Diurnal greenhouse gas emissions and substrate temperatures from green roofs in north-eastern Italy during summer season

Covering building rooftops with vegetation [Green roofs (GR)] holds promise for lowering building temperatures, reducing stormwater runoff, and other ecosystem services, but it is unclear how this will impact greenhouse gas (GHG) emissions. GHG emissions may also be influenced by vegetation type, substrate depth, and irrigation level and we sought to test this by comparing daytime greenhouse gas (GHG) emissions (CH₄, CO₂, and N₂O) and daily temperatures, from 48 GR microcosms in North-eastern Italy during a dry-hot summer season (June to September). The microcosms were planted with Sedum spp., cold season grasses, warm season grasses, or wildflowers to a substrate depth of 8 cm or 14 cm, and with an average irrigation level of 1 or 2 mm d^-1 for a total of 16 treatments with 3 replicates. We found that vegetation type had a significant effect on temperature [median temp. of 24.8 °C (Sedum) vs 25.5 °C (warm season grasses)] and CH₄, CO₂, and N₂O emissions. While all species emitted net CO₂ (median values from 147 to 671 mg m^-2  h^-1) and captured net N₂O (median values were negative from -0.06 to -0.28 mg m^-2  h^-1)— highlighting a potential GR ecosystem service with a beneficial effect on the environment— CH₄ had net negative values (capture) only in microcosms with wildflowers (-0.07 mg m^-2  h^-1) whereas other treatments had a median emission of 0.09 mg m^-2  h^-1 . Substrate depth significantly affected CO₂ and N₂O fluxes with deeper substrate leading to higher CO₂ emission (+ 60.7%) and greater N₂O uptake (+ 30.8%). Irrigation level only significantly influenced N₂O fluxes with a significantly higher uptake supplying 2 mm  (-0.20 mg m^-2  h^-1) than in the 1 mm  (-0.09 mg m^-2  h^-1) irrigation treatments. Our study suggests that, in a hot summer season, GRs can improve N₂O and CH₄ capture, but will likely increase CO₂ emissions and that vegetation type and substrate depth can significantly alter emissions and are thus important design parameters.