Print PageA third of the terrestrial biosphere is made up of agricultural ecosystems, making understanding their ecology crucial to our understanding of the global carbon cycle. One key, poorly constrained gas exchange parameter is the terrestrial biosphere’s oxidative ratio (OR). OR is a property of gas exchange fluxes between the atmosphere and the biosphere and is defined as the ratio of O2 released to CO2 fixed during net primary production. Constraining the OR of the terrestrial biosphere is critical to estimating the sizes of the terrestrial biosphere and ocean carbon sinks in anthropogenic carbon dioxide apportionment calculations (Keeling et al. 1996). Current methods to estimate OR are prone to error (Seibt et al. 2004), which can propagate into larger errors in the estimates of the sizes of the carbon sinks (Randerson et al. 2006). We have developed more accurate and precise methods (using elemental analysis and 13C nuclear magnetic resonance spectroscopy) to estimate OR from biomass chemistry (Masiello et al. 2008; Hockaday et al. 2009). To improve our estimate of terrestrial biosphere OR, OR measurements need to be made across a range of ecosystems.
Results/Conclusions
We show that agricultural ecosystem OR varies significantly with crop type, from 1.030±0.002 and 1.035±0.001 for corn and wheat crops, respectively, to 1.112±0.003 for soybean. The grass crops have significantly lower OR values compared to the Intergovernmental Panel on Climate Change (IPCC)-used OR value, used in apportionment calculations, of 1.10 ± 0.05. In addition, our method of estimating OR from biomass, rather than from gas exchange studies, has improved the error surrounding OR measurements by an order of magnitude. We also present a preliminary estimate of the average U.S. agricultural OR for 2003-2005 of 1.06, and estimate how this may have differed in the past. We found that U.S. agricultural OR has increased from 1.04 in the 1930s to 1.06 today, and this shift is primarily controlled by changes in cropland composition, with the major driver being the increase in soybean acreage beginning in the 1960s.
