Nitrogen cycling players and processes in green roof ecosystems

•Improving green roof nitrogen (N) retention will help reduce N runoff and fertilizer use.•Nitrate runoff and N2O losses from several green roofs suggest active nitrification.•Ammonia oxidizing (AO) archaeal sequences were much more prevalent than AO bacteria.•Bacterial orders associated with root nodulation and N fixation were common.•Use of N-fixing plants or nitrification inhibitors may improve green roof N retention.