Biochar as soil amendment: Syngas recycling system is essential to create positive carbon credit.

Abstract Biochar utilization is accepted as the most cost-effective practice to mitigate global warming via increase in soil C stock. However, its utilization effect on greenhouse gas (GHG) fluxes was evaluated only within land application without considering industrial processes. To evaluate the net effect of biochar utilization on global warming within whole system boundary, swine manure-saw dust mixture was pyrolyzed under four different temperatures, and GHG fluxes were characterized under with/without syngas recycling systems. To determine GHG fluxes from biochar amended soil, 40 Mg ha−1 of biochar was mixed with soil and incubated under the flooded and the dried soil conditions. Finally, the effect of biochar utilization was generalized using net global warming potential (GWP) from industrial process to land application. Under without syngas recycling system, huge amounts of GHGs were emitted during pyrolysis, and GHG fluxes highly increased with increasing pyrolysis temperature, due to direct and indirect GHG emissions from feedstock combustion and electricity, respectively. However, syngas recycling system removed most of GHGs, except for direct N2O and indirect GHG emissions from electricity. Biochar application was very effective to mitigate GHG emissions within soil system boundary, and biochar produced at higher pyrolysis temperature showed higher effectivity in decreasing GHG fluxes. Within the whole system boundary from pyrolysis to soil application, without the installation of syngas recycling system, raw manure application was more effective than biochar to reduce GHG emissions, regardless of soil water conditions. However, with the installation of syngas recycling system, biochar application was much more effective than fresh manure to decrease GHG fluxes. Biochar produced at higher temperature had higher effectivity to mitigate global warming impacts. In conclusion, to functionally mitigate global warming in soils, biochar should be produced in pyrolysis reactors equipped with syngas recycling system under higher temperature.