Modeling and regional assessment of soil carbon: A case study of the conservation reserve program

Regional assessments of soil C sequestration are needed to quantify potentials at state and national levels and to identify how C sequestration rates may vary regionally as a function of climate, soil, land use history, and other factors. This information can help guide policy and management decisions to maximize the environmental and societal benefits from C sequestration. We employ an approach combining an ecosystem simulation model and a set of geographically distributed databases that provide the driving variables for the model. In an application of this approach, we assessed C sequestration in Conservation Reserve Program (CRP) grasslands during the first 10-yr contract period, for a 16-state region in the central and northwestern USA, containing about 70% of the CRP acreage nationwide. Climate, soil and land use data were used to delineate 59 climate/land use zones each including multiple soil textural classes. The model results suggest that CRP lands are sequestering C and that regional differences in rates of C storage are controlled mainly by differences in primary production rates and secondarily by abiotic and soil factors influencing decomposition. Simulated rates of soil organic matter (SOM) C accumulation under CRP ranged from less than 10 to more than 40 g C m(-1) yr(-1), with the highest rates in the most humid regions. Total below-ground C accumulation rates (including roots) ranged from 25 to 135 g C m(-2) yr(-1). For the study region, predicted C increases for the first 10-yr period of the CRP were on the order of 25 Tg (10(12) g) for SOM C alone or up to 69 Tg if plant and litter C stocks are included. These levels of C sequestration could contribute significantly to the overall sequestration potential in the agricultural sector. However, C sequestered on CRP grasslands is vulnerable to loss with reversion to annual cropping and the effects of future management of previous and newly contracted CRP lands on C sequestration needs further evaluation.