Effects of Soil Moisture on the Temperature Sensitivity of Soil Heterotrophic Respiration: A Laboratory Incubation Study

The temperature sensitivity (Q(10)) of soil heterotrophic respiration (Rh) is an important ecological model parameter and may vary with temperature and moisture. While Q(10) generally decreases with increasing temperature, the moisture effects on Q(10) have been controversial. To address this, we conducted a 90-day laboratory incubation experiment using a subtropical forest soil with a full factorial combination of five moisture levels (20%, 40%, 60%, 80%, and 100% water holding capacity - WHC) and five temperature levels (10, 17, 24, 31, and 38 degrees C). Under each moisture treatment, R-h was measured several times for each temperature treatment to derive Q(10) based on the exponential relationships between R-h and temperature. Microbial biomass carbon (MBC), microbial community structure and soil nutrients were also measured several times to detect their potential contributions to the moisture-induced Q(10) variation. We found that Q(10) was significantly lower at lower moisture levels (60%, 40% and 20% WHC) than at higher moisture level (80% WHC) during the early stage of the incubation, but became significantly higher at 20% WHC than at 60% WHC and not significantly different from the other three moisture levels during the late stage of incubation. In contrast, soil R-h had the highest value at 60% WHC and the lowest at 20% WHC throughout the whole incubation period. Variations of Q(10) were significantly associated with MBC during the early stages of incubation, but with the fungi-to-bacteria ratio during the later stages, suggesting that changes in microbial biomass and community structure are related to the moisture-induced Q(10) changes. This study implies that global warming's impacts on soil CO2 emission may depend upon soil moisture conditions. With the same temperature rise, wetter soils may emit more CO2 into the atmosphere via heterotrophic respiration.