Measured and Simulated Carbon Dynamics in Midwestern US Corn-Soybean Rotations

Corn (Zea mays L.) and soybean (Glycine max [L.] Merr.) production dominate Midwestern U.S. agriculture and impact the regional carbon and nitrogen cycles. Sustaining soil carbon is important for corn-soybean production (CS); however, quantifying soil carbon changes requires long-term field measurements and/or model simulations. In this study, changes in soil organic (SOC), inorganic (SIC), and total (TC) carbon; pH; total nitrogen (TN); and net ecosystem production (NEP) were measured in a conventional corn-soybean rotation in central Iowa. Soil samples (n = 42; 0-120 cm depth) were collected from two adjacent fields in 2005 and 2016. Eddy-flux stations set up in the fields continuously monitored NEP from 2005-2016, and net biome production (NBP) was calculated using yield records. The DayCENT model was used to simulate the effects of conventional management practices on soil carbon and calibrated with field-measured NEP and SOC. Measured soil TC (0-120 cm) decreased by -14.19 +/- 6.25 Mg ha(-1), with highest reductions in SOC and SIC (p < 0.05) at 0-15 and 90-120 cm, respectively. Measured TN decreased by -0.7 +/- 0.29 Mg ha(-1) with N-accumulation at 60-90 cm (p < 0.05). Eddy-flux NBP decreased by -13.19 +/- 0.05 Mg ha(-1). Soil and eddy-flux records show a carbon reduction by -1.14 +/- 0.63 and -1.20 +/- 0.06 Mg ha(-1) yr(-1), respectively. The validated DayCENT model suggests that all SOC pools declined. We postulate that conventional CS production has adverse effects on C and N dynamics in Midwestern United States.

Automated summary

This study conducted a long-term investigation on soil carbon and nitrogen changes in a corn-soybean rotation field in central Iowa, showing a decreasing trend in soil carbon and nitrogen content. The findings suggest that conventional corn-soybean production may have adverse effects on regional carbon and nitrogen dynamics in the Midwestern United States.