Critical evaluation of biochar utilization effect on mitigating global warming in whole rice cropping boundary

Biochar and compost were accepted as a stable organic amendment to increase soil C stock as well as to decrease greenhouse gas (GHG) emissions in rice paddy soils. However, in most studies, their effect on GHG flux was evaluated only within the cropping boundary without considering industrial processes. To compare the net effect of these organic amendment utilizations on global warming within the whole rice cropping system boundary from industrial process to cropping, fresh, compost, and biochar manures were applied at a rate of 12 Mg ha⁻¹ (dry weight) in a rice paddy, and total GHG fluxes were evaluated. Compared with fresh manure, compost utilization decreased net global warming potential (GWP) which summated GHG fluxes and soil C stock change with CO₂ equivalent by 43% within rice cropping boundary, via a 25% decrease of CH₄ flux and 39% increase of soil C stock. However, 34 Mg CO₂-eq. of GHGs were additionally emitted during composting to make 12 Mg of compost and then increased the net GWP by 34% within the whole system boundary. In comparison, biochar changed paddy soil into a GHG sink, via 56% decrease of CH₄ flux and 13% increase of soil C stock. However, pyrolysis emitted a total of 0.08 and 19 Mg CO₂-eq. of GHGs under with and without syngas recycling system, respectively, to make 12 Mg of biochar. As a result, biochar utilization decreased net GWP by approximately 28–70% over fresh manure within the whole system boundary. Rice grain productivity was not discriminated between biochar and compost manures, but compost considerably increased grain yield over fresh manure. Consequently, biochar utilization significantly decreased GHG intensity which indicates net GWP per grain by 33–72% over fresh manure, but compost increased by 22%. In conclusion, biochar could be a sustainable organic amendment to mitigate GHG emission impact in the rice paddy, but compost should be carefully selected.