Older, established green roof plant communities are more likely to absorb nutrients from rainfall and media than younger communities. Green roofs can act as either sources or sinks of nutrient runoff. Low to medium dosages of controlled release fertilizers, use of species that require little to no fertilization, using less nutrient-rich organic matter amendments, and reducing irrigation will improve a green roof's ability to act as a nutrient sink.
Collins, Kelly A., Timothy J. Lawrence, Emilie K. Stander, Robert J. Jontos, Sujay S. Kaushal, Tamara A. Newcomer, Nancy B. Grimm, and Marci L. Cole Ekberg. (2010). Opportunities and challenges for managing nitrogen in urban stormwater: A review and synthesis. Ecological Engineering, 36(11), 1507-1519.