Biofuel Cropping Systems for Feedstock Production and Greenhouse Gas Mitigation.

The Comparison of Biofuel Cropping Systems (COBS) project at Iowa State University is designed to provide a quantitative, side-by-side comparison of corn and multi-species perennial cropping systems. We are making comprehensive, long-term comparisons of contrasting biomass feedstock production systems with respect to: (a) potential for biomass production, fossil-fuel replacement, and net energy returns, (b) potential to reduce greenhouse gas emissions and to increase belowground carbon storage, and (c) potential to maintain soil quality and reduce water-quality impacts of nutrient exports. The COBS project investigates trade-offs and opportunities to optimize system performance relative to multiple criteria.

The experiment is large-scale (24 plots, each 27 m x 61 m, four replications of each system) and compares biomass production, fossil fuel replacement value, and environmental impacts for continuous corn grown for grain and stover removal with and without a rye cover crop; multi-species perennial crops grown for aboveground biomass with and without fertilizer; and a conventional corn-soybean grain system, used as a comparison baseline.

Aboveground biomass. Over four growing seasons, multi-species perennial systems fertilized with a moderate rate of N provided about 70% of the aboveground biomass of corn that had been fertilized at substantially higher N rates. Multi-species perennial plots not receiving N fertilizer produced about 70% of the aboveground biomass of the fertilized perennials and about 50% of the biomass of corn. A cool season (C3) grass, Canada wild rye, and multiple non-legume forbs dominated the fertilized perennial treatment, whereas warm season (C4) perennial grasses, especially big bluestem and Indiangrass, dominated the unfertilized perennial  treatment. C4 grasses have a higher liquid-fuel conversion ratio and a higher heating value than do C3 grasses, and a high energy return on investment might be obtained from multi-species perennial plots that do not receive energy-intensive N fertilizer, but which do produce high-quality C4 grass feedstocks.

Belowground biomass. Root growth rates were estimated by using root in-growth cores. In addition, root biomass to a depth of 100 cm in the research plots was quantified. Multi-species perennial plots had approximately 8 – 12 times more roots and root carbon than corn plots. The unfertilized perennial-system plots produced more root biomass than fertilized perennial-system plots, despite having lower aboveground biomass. We investigated the potential for microbial degradation of cellulose in the crop roots by assaying glucosidase activity in several aggregate size fractions of soil of the experimental treatments. The greater enzyme activity in the multi-species perennial crops likely resulted from the greater presence of C, specifically cellulose-containing plant tissues including roots. N fertilization did not increase C-hydrolyzing enzyme activity in the perennial treatments.

Greenhouse gas emissions.During the growing seasons, CO₂ emissions from the cropping systems plots varied from one another primarily in the early and middle parts of the growing season. In general, the multi-species perennial systems emitted more CO₂ than the other treatments, and the non-fertilized perennial system emitted the most CO₂. The main reason for the relatively large CO₂emissions from the perennial systems is that the perennial systems had greater root masses than the other treatments, and the non-fertilized perennial system had the greatest root mass.

Preliminary assessment of the 2011 growing season shows that corn, continuous corn, and continuous corn with a winter cover crop produced 17, 15, and 13 times more N₂O than the unfertilized multi-species perennial treatment. In contrast, soybeans and the fertilized perennial system produced only 2 and 3 times as much N₂O as the unfertilized perennial system. Even when fertilized, cumulative N₂O emissions from the perennial system were significantly less than those from corn systems.