University of Illinois at Urbana-Champaign, United States
Abstract: Bioenergy crop production holds promise for supporting both the ecological and economic sustainability of a bioeconomy that can displace fossil fuel use. Energy sorghum (Sorghum bicolor) is an emerging bioenergy feedstock with high biomass yield potential that is grown as an annual crop that can be more readily integrated into existing landscapes of annual row crop rotations than perennial crops. However, as an annual crop that requires fertilizer inputs, it exhibits high soil nitrous oxide emissions and nitrate leaching that detract from its ecological sustainability. Despite lower fertilization rates for energy sorghum than for maize, soil nitrous oxide emissions are comparably high for these two crops, highlighting the need to improve nitrogen management of energy sorghum cropping systems. Cover cropping, in which non-cash crops are planted after cash crop harvest, can reduce non-growing season nitrogen losses from agricultural fields. Poor establishment of cover crops before winter can lead to smaller ecosystem benefits, but the planting date is typically constrained by the cash crop harvest date. Since energy sorghum is harvested green, this allows for flexibility in the harvest date which is typically in October. We conducted a two-year field trial at the University of Illinois Energy Farm in Urbana, IL to evaluate how sorghum harvest and cover crop planting date affects cereal rye cover crop biomass and nitrogen loss reductions. We measured aboveground sorghum and rye biomass, plant C:N ratios, soil nitrous oxide emissions, soil nitrate leaching rates via resin lysimeters, and net nitrogen cycling rates (mineralization and nitrification).
In the first year of the study with wetter conditions in the late summer and fall, harvesting energy sorghum one month early to plant a cereal rye cover crop increased cover crop biomass but later sorghum harvest led to the lowest agroecosystem nitrogen losses. September sorghum harvest led to the highest cereal rye cover crop biomass because sorghum regrowth after August harvest shaded out the rye. Nitrate leaching decreased with later sorghum harvest date and was not affected by cover crops. Cover crops reduced soil nitrous oxide emissions only when sorghum was harvested in September and October. Drier conditions suppressed plant productivity without sorghum re-growth after early harvest in the second year of the study (2022-2023). We conclude that the ideal sorghum harvest date that maximizes the efficacy of cover cropping in improving the nitrogen sustainability energy sorghum varies inter-annually depending on precipitation.