores (13.7 ± 2.4%) over the course of the growing season. Clearly, higher rates of overall decomposition occurred in the control cores, which contained more moisture. Stable isotopic analyses [in progress] will allow us to specifically test whether drying enhances the low-concentration constraint on decomposition. In conclusion, soil moisture plays a crucial role in decomposition by allowing diffusion of enzymes, substrates and degradation products within the soil matrix. When a system becomes limited by water potential, this may further favor the decomposition of those substrates that are high-enough in concentration to be encountered by microbial decomposers, and allow for sufficient diffusion of degradation products.