Carbon Sequestration Strategies in Soil Using Biochar: Advances, Challenges, and Opportunities

Biochar, a carbon (C)-rich material obtained from thethermochemicalconversion of biomass under oxygen-limited environments, has beenproposed as one of the most promising materials for C sequestrationand climate mitigation in soil. The C sequestration contribution ofbiochar hinges not only on its fused aromatic structure but also onits abiotic and biotic reactions with soil components across its entirelife cycle in the environment. For instance, minerals and microorganismscan deeply participate in the mineralization or complexation of thelabile (soluble and easily decomposable) and even recalcitrant fractionsof biochar, thereby profoundly affecting C cycling and sequestrationin soil. Here we identify five key issues closely related to the applicationof biochar for C sequestration in soil and review its outstandingadvances. Specifically, the terms use of biochar, pyrochar, and hydrochar,the stability of biochar in soil, the effect of biochar on the fluxand speciation changes of C in soil, the emission of nitrogen-containinggreenhouse gases induced by biochar production and soil application,and the application barriers of biochar in soil are expounded. Byelaborating on these critical issues, we discuss the challenges andknowledge gaps that hinder our understanding and application of biocharfor C sequestration in soil and provide outlooks for future researchdirections. We suggest that combining the mechanistic understandingof biochar-to-soil interactions and long-term field studies, whileconsidering the influence of multiple factors and processes, is essentialto bridge these knowledge gaps. Further, the standards for biocharproduction and soil application should be widely implemented, andthe threshold values of biochar application in soil should be urgentlydeveloped. Also needed are comprehensive and prospective life cycleassessments that are not restricted to soil C sequestration and accountfor the contributions of contamination remediation, soil quality improvement,and vegetation C sequestration to accurately reflect the total benefitsof biochar on C sequestration in soil.