The harvesting of plant biomass in excess of the soil organic carbon (SOC) maintenance requirement can produce short-term economic benefits at the cost of long-term sustainability. The objective of this study was to assess the impact of corn ( Zea mays L.) harvesting approach, N rate, and simulated landscape positions on estimated long-term SOC maintenance, profitability, and the energy efficiency of no-till corn grown in eastern South Dakota. The 3-yr experiment (2002-2004) contained four N rates (0, 56, 112, and 168 kg N ha -1), two simulated landscape positions (shoulder and backslope), and two harvesting methods (grain with 100% stover returned or grain+40% corn stover returned). No-tillage was used at the site. Energy gains (out-input), for a cropping system where corn grain or corn grain plus stover was sold for ethanol production, were calculated. Profitability was estimated and SOC turnover was simulated using the annual time-step model, SOC t=SOC t-1+k NHCNHC-k soc SOC t-1, where SOC t was SOC at time t, SOC t-1 was SOC at the sampling date before time t, k NHC was rate that nonharvested carbon (NHC) was converted to SOC, and k soc was the rate that SOC was converted to CO 2 Tillage impacts on k SOC was estimated with the model k soc [g SOC-C (g SOC year) -1]=0.0115+0.00631(tillage events). When only grain was harvested, the highest and lowest energy gains and financial were associated with the 112 kg N ha -1 (46.6 GJ ha -1 and $427 ha -1) and the 0 kg N per ha -1 (37.5 GJ ha -1 and $192 ha -1) treatments, respectively. Applying more than 112 kg N ha -1 did not increase energy gains or financial returns. Profits were increased by 60% when corn stover was harvested for ethanol production and lower yielding simulated shoulder/summit position had a lower energy gain (59.3 GJ ha -1 and $425 ha -1) and financial return than the backslope position (66.3 GJ ha -1 and $614 ha -1). The SOC sustainability analysis suggests that the ability of a system to maintain SOC depends on many factors including the amount of nonharvested carbon returned to the site, and the amount of carbon contained in the soil.
