Toward a Better Assessment of Biochar–Nitrous Oxide Mitigation Potential at the Field Scale

Through meta‐analysis, we synthesize results from field studies on the effect of biochar application on N₂O emissions and crop yield. We aimed to better constrain the effect of biochar on N₂O emissions under field conditions, identify significant predictor variables, assess potential synergies and tradeoffs between N₂O mitigation and yield, and discuss knowledge gaps. The response ratios for yield and N₂O emissions were weighted by one of two functions: (i) the inverse of the pooled variance or (ii) the inverse of number of observations per field site. Significant emission reductions were observed when weighting by the inverse of the pooled variance (−18.1 to −7.1%) but not when weighting by the number of observations per site (−17.1 to +0.8%), thus revealing a bias in the existing data by sites with more observations. Mean yield increased by 1.7 to 13.8%. Our study shows yield benefits but no robust evidence for N₂O emission reductions by biochar under field conditions. When weighted by the inverse of the number of observations per site, N₂O emission reductions were not significantly affected by cropping system, biochar properties of feedstock, pyrolysis temperature, surface area, pH, ash content, application rate, or site characteristics of N rate, N form, or soil pH. Uneven coverage in the range of these predictor variables likely underlies the failure to detect effects. We discuss the need for future biochar field studies to investigate effects of fertilizer N form, sustained and biologically relevant changes in soil moisture, multiple biochars per site, and time since biochar application. CORE IDEAS: Biochar reduced mean N₂O emissions by 12.4% when weighted by the inverse of the pooled variance. Biochar did not significantly reduce N₂O emissions when weighted by the inverse of the number of observations per site. The effect of weighting function suggests site‐dependence and dataset limitations on the potential of biochar to mitigate N₂O. Biochar increased yield for both weighting functions.