Impacts of Woodchip Biochar Additions on Soil Carbon Net, CH4 Oxidation and Sorption/Degradation of Two Herbicides in a Minnesota Soil

Authors: Spokas, Kurt; Koskinen, William; Baker, John; Reicosky, Donald

Submitted to: Chemosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/22/2009
Publication Date: 7/31/2009
Citation: Spokas, K.A., Koskinen, W.C., Baker, J.M., Reicosky, D.C. 2009. Impacts of Woodchip Biochar Additions on Soil Carbon Net, CH4 Oxidation and Sorption/Degradation of Two Herbicides in a Minnesota Soil. Chemosphere. 77(4):571-581.

Interpretive Summary: Elevated atmospheric carbon dioxide, potential global warming concerns and prospective use of soil as a sink for carbon has attracted interest from farmers and land managers. The recent potential of converting biomass into charcoal represents one potential mechanism to reduce the atmospheric carbon dioxide levels by returning this carbon back to the soil to increase soil fertility. This charcoal is more stable than the original biomass residues, thus sequestering the atmospheric carbon dioxide into a more stable form. This research examined the impacts of charcoal amendments (derived from woodchips) on carbon dioxide production, methane oxidation, as well as the impact of charcoal on the sorption and degradation of two common herbicides (acetochlor and atrazine). The results of this research observed increased levels of carbon dioxide production resulting from charcoal additions as well as a reduction in the observed methane oxidation potentials. The sorption of the two herbicides was increased and dissipation rates were decreased as a consequence of the charcoal addition. These impacts on the herbicides could reduce herbicide efficacy in charcoal amended soil. These results are significant to farmers and policy makers in that woodchip charcoal appear to sequester carbon into a form that is more resistant to microbial degradation. This information will assist scientists and engineers in developing improved mechanisms of biochar additions to minimize the gaseous loss and to improve soil carbon management.

Technical Abstract: A potential abatement to increasing levels of carbon dioxide (CO2) in the atmosphere is the use of pyrolysis to convert vegetative biomass into a more stable form of carbon (biochar) that could then be applied to the soil. However, the impacts of pyrolysis biochar on the soil system need to be assessed before initiating large scale biochar applications to agricultural fields. We compared CO2 respiration, nitrous oxide (N2O) production, methane (CH4) oxidation, and herbicide retention and transformation through laboratory incubations at field capacity in a Minnesota soil (Waukegan silt loam) with and without added biochar. CO2 originating from the biochar needs to be subtracted from the soil-biochar combination in order to elucidate the impact of biochar on soil respiration. After this correction, biochar amendments reduced CO2 production for all amendment levels tested (2, 5, 10, 20, 40 and 60% w/w; corresponding to 24 t ha-1 to 720 t ha-1 field application rates). In addition, biochar additions suppressed N2O production at all levels. However, these reductions were only significant at biochar amendment levels >20% w/w. Biochar additions also significantly suppressed ambient CH4 oxidation at all levels compared to unamended soil. The addition of biochar (5% w/w) to soil increased the sorption of atrazine and acetochlor compared to non-amended soils, resulting in decreased dissipation rates of these herbicides. The recalcitrance of the biochar suggests that it could be a viable carbon sequestration strategy, and might provide substantial net greenhouse gas benefits if the reductions in N2O production are lasting.