Nutrient Cycling in Green Roof Ecosystems

In this chapter we consider the cycling of Carbon (C), Nitrogen (N) and Phosphorus (P) in green roof ecosystems. The focus is placed primarily on N and P because these are the nutrients most often limiting to plant growth in terrestrial ecosystems, and because leaching of these elements to downstream aquatic ecosystems is a concern due to their potential to contribute to eutrophication. Extensive green roofs are commonly sources of phosphorus and dissolved organic carbon in runoff, while they may be either a source or a sink for nitrogen. Plant communities, substrate characteristics, substrate depth, and roof age all play a role in regulating nutrient export. Seasonal variation in runoff nutrient concentrations suggests the importance of temperature and light-mediated processes. Nitrogen leaching may drop off rapidly with the age of the ecosystem and vary with new inputs (atmospheric deposition of N, new fertilizer additions), while roofs leach out P for years or decades under current construction regimes, likely resulting from mineralization of P-rich organic matter in the roof substrate. Conceptual models of nutrient cycling developed from natural terrestrial ecosystems provide a useful starting point for interpreting the important nutrient cycling processes on green roofs. However, the engineered nature of green roof ecosystems, often with a high-nutrient substrate coupled to plants adapted to low-nutrient, extreme environments, gives rise to unique characteristics. There is still little known of the dynamics of important processes for recycling of nutrients within green roof ecosystems, and more studies which include modeling, full roof-scale experiments, and long-term monitoring are needed for improved understanding of these ecosystems.