Tuesday, November 3, 2009
Convention Center, Exhibit Hall BC, Second Floor
Globally, mechanized farming contributes 13% of annual greenhouse gas emissions (CO2, CH4, and N2O). Tillage is frequently identified as the primary driver of these high CO2 emissions as it increases soil carbon (C) losses by altering the soil structure resulting in greater C losses as CO2. In light of developing C credit markets, which promote management practices based on their soil C storage potential, we must improve our understanding of management system (comprising tillage, fertilization and rotation practices) effects on soil C cycling. Our research addresses the uncertainty surrounding management system effects on soil C storage by measuring gaseous C losses from recent organic matter inputs to the soil across five management systems commonly used in the Upper Midwest. We used 13C- and 15N-labeled corn root residue to track gaseous emissions of corn root C and N from the soil as CO2, CH4, and N2O over two growing seasons. In the early growing season, root residue C accounts for 10-15% of total gaseous C losses, decreasing on average to 5-10% by the end of the growing season. Changes in the contribution of root derived C to total C losses are largely driven by changes in live root respiration, as the seasonal variation in root residue C losses is small (growing season range is 0.1-0.4 g root C/m2/d). We found that management systems (varying in tillage, fertilization and rotation practices) explain only a small proportion of the variability in both the proportion of total soil C losses derived from recent root residue and the cumulative losses of soil C. These results indicate that management systems are not the primary factor controlling the retention of recent soil C inputs and indicate the need for further investigation of controls on C dynamics in managed soils.