Carbon stability and mitigation of fertilizer induced N₂O emissions in soil amended with biochar

Biochar is a promising tool for an efficient and low environmental impact agriculture since can offer both soil carbon (C) sequestration and mitigation of nitrous oxide (N₂O) emissions. The extent of biochar C stability after soil amendment and efficiency in reducing N₂O emissions from an external nitrogen (N) source were accessed through laboratory incubations. A clay loam soil was amended with chicken manure (CM), sewage sludge (SS), eucalyptus sawdust (ES) and filter cake (FC) feedstocks and corresponding slow-pyrolysis (400°C) biochars at 5gCkg^-1 soil in combination with two N fertilizer rates (0 and 140mgNkg^-1 soil). Carbon dioxide (CO₂) and N₂O emissions were measured during 60days. Biochars and feedstocks CO₂ emissions were described by an exponential first order kinetics model. For C mineralization an interaction effect was observed for feedstock source and organic amendment. Lower values of mineralizable C was found for biochars than corresponding feedstocks, except for ES. Carbon losses in 60days of incubation totaled between 0.8 and 9.4% and 2.4 and 32% for biochars and feedstocks, respectively. Regarding to N₂O emissions, only CM-biochar impacted emissions with a two-fold increase in non-fertilized soil. When NH₄NO₃ was co-applied, biochars reduced fertilizer induced N₂O emissions, reaching a seven-fold reduction in SS-biochar treatment. The fertilizer emission factor (EF) decreased with biochar amendments as well, varying between 0.01 and 0.08% of the fertilizer N emitted as N₂O, which shows the biochar potential to reduce fertilizer induced N₂O emissions, with major reduction by SS-biochar mitigating 87% of the soil-fertilizer emissions. Such potential could be explored by designing biochars based on feedstock chemical and structural properties, including a mixed feedstock source biochar that promotes C sequestration and mitigates N₂O emissions.