Woody biochar's greenhouse gas mitigation potential across fertilized and unfertilized agricultural soils and soil moisture regimes

Biochar has been widely researched as an important technology for climate smart agriculture, yet work is still necessary to identify the magnitude of potential greenhouse gas (GHG) mitigation and mechanisms involved. This study measured slow-pyrolysis wood-derived biochar's impact on GHG efflux, mineral N dynamics, and soil organic C in a series of two incubations across fertilized and unfertilized agricultural soils and soil moisture regimes. This research explored the magnitude of biochar's full GHG mitigation potential and drivers of such impacts. Results of this incubation indicate slow-pyrolysis wood-derived biochar has potential to provide annual emission reductions of 0.58-1.72 Mg CO2-eq ha(-1) at a 25 Mg ha(-1) biochar application rate. The greatest GHG mitigation potential was from C sequestration and nitrous oxide (N2O) reduction in mineral N fertilized soils, with minimal impacts on N2O emissions in unfertilized soils, carbon dioxide (CO2) emissions, and methane (CH4) uptake. Analysis of mineral N dynamics in the bulk soil and on biochar isolates indicated that neither biochar impacts on net mineralization and nitrification nor retention of ammonium (NH4+) on biochar isolates could explain biochar's N2O reduction. Instead, biochar amendments exhibited consistent N2O emission reductions relative to the N2O emission in the control soil regardless of soil type and fertilization. Results across a soil moisture gradient suggest that woody biochar may aerate soils shifting redox conditions and subsequent N2O production. Understanding the magnitude of biochar's GHG reduction potential and the mechanisms driving these effects can help inform biochar modeling efforts, explain field results and identify agricultural applications that maximize biochar's full GHG mitigation potential.