The effect of warming on belowground carbon dynamics
Machmuller, Megan Beth
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This dissertation explores the effects of warming on soil respiration and soil organic matter decomposition in forest soils. I utilized a field warming experiment located in the Southeastern US Piedmont, the first of its kind on highly-weathered soils. Both field and laboratory approaches were utilized to assess the microbial (heterotrophic) response to increased temperature. My results suggest that warming results in no significant change in soil respiration (carbon dioxide efflux), soil carbon stocks, or soil organic matter chemistry, unlike previous studies. Hence, rising temperatures will not universally increase carbon dioxide efflux from forest soils. To explain the lack of a response, I performed a fully-factorial laboratory incubation to determine the importance of soil moisture in regulating heterotrophic respiration response to temperature but the nature of the interaction varies with season. I observed that moisture and temperature interact to regulate heterotrophic respiration. The temperature sensitivity of heterotrophic respiration does not change with moisture treatments but increases ~ 30% from winter to summer. This finding demonstrates that key ecosystem processes vary by season and modulate the response of heterotrophic respiration. By including moisture and temperature into models of heterotrophic respiration, we improved predictions of field respiration and explained 50-70% of the variability associated with in situ soil respiration. Finally, I investigated how in situ warming affected six different extracellular enzyme activities involved in soil organic matter decomposition. The microbial response to soil warming was minor relative to seasonal variation in microbial activity. We observed significant seasonal variation in enzyme activities, temperature sensitivities and microbial nutrient acquisition ratios, but the response was inconsistent across enzyme type. Additionally, we found low C:P enzyme ratios suggesting P limitation in this ecosystem. Our study suggests that a major effect of climate change on soil organic matter decomposition may be the result of changes in the seasonal patterns rather than directly altering the kinetics of enzymes in the soil through warming alone.