Nitrogen cycling players and processes in green roof ecosystems

Green roofs typically consist of an engineered soil paired with drought-tolerant plants installed on a rooftop. These emerging ecosystems are installed for the services they provide, which in many cases depend on plant performance. In terrestrial ecosystems, nitrogen (N) is most often the limiting nutrient for plant productivity and its availability is heavily influenced by soil microorganisms. However, little is known about the green roof microbiome or their influence on N availability. We employed 16S rRNA and nifH gene sequencing to assess the presence of nitrifying and nitrogen-fixing prokaryotes and their relationships to evidence of nitrification (runoff fluxes of nitrate and gaseous fluxes of nitrous oxide) and N fixation (presence of plants with symbiotic nitrogen-fixers) on 6 green roofs near Cincinnati, Ohio, USA. Our data indicate that green roofs in this region are a net source of reactive N: the majority of roofs released elevated nitrate levels in runoff relative to precipitation inputs, and also emitted N₂O, although at low rates comparable to those in natural ecosystems. We also recovered abundant nifH genes affiliated with Rhizobiales and observed root nodules on leguminous plants on 5 of the 6 studied roofs, together indicating active N fixation. Our findings suggest N loss from green roofs could be alleviated by applying nitrification inhibitors, and that fostering plant-microbe N fixing communities could reduce the need for fertilizer amendments. Our data also highlight the role of microbial N cycling on green roofs and underscore the need for further work to quantify N-processing rates.