Biochar's role in mitigating soil nitrous oxide emissions: A review and meta-analysis

•The results of a meta-analysis show that addition of biochar decreased N2O emissions from soils by an average of 54%.•Factors for mitigation (biochar C/N, pyrolysis conditions, application rate, N fertilizer, soil texture and pH) were identified and discussed.•Recommendations are given to address future research needs to improve our understanding of biochar's role in N2O emissions from soil. More than two thirds of global nitrous oxide (N2O) emissions originate from soil, mainly associated with the extensive use of nitrogen (N) fertilizers in agriculture. Although the interaction of black carbon with the N cycle has been long recognized, the impact of biochar on N2O emissions has only recently been studied. Herein we reflect on proposed hypotheses to explain N2O decrease with biochar, linking them to specific mechanisms for N2O formation and consumption in soil. Moreover, to assist in elucidating key mechanisms in which biochar may act in mitigating emissions of N2O, we undertook a meta-analysis using published literature from 2007 to 2013. This quantitative analysis used 30 studies with 261 experimental treatments. Overall, we found that biochar reduced soil N2O emissions by 54% in laboratory and field studies. The biochar feedstock, pyrolysis conditions and C/N ratio were shown to be key factors influencing emissions of N2O while a direct correlation was found between the biochar application rate and N2O emission reductions. Interactions between soil texture and biochar and the chemical form of N fertilizer applied with biochar were also found to have a major influence on soil N2O emissions. While there is clear evidence that, in many cases, emissions of N2O are reduced, there is still a significant lack in understanding of the key mechanisms which result in these changed emissions. As such, we have guided readers with suggestions to address specific research gaps, which we anticipate will enhance our knowledge and understanding of biochar's N2O emission mitigation potential.