Author
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VARVEL, GARY |
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Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 12/15/2005 Publication Date: 2/2/2006 Citation: Varvel, G.E. 2006. Soil organic carbon changes in diversified rotations of the western corn belt. Soil Science Society of America Journal 70:426-433. Interpretive Summary: Storage of carbon by agricultural soils has been cited as one solution to soil degradation and global climate change. However, carbon sequestration in soils is a very slow and dynamic process. This necessitates the need to evaluate the effects of management practices such as crop rotation and N fertilizer management on changes in soil organic carbon levels in long-term studies. One such study including 7 cropping systems (three monoculture, two 2-yr, and two 4-yr rotations) with three levels of N fertilizer where these determinations can be made has been conducted for the past 18 years. In this study, soil samples were taken in the spring in 1984, 1992, 1998 and 2002 to a depth of 30-cm in 0 to 7.5, 7.5 to 15, and 15 to 30-cm increments. These samples were analyzed for soil organic carbon, essentially total carbon, since all soil ph values were below 7 and no carbonates were present. No differences were obtained in soil organic carbon at the initiation of the study in 1984, but rotation significantly affected soil organic carbon levels in the 0 to 7.5-cm (surface) depth at all of the other sampling times and at lower depths in 1998 and 2002. Nitrogen fertilizer level also significantly affected soil organic carbon levels in the surface samples in 1998 and 2002. More importantly, after 18 years, soil organic carbon changes in the surface 30-cm indicated significant losses were occurring in all monoculture and 2-yr cropping systems, but not in the 4-yr cropping systems. These 4-yr systems with an oat+clover crop once every four years may be contributing a much greater amount of belowground biomass than any of the monocultures or 2-yr cropping systems, which may significantly increase or at a minimum, reduce losses of soil organic carbon. The results also indicated that when tillage depth was increased, soil organic carbon levels decreased in several of the cropping systems from that point on. Sometimes, apparent small changes in management can have significant long-term results, but long-term experiments are required where these changes can be detected and measured under current climate conditions. Technical Abstract: Sequestration and storage of carbon (C) by agricultural soils has been cited as one potential part of the solution to soil degradation and global climate change. However, C sequestration in soils is an extremely slow and dynamic process. As a result, the objective was to evaluate the effects of crop rotation and N fertilizer management on changes in soil organic C (SOC) levels throughout eighteen years in a long-term study in the Western Corn Belt. Seven cropping systems (three monoculture, two 2-yr, and two 4-yr rotations) with three levels of N fertilizer were compared. Soil samples were taken in the spring in 1984, 1992, 1998 and 2002 to a depth of 30-cm in 0 to 7.5, 7.5 to 15, and 15 to 30-cm increments. No differences were obtained in 1984, but rotation significantly affected SOC levels in the 0 to 7.5-cm depth in 1992, 1998, and 2002 and SOC levels in the 7.5 to 15 and 15 to 30-cm depths in 1998 and 2002. Nitrogen fertilizer level also significantly affected SOC levels in the 0 to 7.5-cm depth in 1998 and 2002. More importantly, SOC changes in the surface 30-cm indicated significant losses were occurring in all monoculture and 2-yr cropping systems, but not in the 4-yr cropping systems after 18 years. The results also indicated that when tillage depth was increased after 8-yrs by simply using a tandem disk with larger diameter disks that it resulted in a loss of SOC in several of the cropping systems from that point on. Sometimes, apparent small changes in management can have significant long-term results, but long-term experiments are required so these changes can be measured under current climate conditions. |
