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GCB Bioenergy

Cover image for Vol. 8 Issue 5

Edited By: Steve Long

Impact Factor: 6.151

ISI Journal Citation Reports © Ranking: 2015: 1/83 (Agronomy); 9/88 (Energy & Fuels)

Online ISSN: 1757-1707

Associated Title(s): Global Change Biology

Biochar weathering alters soil greenhouse gas production


Impact of forest biomass-based electricity generation

Biomass can be converted to bioenergy by pyrolysis, a type of chemical–thermal conversion. Compared to fossil fuels, bioenergy use mitigates climate change by reducing carbon emissions and sequestering carbon (i.e. capturing atmospheric carbon dioxide and storing it long-term). Use of biochar, the solid carbon-rich product of pyrolysis, also has the potential to mitigate climate change. When incorporated into the soil, biochar has the potential to sequester carbon and decrease greenhouse gas (GHG) emission from soils. However, observations of the effects of biochar on net soil GHG production/ emission have been variable, with some biochars suppressing while other biochars stimulate or have no significant effects on GHG production. In addition, the mechanisms behind this biochar-mediated response are complex and have not been fully explained.

In this study, Spokas evaluates the impacts of biochar field aging (e.g. weathering) on observed GHG production/consumption. Hardwood, wood pellet, and macadamia nut shell-derived biochars were applied to an agricultural field and incorporated into the soil by tilling. Three years after incorporation into the soil, the weathered biochar was recovered and analysed.
The elemental composition, pH and microscopic images of weathered and unweathered biochar were assessed. The impacts of weather and unweather biochar application on net soil GHG production/consumption were assessed through laboratory incubations. GHGs considered were nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2).

The author found that while fresh biochar additions supressed N2O production following incorporation, weathering of biochar did not affect N2O production. Amending the soil with fresh biochar samples suppressed soil CH4 oxidation (i.e. increased CH4 emissions), while amending the soil with weathered biochar did not affect CH4 emissions. Fresh biochar amendments had no effect on CO2 production, while weathered biochars enhanced CO2 production.

This study demonstrates that for these three biochars, weathering greatly alters the GHG response of the soil systems to biochar amendments. Of particular importance is the increased N2O production of soil amended with weathered biochar. These results cast doubt on the long-term duration of the mitigation of soil N2O emissions by biochar additions.

Spokas KA (2013) Impact of biochar field aging on laboratory greenhouse gas production potentials. GCB Bioenergy, 5, 165–176. doi: 10.1111/gcbb.12005 Read this paper

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