By virtue of the large fraction of the terrestrial carbon (C) cycle controlled by human activities, agroecosystems are both sources and sinks for greenhouse gases. Their potential role in mitigation of climate change thus depends on a dual strategy of decreasing greenhouse gas emissions while increasing sinks so that the net impact on climate warming is less than at present. Emissions of carbon dioxide, methane and nitrous oxide arise from various agricultural activities, ranging from land clearing to ploughing, fertilization, and animal husbandry. Reductions in these emissions can be achieved by decreasing the heterotrophic conversion of organic C to carbon dioxide, and by better management of agricultural waste streams to minimize release of methane and nitrous oxide. Current sinks include C stored in standing biomass and soil organic matter, and the oxidation of atmospheric methane by soil bacteria. These sinks can be enhanced by increasing net primary productivity, thereby actively withdrawing more carbon dioxide from the atmosphere, and by promoting more oxidation of methane by soils. Judicious biochar management may contribute to both strategies, reductions of emissions by agriculture and active withdrawal of atmospheric carbon dioxide, as part of a comprehensive scheme in agricultural and forestry watersheds. Biochar is amore » carbon-rich organic material generated by heating biomass in the absence, or under a limited supply, of oxygen. This so-called charring or pyrolysis process has been used to produce charcoal as a source of fuel for millennia. Recently, interest has grown in understanding the potential of this process to improve soil health by adding biochar as an amendment to soil, to manage agricultural and forestry wastes, to generate energy, to decrease net emissions of nitrous oxide and methane, and to store carbon (C). The main incentive of biochar systems for mitigation of climate change is to increase the stability of organic matter or biomass. This stability is achieved by the conversion of fresh organic materials, which mineralize comparatively quickly, into biochar, which mineralizes much more slowly. The difference between the mineralization of uncharred and charred material results in a greater amount of carbon storage in soils and a lower amount of carbon dioxide, the major greenhouse gas, in the atmosphere. The principle of creating and managing biochar systems may address multiple environmental constraints. Biochar may help not only in mitigating climate change, but also fulfill a role in management of agricultural and forestry wastes, enhancement of soil sustainability, and generation of energy. Pyrolysis is a comparatively low-technology intervention. Deployment on a global scale, however, must be done carefully if the full mitigation potential is to be reached. Critical aspects of a successful implementation are that: 1) the biochar is sufficiently stable to reduce greenhouse gases in the atmosphere for an appropriate length of time. 2) the storage of carbon as biochar in soil is not offset by greenhouse gas emissions along the value chain of the system, such as mineralization of soil carbon or emissions of other greenhouse gases (e.g., methane and nitrous oxide). 3) net emission reductions are achieved for the entire life cycle of the system including indirect land use. 4) the biochar product does not cause unwanted side effects in soil. 5) the handling and production of biochar are in compliance with health and safety standards and do not pose hurdles to implementation. and 6) the biochar system is financially viable. This chapter discusses these issues in separate sections, identifies knowledge gaps, and proposes a road map to fully evaluate an environmentally and socially safe exploration of the biochar potential to mitigate climate change if adopted widely around the world.« less

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