Role of Biochar in Mitigation of Climate Change.

Large amounts of carbon are assimilated by plants and mineralized in soils of agricultural, forest and residential landscapes each year. Changes in those large carbon fluxes may have a significant effect on atmospheric carbon dioxide contents and global warming. One opportunity to reduce the relatively rapid return of annual dissimilation of plant material is the conversion of biomass into biochar. Biochar is a charcoal-like material which is produced by heating organic matter under complete or partial exclusion of air. During heating, the organic material undergoes complete chemical change and becomes very recalcitrant to microbial decay. Current estimates of mean residence times of biochars in soils exceed 1000 years. Biochars produced from woody material, grasses or manures do not constitute a disposal risk but create value when applied to soil. Biochar applications to highly weathered soils have in many cases been found to improve crop productivity. And analyses of soils world-wide have shown that biochar-type materials are ubiquitous and already account for a significant proportion of soil organic carbon. The question remains whether biochar production and application can be done in an environmentally friendly way that improves ecosystem functions, human health, and food production and at the same time scales to a significant withdrawal of carbon dioxide from the atmosphere. Several opportunities exist that improve soil and human health while preserving natural resources, biodiversity and promote social justice which are framed around small-scale distributed biochar systems. Indeed, life-cycle analyses of the environmental and economic constraints and opportunities show that large-scale biochar systems based on dedicated feedstock production rather than organic by-products are less likely to be viable due to an unfavorable carbon footprint and high costs. While such opportunities may have an important short- and medium-term role in an overall package of approaches to actively withdraw carbon dioxide from the atmosphere, they are not sufficient to substitute for significant efforts to reduce global emissions from energy consumption.