Numerous studies with wide-ranging results have been conducted to resolve if Great Plains soils are a C source or sink. The authors addressed the source/sink question by examining the results from producer soil samples and production surveys that were analyzed and archived by the South Dakota Soil Testing Laboratory. Results showed that between 1985 and 2010, soil organic C content increased at a rate of 326 lb C/A/year, for a total increase of 24%. The increase was attributed to planting better adapted varieties and using better management practices that on average increased corn grain yields 2.29 bu/A/year. Higher soil organic C has impacts on water quality, soil productivity, and plant nutrition. For example, if we assume that the C:N ratio of organic matter is 10:1, then these findings would indicate that soils during this 25-year period were a sink for both C and N, and could have influenced the N needed to optimize crop yields.
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Clay, David E.; Carlson, Gregg C.; Clay, Sharon A.; Stone, James; Reitsma, Kurtis; and Gelderman, Ronald, "Great Plains Soils May be C Sinks" (2012). Agronomy, Horticulture and Plant Science Faculty Publications. 33.