Variation in the Oxidative Ratio of US Agricultural Ecosystems from 1930 to 2010

Gallagher, M.E., C.A. Masiello, W.C. Hockaday, J.A. Baldock, S.S. Snapp, and C.P. McSwiney

Presented at the All Scientist Meeting (2011-04-15 to 2011-04-15 )

Accurately and precisely constraining ecosystem oxidative ratio (OR) is necessary to assess the sizes of the terrestrial biosphere and ocean carbon sinks, as well as the rate at which they are taking up anthropogenic carbon dioxide (CO2). The Intergovernmental Panel on Climate Change (IPPC)-used OR value used in apportionment calculations is 1.10 ± 0.05. Techniques currently used to estimate OR values are prone to error, which can lead to large errors in estimations of the sizes of carbon sinks: for example, an error of ±0.05 OR units is equivalent to an error of 0.5 Pg C/yr or 50 Pg C over 100 years (Randerson et al., 2006). We have developed several methods for estimating OR from biomass chemistry that are more accurate and precise. To better constrain biospheric OR, measurements need to be made across a range of ecosystems, and here we begin with 3 agricultural ecosystems. Here we analyze the top three crops in the United States (soybean, corn, and wheat) and show significant differences between the OR of legumes versus grass crops. Our preliminary estimate of the 2003-2005 average U.S. agriculture OR value of 1.06, an increase from 1.04 in the 1930s. This shift was driven by changes in cropland composition, with the major driver being the increase in soybean acreage between 1963 and the present.

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