The molar H:Corg ratio of biochar is a key factor in mitigating N2O emissions from soil

• An updated meta-analysis including 56 studies finds that biochar reduces soil N2O emissions by 49 ± 5%.
• Field studies show lower reductions in N2O emissions (28 ± 16%) compared to laboratory studies (54 ± 5%).
• The lower the biochar molar H:Corg ratio the higher the N2O mitigation.

A previously published meta-analysis of biochar impacts on soil N2O emissions by Cayuela et al. (2014) found a “grand mean” reduction in N2O emissions of 54 ± 6% following biochar application to soil. Here we update this analysis to include 26 additional manuscripts bringing the total to 56 articles. The updated meta-analysis confirms that biochar reduces soil N2O emissions by 49 ± 5% (mean ± 95% confidence interval). Importantly, this meta-analysis has sufficient data to investigate the impact of biochar under field conditions, showing a statistically significant lower average reduction in the field (28 ± 16%) compared to controlled laboratory studies (54 ± 3%). A key finding is the importance of the molar H:Corg ratio of biochar in determining mitigation of N2O. Biochars with a molar H:Corg ratio <0.3, indicative of a high degree of aromatic condensation, lowered N2O emissions by 73 ± 7% while biochars with a molar H:Corg ratio >0.5 were less effective at 40 ± 16%. Together with previously published information, our new results suggest that a key mitigation mechanism is linked to the degree of polymerization and aromaticity of biochar.