Gallagher, M. E., C. A. Masiello, W. C. Hockaday, J. A. Baldock, S. Snapp, and C. P. McSwiney. 2014. Controls on the oxidative ratio of net primary production in agricultural ecosystems. Biogeochemistry 121:581-594.
Ecosystem-, biome-, and global-level oxidative ratio (OR) estimates are used to understand ecosystem gas exchange processes, and assess the sizes of the terrestrial biosphere and ocean carbon sinks, and the mechanisms controlling them. We have developed analytical methods to measure the OR of terrestrial carbon stocks from biomass chemistry that give accurate and precise estimates of ecosystem atmosphere-biosphere gas flux OR. Here we apply these techniques to agricultural ecosystems to address two uncertainties: (1) whether changing crop species distribution can change ecosystem OR, and (2) whether nutrient status can change ecosystem OR. Analyzing the top three crops in the U.S. (soybean, corn, wheat), we showed differences between the OR of net primary production (ORab) of legumes (1.109 ± 0.003) versus grass crops (corn: 1.028 ± 0.001; wheat: 1.033 ± 0.001). Our preliminary estimate of the average U.S. agriculture ORab is 1.058 in 2010, and extrapolations indicate that the U.S. agricultural ORab has increased from 1.040 in 1930, likely due to shifts in agricultural land use (e.g. oat acreage decreases while soybean acreage increases over time). We also observed that increasing nitrogen fertilization rate led to only a small increase in ORab. Taken together, our results indicate that it is possible for biome-level ORab to vary over time and space. The largest driver of ORab variation in this study was variation in crop distribution, which changed ORab by an order of magnitude more than variations in nitrogen fertilizer application.
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