Greenhouse Gas Emissions from Soils as Affected by Addition of Biochar.

High concentrations of two greenhouse gases, nitrous oxide (N₂O) and carbon dioxide (CO₂), in the stratospheric ozone layer are believed to be responsible for global climate change. We hypothesize that conversion of biomass to liquid fuel stocks by pyrolysis and returning the biochar co-product to the soil is a mean of reducing net greenhouse gas emissions. We studied the effect of biochar additions to the soil on changes in total soil organic C and emissions of N₂O and CO₂ during a one-year soil column incubation study. Four rates of biochar 0, 5, 10, and 20g kg^-1 soil were applied to Clarion fine-loamy soils. Recoveries of biochar C ranged from 98 to 109% indicating that little or no biochar C was mineralized during incubation. Biochar additions reduced N₂O emissions (measured once near the end of the one-year incubation) but consistently increased CO₂ emissions (measured 13 times during the one-year incubation) relative to controls receiving no biochar additions.  The reduced N₂O emissions were correlated with reductions in soil bulk density, suggesting that enhanced aeration of the soil may have contributed to the reductions in N₂O emissions.  Increased CO₂ emissions with biochar additions apparently due to enhanced mineralization of biogenic soil organic matter may partially offset the C sequestered by addition of the biochar.  However, increased respiration also implies enhanced microbial activity and nutrient cycling, which could increase plant biomass production and hence the biological fixation of CO₂-C through photosynthesis.  Biochar may also influence the rate at which organic C from plant and animal residues is transformed into stabilized humic substances.  For the studied system, we conclude that biochar additions sequestered large amounts of highly stable C, reduce N₂O emissions, and increased CO₂ emissions.  Understanding the net effect of biochar additions on levels biogenic soil organic C, however, will require additional study.