MODELING CLIMATE, CO2 AND MANAGEMENT IMPACTS ON SOIL CARBON IN SEMIARID AGROECOSYSTEMS

In agroecosystems, there is likely to be a strong interaction between global change and management that will determine whether soil will be a source or sink for atmospheric C. We conducted a simulation study of changes in soil C as a function of climate and CO2 change, for a suit e of different management systems, at four locations representing a cl imate sequence in the central Great Plains of the US. Climate, CO2 and management interactions were analyzed for three agroecosystems: a con ventional winter wheat-summer fallow rotation, a wheat-corn-fallow rot ation and continuous cropping with wheat. Model analyses included soil C responses to changes in the amount and distribution of precipitatio n and responses to changes in temperature, precipitation and CO2 as pr ojected by a general circulation model for a 2xCO(2) scenario. Overall , differences between management systems at all the sites were greater than those induced by perturbations of climate and/or CO2. Crop resid ue production was increased by CO2 enrichment and by a changed climate . Where the frequency of summer fallowing was reduced (wheat-corn-fall ow) or eliminated (continuous wheat), soil C increased under all condi tions, particularly with increased (640 mu L L-1) CO2. For wheat-fallo w management, the model predicted declines in soil C under both ambien t conditions and with climate change alone. Increased CO2 with wheat-f allow management yielded small gains in soil C at three of the sites a nd reduced losses at the fourth site. Our results illustrate the impor tance of considering the role of management in determining potential r esponses of agroecosystems to global change. Changes in climate will d etermine changes in management as farmers strive to maximize profitabi lity. Therefore, changes in soil C may be a complex function of climat e driving management and management driving soil C levels and not be a simple direct effect of either climate or management.