Allen, L. H. (USDA-ARS & Agronomy Department, University of Florida, P.O. Box –110965, Gainesville, Florida, 32611-0965; Phone: 352-392-8194; Fax: 352-392-6139; Email: LHAJR@mail.ifas.ufl.edu)

Soil Organic Carbon and Nitrogen Accumulation of Rhizoma Perennial Peanut and Bahiagrass Grown under Elevated CO₂ and Temperature

L.H. Allen*, S.L. Albrecht, K.J. Boote, J.M.G. Thomas, K. Skirvin, S.D. Nelson

Carbon sequestration in soils might mitigate the increase of CO₂ in the atmosphere. Rhizoma perennial peanut (RPP) and bahiagrass (BG) were grown at Gainesville, FL in field soil plots at four temperature zones (baseline-ambient, +1.5, +3.0, and +4.5 Celsius) in temperature-gradient greenhouses at 360 and 700 ppm CO₂ from 1995 to 2000. In addition to measurements of growth and herbage yield, soil samples from the top 20 cm of each plot were collected in February 1995 before establishment, and before new growth began each year thereafter, including December 2000 when the study ended. February 1995 and December 2000 samples were analyzed for soil organic carbon (SOC) and nitrogen (SON) for each species, CO₂ concentration, and temperature. First, across all treatments of the six-year period, SOC increased 26% and SON increased 34%. Mean SOC increase was 1.396 and 0.746 g/kg in BG and RPP, respectively, indicating that BG sequestered more SOC than RPP. Mean SOC gain of BG at 700 and 360 ppm CO₂ was 1.450 and 1.343 g/kg, respectively, indicating that elevated CO₂ caused only a slight increase in SOC. However, mean increase of RPP at 700 and 360 ppm CO₂ was 0.949 g/kg and 0.544 g/kg, respectively, indicating that elevated CO₂ caused a significant increase in SOC in RPP plots. No response of SOC to temperature was found in BG, but SOC decreased with temperature in RPP plots. Growth of both forages caused an increase in SOC, but responses to CO₂ and temperature were species dependent. Generally, SON responses were similar to SOC, but SON was much less affected by CO₂ concentration. From these data we calculated a SOC accumulation of 540 kg/ha per year from the 6 years of data. Removing the CO₂ effect gave 425 kg/ha per year of SOC. In 1998, from a side by side comparison of soils under continuous tillage (winter small grain crops or fallow) and under 15-years of continuous unharvested RPP at Gainesville, FL, Allen and Nelson (unpublished), found 370 kg/ha per year accumulation of SOC for RPP. This is consistent with RPP accumulating less SOC than BG. Comparing data, W.A. Albrecht (1938) reported an accumulation of 380 kg/ha per year across a 14-year red clover study in Missouri. K.N. Potter et al. (1999) reported an accumulation of 450 kg/ha per year for a 60-year study of degraded Texas blackland prairie soils returned to grasslands. These SOC accumulation rates are within the range of values found by A.J. Franzluebbers et al. for Southeastern USA data that were presented in several reports at the 2004 meetings of the American Society of Agronomy, October 31-November 4, Seattle, WA.