Abstract
Abstract. Carbon fixed by agricultural crops in the US creates regional CO2 sinks where it is harvested and regional CO2 sources where it is released back to the atmosphere. The quantity and location of these fluxes differ depending on the annual supply and demand of crop commodities. Data on the harvest of crop biomass, storage, import and export, and on the use of biomass for food, feed, fiber, and fuel were compiled to estimate an annual crop carbon budget for 2000 to 2008. With respect to US Farm Resource Regions, net sources of CO2 associated with the consumption of crop commodities occurred in the Eastern Uplands, Southern Seaboard, and Fruitful Rim regions. Net sinks associated with the production of crop commodities occurred in the Heartland, Northern Great Plains, and Mississippi Portal regions. The national crop carbon budget was balanced to within 0.3 to 6.1 % yr−1 during the period of this analysis.
Highlights
A large amount of CO2 is fixed annually by crops through photosynthesis
Changes in crop carbon stocks are not recorded in national greenhouse gas inventories, but changes in soil carbon are reported (IPCC, 2006; EPA, 2010; CCSP, 2007)
Estimating net uptake and release of crop-derived carbon is important for estimating regional carbon sources and sinks, and for comparison to atmospheric measurements and modeling
Summary
A large amount of CO2 is fixed annually by crops through photosynthesis. Most of the fixed carbon is released in 1 to 2 yr following harvest and subsequent decomposition or consumption and respiration by humans and livestock. The global net annual exchange of CO2 from the uptake and release of crop carbon is near zero, with the exception of crop residues that are incorporated into soil. Whether a region is a net source or sink of all carbon (i.e., cropland and non-cropland carbon) depends on the sum of fluxes from croplands, non-cropland ecosystems, and fossil fuel combustion. This analysis considers only carbon uptake and release associated with carbon fixed in US croplands
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