Quantifying and comparing belowground carbon pools and fluxes of two bioenergy crop species: Miscanthus x giganteus and Sorghum bicolor
Quinn, Ryan Kelly
MetadataShow full item record
Agricultural bioenergy crops (“bioenergy”) are a promising renewable fuel source if carbon (C) emitted during the production and combustion of bioenergy is less than emissions associated with fossil fuel analogs. Despite the importance of belowground C sequestration in determining the net C sink potential of bioenergy, belowground C cycling processes in bioenergy crops remains largely uncharacterized. This study seeks to quantify and characterize the response of belowground C pools and fluxes to farm management scenarios (nitrogen (N) fertilization, stand age, and genotype) in two crops proposed as potential sources of bioenergy, Miscanthus x giganteus (Miscanthus) and Sorghum bicolor (Sorghum). This study additionally seeks to compare the belowground C fluxes in two crop species and draw conclusions about the potential for belowground C storage to mitigate carbon dioxide (CO2) emissions associated with the production and combustion of bioenergy derived from these two crop species. We quantified fine root biomass, soil organic carbon (SOC) content, and CO2 emissions associated with root respiration under five nitrogen (N) fertilization levels in Miscanthus and Sorghum. For perennial Miscanthus, we also quantified fine root biomass and root respiration among stands established over three different years to observe how the net belowground C flux changed over time and as a function of establishment year. Both fine root biomass and root respiration rates did not change as a function of fertilization in Sorghum stands, but SOC content in Sorghum was significantly greater in the <0.053 and 2-0.25mm size fractions in unfertilized stands compared to fertilized. In Miscanthus stands, N fertilization did not affect SOC content. Nitrogen fertilization decreased the belowground C storage capacity of Miscanthus by depressing fine root biomass. Simultaneously, N fertilization increased mass-specific rates of root respiration rates in Miscanthus. Despite increased mass-specific root respiration with N fertilizer addition, Miscanthus plot-scale root respiration did not change with increasing N application due to decreased fine root biomass observed with increasing amounts of N fertilization. Fine root biomass was six-fold greater in Miscanthus stands than Sorghum, while mass-specific root respiration rates were lower in Miscanthus stands than Sorghum. When scaled up, plot-scale root respiration emissions were lower in Miscanthus compared to Sorghum stands, while SOC content was greater in Miscanthus stands than Sorghum stands. Our results indicate Miscanthus has greater C sink potential than Sorghum via C allocated belowground to fine root biomass production and lower rates of root respiration.