Greenhouse gas emissions from soil amended with agricultural residue biochars: Effects of feedstock type, production temperature and soil moisture

The conversion of agricultural residues into biochar produces a material with agronomic and environmental benefits. Biochars have been found to positively or negatively affect soil greenhouse gas (GHG) emissions, with the variability being explained by differences in biochar properties and environmental conditions. This study evaluated the effect of soil moisture conditions on the emissions of GHGs from soils amended with different types of biochar. For that, we added biochars to a sandy soil and incubated the samples at 23 °C for 90 days under two moisture conditions. While CH4 fluxes remained not significantly different from zero in all treatments, CO2 effluxes were stimulated at higher soil moisture contents, though the stimulating effect varied with biochar type. Soils amended with biochars with a narrow C:N ratio showed a higher CO2 efflux than soils amended with biochars with a wide C:N ratio. There were significant main effects of pyrolysis temperature and soil moisture conditions on the N2O fluxes from the treatments, however total N2O fluxes from biochar amended soils were overall not significantly different from the control due to treatment interactions. Our study indicates that the main effect of biochars on the GHG balances of soils is related to an increase in soil C stocks, with little short-term changes in soil CH4 and N2O fluxes. Nevertheless, short-term increases in GHG emissions due to biochar addition to soils can be easily mitigated by use of biochars with high C:N ratios produced under high temperatures and by avoiding application to moist soils.