Soil Moisture Sensitivity of Microbial Processing of Soil Organic Carbon

Soil organic carbon (SOC) is the largest terrestrial C pool and understanding SOC decomposition in response to environmental factors is critical for accurate predictions of climatic change. Soil moisture is one of the most important, yet less explored, environmental factors controlling soil microbial respiration. The relationship between soil moisture and respiration also varies with soil texture. Currently, it is difficult to predict feedbacks to climatic changes from changes in soil moisture, as most earth system models lack site-specific, experimentally-derived parameters to represent soil moisture-texture-respiration relationships. The overarching goals of this dissertation are to gain a fundamental understanding of the interactive effect of soil moisture and texture on microbial processing of SOC, and to elucidate microbial response to global changes. To accomplish this, two laboratory experiments and one field study were conducted. Laboratory experiments included (i) 90-day incubation with steady-state moisture, and (ii) 140-day incubation with alternating wetting and drying (transient) and steady state moisture using three soils of different textures (sandy, loamy, and clayey). A field experiment was conducted to understand the effect of moisture to SOC dynamics under in-situ conditions and involved drought, rainfed, and irrigated moisture treatments. The 90-day experiment revealed that texture was the major determinant of SOC cycling despite observing different moisture optima for the highest respiration for different textures. The 140-day incubation aimed to decipher mechanism(s) fueling the Birch effect (a spike in respiration rate upon rewetting of dry soils). I found that different mechanisms caused the Birch effect in different soils. In sandy soil, metabolite accumulation and changes in bacterial community, while in loamy and clayey soils, metabolite accumulation and release of aggregate protected C contributed to the Birch effect. Results imply that response of microbial respiration to changing climate will strongly depend on soil moisture sensitivity to textural differences. Field experiment results showed strong influence of moisture on SOC decomposition, i.e., lower CO₂ emissions from drought plots, however, higher extractable organic C, microbial biomass C and fungal hyphal abundance were observed under drought conditions. The laboratory and field experiment provide critical information for improving the moisture sensitivity of microbial parameters in ecosystem models.