Robert Grant1, T.J. Arkebauer2, A. Dobermann2, K.G. Hubbard3, T.T. Schimelfenig3, A.E. Suyker3, S.B. Verma3, and D.T. Walters2. (1) University of Alberta, Edmonton, AB T6G 2E3, Canada, (2) University of Nebraska, Department of Agronomy and Horticulture, Lincoln, NE 68583-0915, (3) Univ. of Nebraska, School of Natural Resources, Lincoln, NE 68583-0728
Estimates of agricultural C sequestration require an understanding of how net ecosystem productivity (NEP) and net biome productivity (NBP) of key agroecosystems are affected by land use. Such estimates will most likely be made using mathematical models of crop ecosystems that have undergone well-constrained tests against field measurements of CO2 exchange as affected by management. We tested a hydraulically driven soil-plant-atmosphere water transfer scheme in the ecosystem model ecosys against diurnal and seasonal changes in CO2 and energy exchange measured by eddy covariance (EC) over irrigated and rainfed no-till maize – soybean rotations at Mead, NE during 2002 and 2003. Correlations between modelled and measured fluxes (R2 > 0.8) indicated that < 20% of variation in EC fluxes could not be explained by the model. Annual aggregations of modelled fluxes indicated that NEP of irrigated and rainfed soybean in 2002 was -30 and -9 g C m-2 y-1 (net C source) while NEP of irrigated and rainfed maize in 2003 was 615 and 397 g C m-2 y-1 (net C sink). These NEP were within the range of uncertainty in annual NEP estimated from gap-filled EC fluxes. When grain harvests were subtracted from NEP to calculate NBP, both the modelled and measured maize-soybean rotations became net C sources of 40 – 80 g C m-2 y-1 during 2002 and 2003. Long-term model runs under repeated 2001 – 2004 weather sequences indicated that irrigating a maize – soybean rotation would raise SOC by an average of 6 g C m-2 y-1 over rainfed values after 100 years. Modelled and measured results indicated only limited opportunity for long-term soil C storage in irrigated or rainfed maize – soybean rotations under the soil, climate and management typical of intensive crop production in the U.S. midwest.