Professor University of Toronto Toronto, Ontario, Canada
Methane (CH4), accounting for ~30% anthropogenic climate forcing, is produced by anaerobic methanogenic microorganisms in soil that metabolize organic molecules into CH4. Organic mulching is a pervasive soil amendment practice in urban forestry intended to enhance soil quality and control weeds. Prior studies indicate a positive correlation between CH4 emission and soil organic matter in peat soils and disturbed soils in operational forestry. Due to compaction common in urban soils, organic mulching could be a potential source of CH4. Biochar has been found to enhance methane oxidation in agricultural and forest soils.
We conducted a field incubation experiment investigating CH4 emission from woodchip and bark mulches added to urban soil, and tested biochar's potential to reduce CH4 emissions. The experiment was a replicated augmented factorial design, with treatments including mulch additions (woodchip, bark, and controls), and biochar (50t/ha vs. controls), with surface vs. mixed applications for each treatment. A total of 68 experimental units were installed in an urban forest setting at University of Toronto’s downtown campus. Measurements of CH4 flux have been conducted monthly during the 2021-2022 growing seasons using an off-axis integrated cavity output spectroscopy gas analyzer and a pressure equilibrated chamber in a closed dynamic configuration.
Our results for the first growing season indicate that surface application of both woodchips and
bark mulches emit CH4 when amended with urban soils, while adding biochar with woodchips
significantly reduced CH4 emissions, though mixed applications gave inconclusive results. In
case of surface application, mean ( se) CH4 emissions in woodchips treatments were 0.91 0.51,
2.82 1.82 and 0.76 0.38 nmol.m-2.s-1 in August to October 2021, respectively, compared to
0.005 0.10, 0.41 0.33, and -0.03 0.24 nmol.m-2.s-1 with added biochar with woodchips. Mean ( se)
emissions of CH4 from bark mulch treatments were 0.81 0.56, -0.22 0.11 and 0.70 0.56 in August
to October 2021, respectively, and the emissions were -0.07 0.09, -0.02 0.06 and 0.46 0.24 when
biochar was applied with bark mulch). Treatment effects for both woodchip and bark mulches
mixed with soils did not show consistent patterns in the first growing season. Overall, our results
suggest that organic mulching, specifically of woodchips, to urban soils can be a significant source
of atmospheric CH4 emission – but that biochar application reduces this emission significantly