As an important measure of the sponge city, green roofs have received extensive attention in recent years. To investigate the inter-annual changes in runoff quality of green roofs with different vegetation types, three green roofs with different vegetation cover (Sedum lineare, Portulaca grandiflora, and a non-vegetated control) were set up in Beijing. The influences of vegetation and monitoring period on runoff quality from the green roofs were evaluated using the plant growth characteristics and the quality of rainwater and runoff from the green roofs during the rainy season of 2017-2019. The results showed that all three green roofs were the sinks of NH4+-N, and the average mass concentration reduction rates were between 50.1% and 79.2%. However, all three green roofs were sources of PO43--P, DCr, DCu, and DNi. The green roofs covered with S. lineare and P. grandiflora were sinks of NO3--N in 2017, and the average mass concentration reduction rates were 71.4% and 99.5%, respectively, but they became sources of NO3--N in both 2018 and 2019. However, the non-vegetated control was the source of NO3--N in all three rainy seasons. Both vegetation type and length of monitoring period had significant effects on the mass concentrations of NO3--N, PO43--P, DNi, and DCu in runoff from the green roofs (P<0.05) but had no significant effects on the mass concentrations of NH4+-N and DCr in runoff from the green roofs (P>0.05). In 2017-2019, the mass concentrations of NO3--N in runoff from the non-vegetated control and the green roofs covered by S. lineare and the mass concentration of PO43--P in runoff from the green roof covered by P. grandiflora increased yearly. The mass concentrations of DNi and DCu in runoff from all three green roofs increased in 2018 but dropped in 2019. Among the green roofs with different vegetation types, the green roof covered by P. grandiflora showed better NO3--N retention capacity than that of the other green roofs but may have increased the concentrations of PO43--P, DNi, and DCu in the runoff.
Keywords: green roofs; heavy metals; inter-annual changes; nutrients; runoff quality; vegetation.