As ecosystems mature following disturbance, they undergo predictable changes, often including initial increases in soil organic matter and nutrient retention. These initial and medium-term dynamics can depend on disturbance type or severity, but commonly systems progress towards a steady-state condition where nutrient inputs to the ecosystem approximately equal outputs. While these successional changes have been studied in many natural systems, it is unclear if these patterns are to be expected in new, engineered ecosystems such as green roofs where components have not co-developed over time. For example, green roofs, which are composed of a soil-like substrate and drought-tolerant plants installed on a roof, often begin with a nutrient rich substrate that is paired with slow-growing plants adapted to nutrient poor conditions - perhaps resulting in different developmental dynamics than natural ecosystems. We evaluated changes over time in green roof soil nutrient content and export via runoff using two approaches: an evaluation of changes within the first 12 years of green roof ecosystem development following roof installation using 3 intensively sampled green roofs in the Cincinnati, OH, USA region; and an investigation of longer-term changes (1-21 years) using a chronosequence in Malmö, Sweden of similarly designed but different age green roofs.
Results/Conclusions
The relatively young intensively sampled green roofs in the USA exhibited striking seasonal patterns in their nutrient export that were highly correlated with temperature, with maximum nutrient runoff occurring during the growing season. Nitrogen and phosphorus export from these roofs declined with increasing roof age, attributed to a combined effect of their diminishing availability due to runoff losses and uptake by an establishing plant community. The soil depth of the chronosequence roofs in Sweden increased linearly with roof age (R2 = 0.82, P < 0.001), likely indicating a buildup of organic matter over time. Preliminary nutrient export results from the chronosequence roofs indicate much lower initial nutrient levels in the substrate in these systems compared to the intensively sampled roofs, but both roof sets appear to be approaching a steady-state condition for nitrogen where export in runoff is nearing the level of nutrients added via atmospheric deposition. While the chronosequence substrates are currently being analyzed and additional runoff samples are to be collected, these initial dynamics suggest that regardless of the starting conditions, green roofs may follow similar successional changes as those expected for natural terrestrial ecosystems.