How effective are green roofs as building carbon sinks? Empirical evidence linking substrate, depth, and vegetation dynamics in the U.S. Great Plains

Understanding how green roofs store carbon as building envelope systems is critical for combating climate change across diverse bioclimatic regions of the U.S. Great Plains. During a two-year study (2019-2020), we examined the carbon storage capacity of green roof beds with two soil depths (10 and 20 cm) and two substrate materials: substrate R, characterized by low bulk density, high pore space, and low water-holding capacity; and substrate K, which has a dense, sandy composition. Three plant mixes were tested: Sedum only (A), Sedum + native grasses mix (B), and native grasses + forbs (C). Soil analyses included microbial community composition via phospholipid fatty acid analysis, soil organic carbon and nitrogen, microbial respiration, and root biomass. Statistical analyses used mixed models, repeated-measures ANOVA, three-way ANOVA, Tukey's HSD, and Spearman's rank correlation. The findings indicate that substrate R facilitates greater carbon storage per unit volume compared to substrate K, with vegetation dynamics, especially the presence of native grasses, playing a critical role in modulating substrate-driven carbon retention and microbial activity, thereby achieving the highest overall carbon storage. The results suggest that using a shallower green roof substrate layer with substrate type R may provide a more effective approach for climate change mitigation in the U.S. Great Plains. Overall, the results show that the ability of substrates to retain moisture, combined with the depth of the growing medium and vegetation composition-induced root biomass, plays a key role in determining the effectiveness of green roofs as carbon sinks within building systems.