: Soil Carbon Sequestration with Depth After 9 Years Under Switchgrass and No-till Maize Grown for Bioenergy.

Net benefits of bioenergy crops including maize and switchgrass include the soil organic carbon (SOC) sequestered.  Life cycle assessments (LCA) for bioenergy crops have been conducted using models in which sequestered SOC was based on data from  the top 30 to 40 cm. Information on effects of crop management practices on SOC has been limited so LCA models largely do not include management effects.  In the first nine years SOC sequestration study in Eastern Nebraska, switchgrass and maize with best management had average annual increases in SOC that exceeded 2 Mg C yr^-1 (7.3 Mg CO₂ yr^-1) for the 0 to 150 soil depth. For both crops, over 50% of the increase in SOC was below the 30 cm depth.  SOC sequestration by switchgrass was two- to four-fold greater than that used in models to date which also assumed no SOC sequestration by maize. The results indicate that N fertilizer rates and harvest management regimes can affect the magnitude of SOC sequestration.  Use of uniform SOC effects for bioenergy crops from sampling depths to only 30-40 cm across agro-ecoregions for large scale LCA is questionable.  The switchgrass management with greatest biomass yields during the 1998 to 2007 was 120 kg ha^-1 N and harvest after killing frost.  The average annual increase in SOC from 0-150 cm for this treatment was 2 Mg ha^-1 yr^-1.  At the same fertilizer rate, maize grain yields were greatest with no stover harvest. Annual increase in SOC was 2.6 Mg ha^-1 yr^-1 and was equivalent to harvested grain C yield.  At the 120 kg ha^-1 N rate, stover harvest yielded 1.3 Mg ha^-1 yr^-1 of harvested C and resulted in a reduced, but not statistically different, SOC sequestration rate when compared to no stover harvest.