Saturday, 15 July 2006

Changes in Total Soil Organic Carbon Due to Crop Rotation and Bio-Cover under No-Tillage Production.

Jason P. Wight, Fred Allen, and Donald D. Tyler. Univ of Tennessee, Dept of Plant Sciences, 2431 Joe Johnson Dr., Knoxville, TN 37996-4561

Farmland under no-tillage production can sequester atmospheric carbon. However, the rate of carbon storage in any given acre is uncertain because climate, soil texture, fertilization, crop rotation, and bio-cover can all affect carbon cycling. The objective of this research is to compare temporal changes in total Soil Organic Carbon (SOC) among different cropping systems in two regions of Tennessee under no-tillage production. The experiment used a split-block treatment design with four replications at each location. The whole-block treatment was differing cropping sequences of Roundup ReadyŽ corn (Zea mays), soybeans (Glycine max), and cotton (Gossypium hirsutum) combined with a split-block treatment of bio-covers using winter wheat (Triticum aestivum), hairy vetch (Vicia villosa), poultry litter, and winter weeds. Treatment combinations were applied to fields at the University of Tennessee's Research and Education Center at Milan (RECM), and the Middle Tennessee Research and Education Center (MTREC). The two sites are located in different physiographic regions of Tennessee. RECM is situated on the Gulf Coastal Plain while MTREC is in the Central Basin. Soil samples were taken before cropping sequence and bio-cover treatments began and after treatments had been applied. SOC was measured at two depths, the surface 0-5 cm and subsurface 5-15 cm. Changes in SOC were calculated after three years under the bio-cover and two years under cropping sequence treatments. Overall, both locations showed small but consistent loss in carbon over all treatments during the two years. Losses were largest in the 0-5 cm surface layer, and at the RECM site. Mean SOC loss at RECM surface layer was 1.47 Mg ha -1 while that of MTREC was 1.28 Mg ha-1. The subsurface layer showed a similar trend, with mean SOC loss at RECM being 0.65 Mg ha-1 and that at MTREC being 0.55 Mg ha-1. Although there were no significant differences between the crop sequence treatments, trends were evident. Continuous cotton and corn sequences had trends of losing more SOC at the surface than did continuous soybean treatments at both locations. The pattern was reversed at the 5-15 cm depth, with continuous soybeans tending to lose more SOC than either continuous corn or cotton. However, experimental variation obscured treatment significance. A significant effect was seen in the top 5 cm layer at the RECM site due to bio-cover. Poultry litter and wheat lost significantly less carbon than hairy vetch and winter weeds (P<0.001). Mean SOC loss was 0.9 Mg ha-1 and 1.27 Mg ha-1 for poultry litter and wheat, respectively, while that of winter weeds and hairy vetch were 1.87 and 1.88 Mg ha-1, respectively. Similar trends were seen in plots under the bio-cover treatments in the subsurface at both the RECM and MTREC sites, but were not statistically significant. Three years is a relatively short time frame in which to examine changes in total organic carbon, which may account for the lack of statistical significance among the cropping sequence treatments and interactions. The experiment is ongoing so that additional changes may be observed.

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