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Agricultural soils represent the main source of anthropogenic N2O emissions. Recently, interactions of black carbon with the nitrogen cycle have been recognized and the use of biochar is being investigated as a means to reduce N2O emissions. However, the mechanisms of reduction remain unclear. Here we demonstrate the significant impact of biochar on denitrification, with a consistent decrease in N2O emissions by 10–90% in 14 different agricultural soils. Using the 15N gas-flux method we observed a consistent reduction of the N2O/(N2 + N2O) ratio, which demonstrates that biochar facilitates the last step of denitrification. Biochar acid buffer capacity was identified as an important aspect for mitigation that was not primarily caused by a pH shift in soil. We propose the function of biochar as an “electron shuttle” that facilitates the transfer of electrons to soil denitrifying microorganisms, which together with its liming effect would promote the reduction of N2O to N2.

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The aim of this study was to evaluate the use of biochar (produced by slow pyrolysis of Eucalyptus grandis biomass) as bulking agent for the composting of poultry manure. Three composting mixtures were prepared by the turned-pile system by mixing poultry manure with different organic wastes used as bulking agent (biochar, coffee husk and sawdust) in a proportion of 1:1 (fresh weight). Despite the inert nature of biochar, the composting mixture prepared with biochar underwent an organic matter degradation of 70% of the initial content. The organic matter of the poultry manure-biochar mixture was characterised by a high polymerisation degree of the humic-like substances, with a relative high proportion of humic acids in relation to fulvic acids. At the end of the composting process, the humic acid fraction represented more than 90% of the alkali extractable fraction, reflecting the intense humification of this material. Enrichment of poultry manure with biochar reduced the losses of nitrogen in the mature composts, although the use of sawdust would be more efficient in preserving the organic matter and nitrogen in the mature compost.

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