Changing Global Climate: Historical Carbon and Nitrogen Budgets and Projected Responses of Ohio’s Cropland Ecosystems

As the evidence of global climate change continues to mount, its consequences for cropland productivity assume particular significance. Against the backdrop of past agricultural practices, simulation models offer a glimpse into the future, showing the effect of temperature changes on crop production. In this study, we first quantified the carbon (C) and nitrogen (N) budgets of Ohio’s cropland ecosystems using inventory yield data of corn for grain, oat, and all wheat for the period 1866–1996 and soybean for the period 1924–96. Then we explored the responses of Ohio’s continuous soybean croplands to changes in temperature, carbon dioxide (CO2) concentration, initial soil organic C and N (SOC-N) pools, soil texture, and management practices by developing a simple cropland ecosystem model (CEM) and performing a long-term sensitivity analysis. Finally, CEM simulations were evaluated against independent observations of SOC values (0–19 cm) averaged over 470 northwest Ohio sites between 1954 and 1987 under conventional tillage and rotations of corn–soybean–winter wheat by using the historical yield data (r2 = 0.8). The C contents per hectare of crop harvests increased by 178% for oats, 300% for corn for grain, and 652% for all wheat between 1866 and 1996 and by 305% for soybean between 1924 and 1996. Ohio croplands acted as C–N sources, releasing average net ecosystem emissions (NEE), including the removal of harvested C–N, of 4,598 kg CO2 ha−1 and 141 kg N ha−1 in 1886 and 205 kg CO2 ha−1 (except for the corn-for-grain cropland) and 39 kg N ha−1 in 1996. The continuous corn croplands continued to become a C sink, sequestering 255 kg C ha−1 in 1996. Results of the sensitivity analysis for Ohio’s continuous soybean croplands revealed that the SOC pool increased by 6.9% and decreased by 7.5% in response to a doubled CO2 concentration and a temperature increase of 2.8°C over 100 years, respectively. The sequestration potential of the SOC pool increased by 6.5% at a rate of 24.6 kg C ha−1 y−1 for the same period with finer soil texture (loam to silty clay loam). The shift from conventional to conservation residue practice led to an 11% increase in the steady-state SOC storage at a rate of 42 kg C ha−1 y−1 for 100 years.