Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/3/2006
Publication Date: 1/10/2007
Citation: Baker, J.M., Ochsner, T.E., Venterea, R.T., Griffis, T.J. 2007. Tillage and carbon sequestration: what do we really know? Agriculture, Ecosystems and Environment. 118:1-5. Interpretive Summary: There is much interest in finding ways that agricultural producers can help mitigate rising atmospheric CO2 levels by storing C in soil. Encouraging reports have suggested that significant C storage can result simply by changing from conventional plowing to conservation tillage. However, before conclusions are drawn or policies are enacted it is important to critically assess the available data. We have examined this issue and have found that in nearly all cases where conservation tillage was found to store C, soils were only sampled to a depth of 30 cm or less, despite the fact that crop roots often extend much deeper. In the relatively fewer studies where sampling extended to a depth greater than 30 cm, conservation tillage has actually resulted in a loss of C relative to conventional tillage in the majority of reported cases. This may be caused by differences in soil physical properties that cause shallower rooting in reduced tillage than in conventionally plowed fields. For example, reduced tillage usually results in cooler soils, and soils with higher bulk density and higher penetration resistance. While there are good reasons to promote reduced tillage, particularly to reduce erosion, it does not appear to be a consistently effective means for storing C. We conclude that other management practices may ultimately be more fruitful avenues for agricultural C storage. These findings have direct implications for scientists and resource managers involved in developing strategies for optimizing agriculture's contribution to the greenhouse gas budget.
Technical Abstract: It is widely accepted that substantial amounts of carbon can be sequestered in agricultural soils by changing tillage practices from conventional plowing to less intensive methods, loosely known as conservation tillage. This view is based on experiments in which relative carbon changes have been estimated through soil sampling of long-term tillage trials. However, an examination of these experiments shows that sampling protocol may have biased the results. In nearly all cases where conservation tillage was found to sequester C, soils were only sampled to a depth of 30 cm or less, despite the fact that crop roots often extend much deeper. In the relatively fewer studies where sampling extended to a depth greater than 30 cm, conservation tillage has actually resulted in a loss of C relative to conventional tillage in the majority of reported cases. These contrasting results may be due to well known tillage-induced differences in soil thermal and physical properties that affect root distribution patterns, causing shallower rooting in reduced tillage situations. Recently reported continuous eddy covariance measurements of net ecosystem exchange of CO2 in different tillage systems have also been unable to detect any C gain due to reduced tillage. Though there are other good reasons, such as erosion prevention, to use conservation tillage practices, the evidence that it promotes C sequestration is not compelling.