EXPERIMENTAL STUDY OF SOIL CO2-FLUX DEPENDENCE ON TEMPERATURE AND MOISTURE: PRELIMINARY RESULTS

Organic matter stored in soil (SOM) is an essential reservoir of carbon derived from the biosphere that forms a buffer in the organic carbon cycle by delaying its return to the atmosphere. Hence, full understanding of the balance between primary productivity and SOM decomposition as climate changes is essential to modeling future atmospheric composition (pCO₂ levels). Multiple studies have examined rates of SOM decomposition and the consequences of climate change on these rates. Most studies to date, however, have focused on temperature change and/or been conducted in the field. Therefore, we examined the effects of both temperature and soil moisture in a strictly controlled laboratory setting by creating pots with a 10-cm deep uniform mix of commercial topsoils lacking plants, visible fungi or any macrofauna, and an organic C content of 11.5%. We created multiple temperature levels (ca. 17 to 40 ^oC), including ambient and multiple elevated temperatures using heated mats. After allowing the soils to dry completely, the moisture levels were increased through weekly addition of 400mL or 800mL water. The soil CO₂ flux of the pots was measured biweekly with a soil CO₂ chamber system, with each pot measured in triplicate one full week following the last watering. Examination of preliminary results after six months (ca. 1000 measurements) demonstrates the combined dependence of soil CO₂ flux on both temperature and soil moisture. Soil CO₂ flux rates are negligible at very low moisture levels regardless of temperature. For pots receiving weekly additions of water, with a soil saturation range of ~6% to 18%, soil CO₂ flux correlates positively, but weakly with both temperature and moisture. For the regression of CO₂ flux against moisture, r² = 0.04; for flux against temperature, r² = 0.12. The correlations are stronger if the low flux rates at very low (<2%) soil moisture are included. For the regression of CO₂ flux against temperature, r² = 0.1, but for flux against moisture, r² = 0.29. These results suggest the presence of a moisture threshold that activates microbial decomposition. Thus, estimating the effects of climate change must account for decreasing CO₂ emissions in soils undergoing aridification, in addition to increased emissions in regions where soil moisture and temperature increases.