Changes in Soil Carbon in a Continuous Corn-Soybean Rotation in the Midwest, 2005 – 2016.

Comprehensive carbon budgets are essential to understand changes in carbon pools and fluxes in agricultural soils, and to assess the sustainability of agricultural production with respect to carbon sequestration. However, that requires long-term experimental setups, due to the slow response and high variability of soil carbon affected by land management. We investigated the impact of a continuous corn-soybean rotation (conventional tillage and exclusive of catch crops) on carbon pools and fluxes in Central Iowa from 2005 to 2016. Soil samples in 0 – 120 cm depth were taken at 42 locations in a 50 x 50 m grid in 2005 and 2016. The carbon flux was continuously measured with an eddy-flux station during the same period, and net ecosystem production (NEP) was calculated. In addition, carbon loss by yield removal was recorded. Preliminary results show that mean (±SE) total carbon concentration in 0 – 15 cm depth decreased from 3.50 ± 0.12% to 3.00 ± 0.15% at similar bulk densities of 1.36 ± 0.01 and 1.34 ± 0.02 g cm^-3 in 2005 and 2016, respectively. The mean carbon stock (0 – 15 cm) decreased from 70.1 ± 2.0 Mg ha^-1 to 58.2 ± 2.5 Mg ha^-1, indicating an average loss of soil carbon of 11.9 Mg ha^-1 in an eleven year period of continuous corn-soybean production. The average loss in 0 – 120 cm was 9.9 Mg ha^-1, showing that the bulk of soil carbon reduction was due to mineralization and not percolation to deeper strata. The calculated NEP was 16.7 Mg ha^-1 with positive NEP for corn and near-neutral and negative NEP for soybean-years. The carbon removed as grain and beans summed up to 33.1 Mg ha^-1, which indicates a net loss of 16.4 Mg ha^-1 of carbon. The presented carbon budget stresses the unsustainability of conventional corn-soybean rotation systems in the Midwest.