Soil physical properties, soil carbon dioxide fluxes, and soil drainage dynamics of select bioenergy cropping systems.

摘要

Bioenergy cropping systems have been proposed as a way to enhance United States energy security. However, research on soil physical properties, soil-surface CO2 effluxes, and soil drainage dynamics in such systems is needed to ensure environmental sustainability in the field. The objective of our research was to evaluate soil physical properties and conditions as well as soil-surface CO2 effluxes and soil drainage dynamics of selected annual- and perennial-based biofuel cropping systems with the goal of comparing these systems for the greatest environmental sustainability in regards to soil and water resources. Near Ames, Iowa, six cropping systems of mixed prairie (nitrogen-fertilized and unfertilized), continuous maize with 50 % stover removal (with and without cover crop), and maize-soybean rotation (each crop type grown each year) was initiated with four replications in a randomized complete block design. Soil physical properties evaluating soil structure and water, solute, and gas transfer were evaluated near the soil surface and soil-surface CO2 effluxes, surbsurface drainage quantity and quality, and soil physical conditions were monitored over time. Overall, the environmental sustainability greatly varied among the cropping systems. The removal of corn stover in the early stages after system establishment did not significantly impact soil physical properties, soil-surface CO2 effluxes, or soil water drainage with regards to subsurface drainage flow dynamics or water quality as compared to corn-soybean systems with only grain harvest. However, the incorporation of a winter rye cover crop in continuous corn systems did tend to be more environmentally sustainable as compared to when a cover crop was not used. Though soil physical properties were not improved, spatial variability associated with corn stover removal was reduced, subsurface drainage water quality was significantly improved, and subsurface drainage peak flows were reduced when a winter rye cover crop was incorporated. Similarly, prairie systems were observed to be the most environmentally sustainable bioenergy cropping systems in comparison to corn stover of continuous corn systems and grain only harvested corn-soybean systems. Prairie systems, significantly improved soil physical properties with regards to soil structure, water retention, and water transfer, improved soil aeration and appeared to have soil carbon sequestration potential, improved subsurface drainage water quality, reduced subsurface drainage peak flows, and cumulative drainage as compared to row crop systems. These observations and trends are robust in regards to representing a wide range of climate conditions in Iowa and the Midwest due to the large range of precipitation observed during the study duration. Based on these findings, prairie systems with or without fertilization are recommended based on their high potential for environmental sustainability. However, if prairie systems are not feasible in comparison to continuous corn systems or corn-soybean rotations due to either market prices or proximity to an ethanol production plant or distribution center, the incorporation of a winter rye cover crop to row crop systems is strongly advised to increase environmental sustainability. Future research includes quantification of cropping system evapotranspiration, water balance component partitioning, and calibrating and validating hydrologic and nutrient cycling numerical models.