Parameters influencing green roof carbon and nutrient stocks

Increasing urbanization brings along problems such as elevated CO₂ emissions, eutrophication, air and water pollution, floods, rising temperature and a decrease in biodiversity. Urban green infrastructures, such as green roofs, have the potential to help mitigate those by using the properties of natural ecosystems and the services they provide in a “engineered” way. Green roofs can for example act as buffers and filters for carbon (C), nutrients, such as nitrogen (N) and phosphorus (P), and water. Hereby improving CO₂ concentrations in the atmosphere by capturing it in plant biomass and improving eutrophication by retaining some dissolved organic carbon (DOC) and mineral N and P in the substrate.

In this research we determine which green roof properties affect the C, N and P cycle in a beneficial way. Therefore, we investigate the influence of different parameters (i.e. roof age, roof size, vegetation type (Sedum and herbs vs. Sedum-only), fertilization, substrate depth, substrate water content, substrate bulk density, substrate pH, plant biomass, plant C/N ratio, N mineralization and nitrification) on the C and nutrient stocks of green roofs. We hypothesize that vegetation type and roof age will be the main factors influencing the C and nutrient stocks. A roof with Sedum and herbs will have a higher nutrient and C input resulting in higher stocks compared to a Sedum-only roof because herbs have a higher turn-over rate compared to Sedum-species. Furthermore, older roofs will stock more C. In the beginning C will be mainly sequestered in plant biomass until the roof is densely covered. Here after, green roofs will be able to build up an organic matter layer if the net primary production exceeds decomposition.

To assess the influence of these parameters on the C, N and P stocks, twelve extensive green roofs were investigated in Belgium. The substrate and vegetation of every roof was sampled at four timepoints (spring, summer and autumn of 2019, winter 2020). Substrate samples were analyzed for stocks (total C, total N, total P) along with other abiotic soil parameters as well as some key soil processes (N mineralization and relative nitrification) for soil fertility.

Our first findings show, as expected, that roofs with Sedum and herbs have an increased total C, N and P in their substrate. In addition, C and P stocks are significantly influenced by roof age: while P stocks slightly decrease over time, C stocks only increased transiently —against our predictions—, with a peak at around 9 years old.