The net environmental benefit of bioenergy crops depends in part on how they impact agroecosystem nutrient cycling. The addition of nitrogen (N) fertilizer to crops affects ground and stream water by increasing leaching of nitrate (NO3-), and increases emissions of nitrous oxide (N2O), a potent greenhouse gas. Perennial biofuel crops, such as switchgrass (Panicum virgatum) and Miscanthus x giganteus, produce high yields with low N fertilizer requirements and small NO3- and N2O losses, but their establishment in the Midwestern United States has been limited. As an annual bioenergy crop, high-yielding energy sorghum (Sorghum bicolor) has the potential for more widespread adoption because it readily fits into the annual crop rotation, but little is known about how a transition to sorghum would affect the ecosystem-level N cycle. One potential mechanism is through the exudation of biological nitrification inhibiting compounds (BNIs) from sorghum root hairs, reducing the production of NO3- that leads to both N2O emissions and NO3- leaching. We measured the effect of sorghum root exudates on potential nitrification in rhizosphere soil of four sorghum genotypes across two N fertilization rates (0 and 168 kg N ha-1) at the University of Illinois Energy Farm, Urbana, IL.
Results/Conclusions
Across all dates, sorghum inhibited potential nitrification by 8.6% in the rhizosphere relative to bulk soil (P = 0.028). Mid-growing season, when plants were growing fastest, sorghum inhibited potential nitrification by an average of 16% (± 4.8%). This inhibition was stronger in unfertilized plots (26.8%) compared to fertilized plots (11.6%) (P = 0.025). Across all dates, potential denitrification was stimulated by 36.6% in the sorghum rhizosphere compared to bulk soil (P = 0.01), and N2O flux from sorghum fields was higher than from maize. In sorghum fields, carbon-rich root exudates may have stimulated denitrification, a heterotrophic process, causing higher N2O emissions. Although sorghum has the capacity to limit NO3- production, this effect diminishes at high fertilization levels. The decline of BNI with fertilizer addition indicates that BNI is likely facultatively expressed to reduce N loss as NO3- and increase N retention in the soil. As a result, leaching and gaseous N losses could increase exponentially with fertilizer application rate and reduce the ecological sustainability of the system if high fertilizer inputs are used to maximize yield. Ongoing work includes the quantification of rhizosphere soil and the measurement of leaching rates to determine the effect of BNI on ecosystem-scale NO3- loss.