Atwood, Jay (USDA-NRCS-RIAD, 808 East Blackland Road, Temple, TX, 76502; Phone: 254-770-6632; Fax: 254-770-6561; Email: jatwood@brc.tamus.edu)

Effect of the Conservation Reserve Program (CRP) on Soil Carbon

J.D. Atwood*, S.R. Potter, J.R. Williams, M.L. Norfleet

The CRP effect on the rate of cropland soil carbon (C) storage was estimated. The analysis included the 89.6 percent of the 32.7 million acres classified as CRP in the 1997 National Resource Inventory (NRI) for which successful model simulations could be completed. Two scenarios were simulated: 1) a CRP scenario reflecting management according to the 1997 NRI and contracted CRP practices for the first 12 signup periods; and 2) a Crop scenario that assumed the crop mix reported in the NRI preceding CRP enrollment would have continued, but with tillage, conservation practice, and nutrient management at 1997 levels observed on non-CRP land. Individual NRI CRP sample points were grouped into 2,570 clusters according to climate, soil, and landscape characteristics. For each cluster, multiple crop simulations were defined for the diverse mix of prior crops and tillage systems of the NRI points in the cluster and up to four CRP simulations were defined for native grass, introduced grass, tree, and wildlife habitat cover types. A total of 20,514 simulations were required for the Crop and CRP scenarios. The simulations were 30 years in length and were replicated for five sets of randomly generated daily weather to remove year-to-year variation in model output due to weather patterns rather than due to program effects. The annual model output was divided into three periods for analysis: 1) years 1 to 10 reflecting the duration of most CRP contracts; 2) years 11 to 20 reflecting benefits of CRP re-enrollment as contracts expire; and 3) years 21 to 30 reflecting benefits for longer term CRP easements. The CRP C benefit (C sequestration rate) was calculated as the rate of CRP soil C storage minus the rate of continued cropping soil C storage. On average, the CRP C benefit was 0.90, 0.49, and 0.36 tons per acre per year in years 1 to 10, years 11 to 20, and years 21 to 30, respectively, for total annual C storage increases of 26.2, 14.3, and 10.6 million tons in the three periods for the 29.3 million acres simulated. In years 1 to 10, the CRP C benefit ranged from a low of 0.7 tons per acre per year in the Northeast to a high of 1.1 tons per acre per year in the Upper Midwest. Within the regions, the standard deviation of the CRP C benefit, where the sample was the set of averages calculated by soil and climate class, varied from 0.56 tons per acre per year in the Southern Great Plains to 1.17 tons per acre per year in the Upper Midwest. With continued cropping, 45 percent of the land would lose five percent or more C in years 1 to 10; with conversion to CRP only three percent would lose more than five percent C. With continued cropping in years 21 to 30, the area losing more than five percent of soil C increased to more than 48 percent. With continued cropping approximately 20 percent of the area gained more than five percent carbon in years 1 to 10; whereas with CRP, approximately 85 percent of the land gained more than 5 percent carbon in the first 10 years. The percent of the CRP land gaining more than five percent carbon was about equal at the national level in the second and third periods, near 20 percent, implying a gradual convergence towards new, stable, higher soil C levels sometime after the second or third period. The estimated CRP C storage benefit varied according to tillage method used for the continued cropping. In years 1 to 10, the CRP C storage benefit relative to conventional, mulch, and notill cropping systems for non-forage crops was estimated to be 1.16, 1.02, and 0.50 tons per acre per year, respectively. In years 21 to 30 the CRP C storage benefit was nearly equal across all tillage types.