Experimental warming effects on soil organic matter dynamics at the temperate - boreal forest ecotone

Laboratory and field experiments have both shown that warming will increase soil CO2 flux to the atmosphere, but do not agree on the importance of this feedback as a future climate driver. As such, we lack the process-based understanding necessary to predict how warming will change soil carbon stocks in the future. Here we measured warming effects on soil organic matter (SOM) decomposition in a southern boreal forest warming experiment (B4WarmED) at two sites in northern Minnesota, USA. We used two laboratory incubations and measurements of soil extracellular enzymes to examine mechanisms that are predicted to alter the warming response of soil respiration, including soil microbial community thermal adjustment; differential temperature response of fast and slow cycling carbon pools to warming; and the potential for upland soil drying to reduce the response of respiration to temperature. We measured soil respiration in the first 5-yr of in situ warming at B4WarmED. Total soil respiration increased in the first three years of experimental warming (by 8 and 21%, for the +1.7 and +3.4°C treatments, respectively, relative to ambient temperature treatments), but warming responses decreased substantially in the fourth and fifth years of warming. In contrast, warming effects on root-excluded bulk soil respiration were relatively constant during the five years of treatment: the +1.7°C treatment showed little response to warming, whereas warming of +3.4°C increased bulk soil respiration by 13%. Warming treatments both decreased the long-term temperature response (i.e. Q10 parameter) and increased the soil moisture response for total soil respiration, but not for bulk soil respiration. Yet, in situ soils, even with warming decreases in soil moisture, were rarely dry enough to substantially alter the response of soil respiration to warming treatment. We used two laboratory incubations to test whether soil drying and soil microbial thermal adjustment reduced the temperature response of decomposition in B4WarmED soils. SOM decomposition was less responsive to temperature in dry soils, and in soils incubated at higher temperatures, suggesting that drying and thermal adjustment could reduce the warming effects on decomposition, although the effects of thermal adjustment were small. Finally, in both incubations, we found that the decomposition of slow cycling soil carbon responded more to warming than rapidly cycling carbon.Soil extracellular enzymes are important catalysts of SOM decomposition, as they break down organic matter polymers and increase the carbon substrates available to the soil microbial community. We examined warming effects on four soil extracellular enzymes at the B4WarmEd sites. In contrast to our predictions, we found little evidence of microbial thermal adjustment of enzyme kinetics to warming that would reduce the warming response of SOM decomposition. In summary, in both field and laboratory measurements SOM decomposition increased with experimental warming. Soil drying was found to decrease SOM decomposition and the response of SOM decomposition to increasing temperature, although warming-enhanced soil drying was not found to substantially decrease in situ soil respiration. Finally, in laboratory incubations, slowly cycling SOM was more temperature sensitive than fast cycling SOM. Together, these results suggest that warming will increase SOM decomposition, and that loss of large pools of slowly cycling SOM, in particular, could contribute significant carbon the atmosphere over the next century. Soil drying could, however, moderate the response of soil respiration to warming, especially if increases in evapotranspiration or changes in precipitation results in drier soils than were observed in the field component of this study.