SEPARATING THE ROLES OF TEMPERATURE AND MOISTURE IN CONTROLLING RATES OF DECOMPOSITION OF SOIL ORGANIC MATTER

Soil organic matter (SOM) is an essential reservoir of carbon derived from the biosphere that delays its return to the atmosphere. Hence, full understanding of the balance between the carbon sink of SOM storage and the atmospheric source from SOM decomposition is essential to modeling future atmospheric composition. 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 the impact of temperature change and/or been conducted in the field. We compared the effects of both temperature and moisture in a strictly controlled laboratory setting. We established soil pots with a 10-cm deep mix of commercial topsoils with an organic carbon content of 11.5%, but completely lacking plants, visible fungi or any macrofauna. We created multiple temperature levels, ambient and elevated, using heated mats. After allowing the soils to dry completely, the moisture levels were increased through weekly additions of either 400 mL or 800 mL water. The soil-CO₂ flux was measured with a CO_2-flux chamber system. Measurements of pots initially on an hourly to daily basis demonstrated a strong CO₂ pulse effect that declined within the first 36 to 48 hours. Steady-state measurements were conducted one week after the application of water to capture equilibrium conditions. We investigated the response across varying temperatures and through a wide range of moisture levels, from very low (dry) saturation to a very high saturation. We found no correlation between soil-CO₂ flux and temperature when examined across all moisture levels. In examining flux vs. temperature at all moisture levels (0-57% water filled pore space, or WFPS), the strongest correlation is seen at 20-30% WFPS (r²=0.2). In examining flux vs. moisture across all temperatures (17 °C to 44 °C), we find a reasonably strong correlation (r²=0.4). The correlation of flux vs. moisture is equally strong at <30 °C and >30 °C (r²=0.44 for both). In conclusion, estimating the effects of climate change on SOM decomposition must account for both decreasing CO₂ emissions from soils undergoing aridification, as well as increases from soils in regions experiencing higher precipitation.