Can biochar reduce soil greenhouse gas emissions from a Miscanthus bioenergy crop?

Energy production from bioenergy crops may significantly reduce greenhouse gas (GHG) emissions through substitution of fossil fuels. Biochar amendment to soil may further decrease the net climate forcing of bioenergy crop production, however, this has not yet been assessed under field conditions. Significant suppression of soil nitrous oxide (NO) and carbon dioxide (CO) emissions following biochar amendment has been demonstrated in short-term laboratory incubations by a number of authors, yet evidence from long-term field trials has been contradictory. This study investigated whether biochar amendment could suppress soil GHG emissions under field and controlled conditions in a Miscanthus × Giganteus crop and whether suppression would be sustained during the first 2 years following amendment. In the field, biochar amendment suppressed soil CO emissions by 33% and annual net soil CO equivalent (eq.) emissions (CO, NO and methane, CH) by 37% over 2 years. In the laboratory, under controlled temperature and equalised gravimetric water content, biochar amendment suppressed soil CO emissions by 53% and net soil CO eq. emissions by 55%. Soil NO emissions were not significantly suppressed with biochar amendment, although they were generally low. Soil CH fluxes were below minimum detectable limits in both experiments. These findings demonstrate that biochar amendment has the potential to suppress net soil CO eq. emissions in bioenergy crop systems for up to 2 years after addition, primarily through reduced CO emissions. Suppression of soil CO emissions may be due to a combined effect of reduced enzymatic activity, the increased carbon-use efficiency from the co-location of soil microbes, soil organic matter and nutrients and the precipitation of CO onto the biochar surface. We conclude that hardwood biochar has the potential to improve the GHG balance of bioenergy crops through reductions in net soil CO eq. emissions.