Nitrogen Rate, Landscape Position, and Harvesting of Corn Stover Impacts on Energy Gains and Sustainability of Corn Production Systems in South Dakota

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, SOCₜ = SOCₜ₋₁ + kNHCNHC – kₛₒc SOCₜ₋₁, where SOCₜ was SOC at time t, SOCₜ₋₁ was SOC at the sampling date before time t, kNHC was rate that nonharvested carbon (NHC) was converted to SOC, and kₛₒc was the rate that SOC was converted to CO₂ Tillage impacts on kSOC was estimated with the model kₛₒc [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.