Marc L. Fischer | Geology Reports

USGS2014

Productivity and carbon dioxide exchange of leguminous crops: estimates from flux tower measurements

Net CO 2 exchange data of legume crops at 17 flux tower sites in North America and three sites in Europe representing 29 site-years of measurements were partitioned into gross photosynthesis and ecosystem respiration by using the nonrectangular hyperbolic light-response function method. The analyses produced net CO 2 exchange data and new ecosystem-scale ecophysiological parameter estimates for legume crops determined at diurnal and weekly time steps. Dynamics and annual totals of gross photosynthesis, ecosystem respiration, and net ecosystem production were calculated by gap filling with multivariate nonlinear regression. Comparison with the data from grain crops obtained with the same method demonstrated that CO 2 exchange rates and ecophysiological parameters of legumes were lower than those of maize ( Zea mays L.) but higher than for wheat ( Triticum aestivum L.) crops. Year-round annual legume crops demonstrated a broad range of net ecosystem production, from sinks of 760 g CO 2 m –2 yr –1 to sources of –2100 g CO 2 m –2 yr –1 , with an average of –330 g CO 2 m –2 yr –1 , indicating overall moderate CO 2 –source activity related to a shorter period of photosynthetic uptake and metabolic costs of N 2 fixation. Perennial legumes (alfalfa, Medicago sativa L.) were strong sinks for atmospheric CO 2 , with an average net ecosystem production of 980 (range 550–1200) g CO 2 m –2 yr –1 .

Agronomy Journal↗

USGS2013

CO 2 uptake and ecophysiological parameters of the grain crops of midcontinent North America: estimates from flux tower measurements

We analyzed net CO 2 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 CO 2 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, σ VPD . 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 CO 2 m −2 year −1 . The measurement-based photosynthesis, respiration, and net ecosystem production data, as well as the estimates of the ecophysiological parameters, provide an empirical basis for parameterization and validation of mechanistic models of grain crop production in this economically and ecologically important region of North America.

Agriculture, Ecosystems and Environment↗