Daniel Walters, Daniel Ginting, Kenneth Cassman, Shashi Verma, Achim Dobermann, Haishun Yang, Andrew Suyker, and Adam Liska. University of Nebraska-Lincoln, Department of Agronomy and Horticulture, P.O. Box 830915, Lincoln, NE 68583-0915
Maize-based agroecosystems represent the dominant agricultural land use in the north-central USA. Estimates of the global warming potential (GWP) of these systems requires an accounting of the net ecosystem exchange (NEE) of carbon (C), trace gas emissions (N2O and CH4), fate of exported C and the embodied C-costs of fossil-fuel emissions associated with production costs. Since 2001, we have been measuring these components utilizing tower eddy covariance as well as direct in-field measurements of process level controls on C-dynamics and trace gas emissions. These measurements have been made at the landscape level in three production-scale fields (each ~65 ha) to include: (i) an irrigated continuous maize system, (ii) an irrigated maize-soybean rotation, and (iii) a rainfed maize-soybean rotation. This paper will report on the annual distributions of net ecosystem production (NEP) during the first four years of no-till farming in these three systems and the “C-costs” from fossil fuel use and trace gas emissions. Results show that there are large differences in primary productivity and ecosystem respiration due to differences in water supply and crop rotation. There are also large differences in the embodied “C-costs” that contribute to their overall GWP. Grain C removal as harvest often equals or exceeds NEE resulting in a net loss of C from the boundaries of the field. The intrinsic “C-costs” from trace gas emissions and fossil fuel use comprise 25 to 30 % of annual NEE and exacerbate the GWP of these systems. Grain C export may be utilized for biofuel production (ethanol and biodiesel) and result in the displacement of fossil fuel use from the transportation sector. In our study, we have shown that diversion of grain to biofuel easily displaced the GWP of on-farm trace gas emissions as well as fossil fuel used for grain production and biofuel processing and distribution.