Increased adoption of warm season crops throughout the northern Great Plains is changing traditional cropping practices. Greater inclusions of corn (Zea maize L.) and soybean (Glycine max L.) in crop rotations have the potential to alter the carbon and water balances on agricultural lands. However, we are not aware of any multi-year study that has been conducted in the region to quantify the impacts of these emerging crops on the use of carbon and water. Therefore, we evaluated carbon and water fluxes under business-as-usual management for a no-till, rainfed spring wheat (Triticum aestivum L.) – corn – soybean rotation using eddy covariance methodology near Mandan, ND USA. We partitioned the net ecosystem exchange (NEE) for CO2 into ecosystem respiration (ER) and gross ecosystem production (GEP), and assumed that the net ecosystem production, NEP ≈ −NEE. Carbon use efficiency (CUE) was estimated as the slope of the linear relationship between daily cumulative NEP and GEP, while water use efficiency (WUE) was calculated as the ratio of daily GEP to evapotranspiration (ET).
Results/Conclusions
Annual means of daily carbon and water fluxes indicated that corn had higher carbon uptake (i.e., higher NEP or more negative NEE), ER, and GEP than wheat and soybean. Annual means of carbon fluxes (NEP, ER, GEP) were similar between fields, although a field (H5, 1.15 ± 0.04 mm d-1) had slightly lower ET than another field (I2, 1.23 ± 0.04 mm d-1). Annual sums of NEP (g C m-2 yr-1) suggested that corn (C, 121 ± 32) was a net sink for CO2, whereas soybean (SB, 7 ± 22) and spring wheat (SW, −32 ± 26) were almost balanced with air-CO2. Indeed, the overall mean NEP (32 ± 30 g C m-2 yr-1), across the SW-C-SB rotation in two fields, implied that CO2 fluxes were near-equilibrium with the atmosphere. During the growing season, CUE (unitless) was in the order of wheat (0.42 ± 0.02) > soybean (0.40 ± 0.02) > corn (0.33 ± 0.01), whereas WUE (g C m-2 mm-1) was in the order of corn (2.38 ± 0.06) > wheat (2.26 ± 0.08) > soybean (2.14 ± 0.07). Outcomes from this study emphasize the importance of continuous, long-term observations for monitoring the impacts of cropping systems on carbon and water use efficiencies in a semiarid environment.