Impact of Biochar on Manure Carbon Stabilization and Greenhouse Gas Emissions

Previous studies indicate that biochar additions sometimes increase soil respiration and CO2 emissions which could partially offset C credits associated with soil biochar applications. Little is known, however, about the impact of biochar on the mineralization of manure in soil systems and how interactions between biochar and manure impact C sequestration and greenhouse gas (GHG) emissions from soils. We studied the effect of biochar and dried swine manure additions on changes in soil bulk density (Db), total soil organic carbon (SOC), and emissions of N2O and CO2 during a 500-d soil column incubation study. The addition of biochar to the soil increased SOC content measured after the 500-d incubation by 17.6 to 68.8%, depending on the treatment. Biochar additions reduced N2O emissions measured once near the end of the incubation. The N2O emissions were weakly correlated with Db, suggesting that enhanced soil aeration contributed to the reductions in N2O emissions. Biochar additions consistently increased CO2 emissions (measured 13 times during the incubation) relative to no-biochar controls with cumulative CO2–C emissions equivalent to 17 to 23% of biochar C applied. However, a distinct biochar-by-manure interaction for CO2 flux indicated that biochar either helped stabilize manure C or the presence of manure reduced the effect of biochar on the mineralization of SOC. For the studied system, we conclude that biochar additions sequestered large amounts of highly stable C, reduced N2O emissions, increased CO2 emissions from the soils, and reduced rates of CO2 emissions following a manure addition.