Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/26/2012
Publication Date: 1/1/2013
Citation: Gilmanov, T.G., Wylie, B.K., Tieszen, L.L., Meyers, T.P., Amiro, B.D., Baron, V.S., Bernacchi, C.J., Billesbach, D.P., Burba, G.G., Fischer, M.L., Hatfield, J.L., Prueger, J.H. 2013. CO2 uptake and ecophysiological parameters of the grain crops of midcontinent North America: estimates from flux tower measurements. Agriculture, Ecosystems and Environment. 164:162-175. Interpretive Summary: Grain crops contribute a significant portion of global food production and account for a massive amount of carbon removal from the atmosphere. Most of this carbon is dedicated to the harvestable component of agriculture but a significant portion can remain as stored carbon in the soil. There is tremendous uncertainty surrounding the magnitude of carbon that is scrubbed from the atmosphere by photosynthesis of these crops. This paper addresses the issue of modeling carbon uptake by grain crops in the Midwestern US through a modeling approach that relies on the state-of-the-art data collection techniques employed throughout the region. The results of this paper provide an improvement of the parameters needed to model these carbon fluxes from the atmosphere into crop ecosystems, thereby improving the prediction of carbon uptake and storage within this agriculturally rich region.
Technical Abstract: We present net CO2 exchange data from 13 flux tower sites with 27 site-years of measurements over maize and wheat fields across midcontinent North America. A numerically robust “light-soil temperature-VPD”-based method was used to partition the data into photosynthetic assimilation and ecosystem respiration components. Year-round ecosystem-scale ecophysiological parameters of apparent quantum yield, photosynthetic capacity, convexity of the light response, respiration rate parameters, ecological light-use efficiency, and the curvature of the VPD- response of photosynthesis for maize and wheat crops were numerically identified and interpolated/extrapolated. This allowed us to gap-fill CO2 exchange components and calculate annual totals and budgets. VPD-limitation of photosynthesis was systematically observed in grain crops of the region (occurring from 20 to 120 days during the growing season, depending on site and year), determined by the VPD regime and the numerical value of the curvature parameter of the photosynthesis- VPD-response, sVPD. In 78% of the 27 site-years of observations, annual gross photosynthesis in these crops significantly exceeded ecosystem respiration, resulting in a net ecosystem production of up to 2100 g CO2 m-2 yr-1. On an annual scale, maize crop gross primary production (GPP) is statistically related to annual sum of temperatures above 5 °C (Tsum5) and the hydrologic year precipitation (PCPNhyd). At larger geographical scales (aggregation in 4°×4° cells or at State level), an empirical climate-based GPP(Tsum5, PCPNhyd) model explained more than 50% of the variance of official USDA maize grain yield data. The measurement-based photosynthesis, respiration, and net CO2 flux data and the numerical estimates of the major ecophysiological parameters of the grain crops of the region provide a solid empirical basis for parameterization and validation of mechanistic models of grain crop production in this economically and ecologically important region of North America.