Influence of Biochar Organic and Inorganic Carbon on Soil Greenhouse Gas Emissions.

Application of biochar, the solid condensed aromatic carbon-rich co-product of biomass pyrolysis, to soil has been proposed as a means to sequester carbon in soil. While the majority of studies show that biochar carbon is highly stable in soil, its impact on soil greenhouse gas (GHG) emissions is less certain, and appears to depend on multiple factors – including soil properties, biochar properties, and the presence of co-amendments. It was hypothesized that both labile organic carbon and inorganic carbon (carbonates) in biochar would influence soil GHG emissions. Six biochars – four corn stover biochars, a mixed wood biochar and a hardwood biochar variously produced by fast pyrolysis, slow pyrolysis, or gasification – were incubated with two soils and with a 50/50 (wt/wt) mixture of silt and sand-sized quartz. Soil controls were amended with CaCO₃ at rates corresponding to the carbonate contents of the biochars. Gas samples were collected from the headspaces regularly for six months and analyzed for GHGs by GC. Prior to the application of fertilizer, N₂O emissions were negligible, and most of the biochars induced a slight increase in CO₂ emissions which was positively correlated with their bicarbonate-extractable organic carbon content. During the first 15 days following fertilizer application, total CO₂ emissions from the two soils and quartz were significantly correlated with the carbonate contents of the biochars, while the influence of the biochars on N₂O emissions was negligible. However, these effects on CO₂ emissions did not persist through subsequent gas flux measurements. The results support previous studies showing initial rates of greenhouse gas emissions from biochar-amended soils to be sensitive to the labile organic and inorganic carbon contents of biochar.