Greenhouse gases and green roofs: carbon dioxide and methane fluxes in relation to substrate characteristics

Green roof systems have been increasingly implemented to enhance vegetation cover and associated ecosystem services in urban spaces, with primary goals being the reduction of peak surface runoff, enhanced water quality, and mitigation of urban heat island effects. Recently, green roofs have also received attention as a means to enhance carbon sequestration, but direct measurements of greenhouse gas fluxes from established green roof systems are largely lacking. Here we present observations of CO₂ and CH₄ fluxes from substrates of experimental extensive green roof units that varied in vegetation type (Sedum spp., and a native meadow species mix), substrate depth, substrate type (high vs. low organic matter content), and irrigation. We predicted that substrate CO₂ effluxes would be higher in high-organic-matter substrates and that systems with high organic matter would potentially act as CH₄ sources. Substrate fluxes were low compared to natural soils, with seasonal means ranging from an efflux of 0.1–0.4 µmol CO₂ m^-2.s^-1 and uptake of ~0.00–0.04 nmol CH₄ m^-2.s^-1, with higher fluxes late in the growing season. CO₂ fluxes showed large increases in response to irrigation and were higher from the high-organic-matter substrate and with increased substrate depth. The strength of the CH₄ sink increased in response to prior irrigation treatments, and CH₄ emissions were detected only on low-organic-matter substrates early in the growing season. No effects of vegetation type were detected for either CO₂ or CH₄ flux. Our results indicate that high levels of organic matter in green roof substrates may enhance aerobic soil respiration but are not associated with CH₄ emissions, which instead were only detected in low-organic-matter substrates.