201.
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.
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