As earth's climate continues to warm, it is vital to understand how the capacity of terrestrial ecosystems to retain carbon will be affected. At a decades-long soil warming experiment in central Massachusetts, soil respiration has shown a triphasic pattern, increasing with the onset of warming, decreasing after a decade only to increase again a number of years later. This secondary increase in soil respiration occurred despite evidence for depleted soil organic matter carbon pools, and concurrent with changes in the structure of microbial communities. Here we set out to assess the hypothesis that warming has increased microbial access to remaining soil organic matter via increased extracellular enzyme activity and/or reduced physical protection. Microbial biomass, soil respiration, and potential activities of enzymes for litter decomposition were measured six times throughout the growing season, while changes in the isotopic ratio and physical structure of soil organic matter pools were assessed in mid summer. Both the forest floor (organic horizon) and the upper mineral horizon were sampled.
Soil carbon stocks were smaller and relatively depleted of biopolymers characteristic of plant litter in the mineral horizon, while total microbial biomass was lower in the organic horizon. Potential extracellular enzyme activity was generally unaffected by warming treatment, although activity per unit microbial biomass at in-situ temperatures was higher. We found a depletion of carbon in the mineral-bound organic matter fraction with warming, although there was no change in the amount of carbon physically protected from decomposition within microaggregates. These results indicate that chronic warming has depleted the soil of organic matter, selecting for a smaller, more active community. Future work on the thermodynamics of decomposition of remaining soil carbon will provide further insight into the long-term stability of soil organic matter pools in temperate forest soils of the Northeastern United States.