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Greenhouse gas production in mixtures of soil with composted and noncomposted biochars is governed by char-associated organic compounds

Borchard, Nils, Spokas, Kurt A., Prost, Katharina, Siemens, Jan
European journal of soil science 2014
animal manures, aromatic hydrocarbons, biochar, carbon dioxide, carbon sequestration, composting, emissions, gasification, greenhouse gases, hardwood, methane, nitrous oxide, organic matter, oxidation, soil, soil amendments, soil productivity, soil treatment, straw, volatile organic compounds
Biochar application to soil has the potential to increase soil productivity while reducing anthropogenic net greenhouse gas (GHG) emissions to the atmosphere by sequestering carbon that has been assimilated by plants in a stabilized form. Techniques for conditioning this material as a soil amendment for maximizing its beneficial effects still require elucidation. This work examined the alteration of sorbed organic matter on two different biochars derived from hardwood during composting and the resulting effect on the net GHG production of biochar-soil mixtures in laboratory incubation experiments. A six months composting of biochar within a mixture of livestock manure and straw did not changed the aromatic content as determined by benzene-carboxylic acid markers, but both biochars sorbed significantly amounts of soluble N (inorganic and organic) and soluble organic C. The process of composting decreased the amount of volatile organic compounds (VOC) that are thermally released from the biochars and affected the molecular nature of these released compounds. Initially, VOCs were dominated by aliphatic hydrocarbons that changed to more aromatic hydrocarbons during composting. Composting of biochars prior to the application to soil resulted in three major observations: i) reduced net methane oxidation to ~50% of net oxidation capacity observed in non-composted biochar-soil mixtures, ii) did not alter observed CO2 production, suggesting that the organic C sorbed by the composted biochars was to a large extend protected against immediate degradation, and iii) increased net nitrous oxide (N2O) emissions by 60% for the gasification coke and by 440% for the charcoal. However, this increased N2O accounted for only a minor fraction of the soluble N sorbed to the composted biochars.