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

Authors: BORCHARD, NILS - University Of Bonn; Spokas, Kurt; PROST, KATHARINA - University Of Bonn; SIEMENS, JAN - University Of Bonn

Submitted to: European Journal of Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2013
Publication Date: 5/9/2014
Citation: Borchard, N., Spokas, K.A., Prost, K., Siemens, J. 2014. Greenhouse gas production in mixtures of soil with composted and noncomposted biochars is governed by char-associated organic compounds. European Journal of Soil Science. 43(3):971-979.

Interpretive Summary: A potential abatement strategy to increasing atmospheric levels of carbon dioxide (CO2) is to sequester atmospheric CO2 into a more stable form through the use of pyrolysis. Biomass feed stocks are used to generate a more stable carbon form (biochar) that is returned to the soil sequestering atmospheric carbon into a slower cycling pool, resulting in alterations in plant growth/yield and microbial soil processes. However, the mechanisms behind the “biochar effect” have not been fully elucidated. In the present work, we have examined the impacts of pre-composting the biochar prior to soil application, specifically in regards to the sorbed volatile organic compound fingerprints and the results impact on greenhouse gas emissions. After composting, both biochars significantly sorbed additional organic C and N during the composting operation. Composting also decreased the sorbed organics present on the original biochar. When this composted biochar was added to soil, there were three significant alterations observed in the soil system response. First, the composting reduced the methane oxidation capacity approximately in half compared to the un-composted biochars. Secondly, there was no increase in the observed production of CO2 of the composted biochar compared to the uncomposted biochar, indicating that the sorbed organic C was not immediately available. Lastly, the composted biochars stimulated N2O production (as high as 4.4 times) indicating that the sorbed N compounds were microbial available. These findings could provide additional insight and direction in the focus of the benefits of biochar additions on plant and soil microbial communities. These results are significant to farmers and policy makers and will assist scientists and engineers in developing improved biochars based on properties to minimize greenhouse gas implications and improve soil carbon management.

Technical Abstract: 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.