Greenhouse gas fluxes from green roof substrates and biochar as a mitigation tool

Abstract:

Green roofs (GRs) are an emerging approach to enhance climate resilience of urban areas, including mitigation of stormwater discharge and urban heat island effects. There has also been recent interest in GRs to enhance C sequestration, but direct measurements of greenhouse gas (GHG) fluxes from established GR systems are lacking. To better understand the relationships between GR vegetation and substrate with GHG fluxes, we measured CO₂ and CH₄ fluxes, using a dynamic closed chamber technique, from substrates of experimental GR units in Toronto in the early, mid, and late growing seasons of 2018. This measurement included extensive systems with mixes of Sedum species and intensive systems using a native species mix, with both vegetation types implemented in a factorial design with substrate depth and type (high vs. low organic matter [OM] content) as factors. We hypothesized that CO₂ effluxes would be higher in high OM substrates, and that systems with low vegetation cover and high OM would act as CH₄ sources. Substrate CO₂ efflux varied from 0.1 – 0.51 µmol m^-2s^-1, with pronounced differences in flux rates related to substrate type and depth (higher with increased substrate depth) but had little effect of vegetation type. Substrate CH₄ fluxes included both net efflux and uptake (-0.1 to 0.14 nmol m^-2s^-1). Net uptake was found to be associated with high vegetation cover, particularly of native species. Results suggest that high OM in GR substrates, while enhancing plant growth, can result in appreciable C losses, partly as CH₄.

Studies have explored various GR substrates to enhance C sequestration of GRs. Biochar, usually derived from pyrolyzed waste biomass, can be a potential amendment to GR substrates to mitigate C loss. Recalcitrant biochars can increase soil water holding capacity and plant growth, and also can suppress soil GHG fluxes in managed systems. However, the biochar effects on GHG fluxes of GR substrates have not previously been evaluated. In a similar experimental setup as above, we examined the effects of biochar addition (20 t.ha^-1, sugar maple biochar) to a high-OM GR substrate. Initial results indicate that biochar addition can significantly reduce C loss from GR, mainly by suppressing CH₄ effluxes. Overall, our studies suggest that biochar can be a sustainable amendment to GR substrates to mitigate C loss.