Ecosystem engineering, such as green roof, provides numerous key ecosystem functions dependent on both plants and environmental changes. In the recent years, global nitrogen (N) deposition has become a hot topic with the intensification of anthropogenic disturbance. However, the response of green roof ecosystems to N deposition is still not clear. To explore the effects of N addition on plant ecological strategy and ecosystem functioning (biomass), we conducted a 3-month N addition simulation experiment using 12 common green roof species from different growth forms on an extensive green roof in Tianjin, China. The experiment included three different N addition treatments (0, 3.5, and 10.5 gN m–2 year–1). We found that plants with the resource-acquisitive strategy were more suitable to survive in a high N environment, since both aboveground and belowground traits exhibited synergistic effects. Moreover, N addition indirectly decreased plant biomass, indicating that ecosystem functioning was impaired. We highlight that there is a trade-off between the survival of green roof species and keeping the ecosystem functioning well in the future N deposition. Meanwhile, these findings also provide insights into how green roof species respond to global climate change and offer important information for better managing and protecting similar ecosystem engineering in the background of high N deposition.

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Aims Nitrogen (N) is one of the limiting nutrients for plant growth in terrestrial ecosystems. Numerous studies that have explored the effects of N addition on the eco-physiological traits and biomass production of plants, but the underlying mechanism of N deposition on biomass allocation has not been clarified, especially for urban greening trees. Methods A greenhouse simulated experiment was conducted by two dominating urban street trees in North China, including conifer Pinus tabuliformis and broadleaved Fraxinus chinensis. We set up three levels of N addition: ambient, low N addition, and high N addition (0, 3.5, and 10.5 gN m− 2 year− 1) and determined the biomass distribution, plant functional traits, and soil nutrient traits of the two trees. Results Our results showed that N addition had positive effects on the aboveground and belowground biomass of P. tabuliformis, which also shifted its functional traits to fast. While F. chinensis only increased root biomass distribution and root acquisitive traits as N increased. Furthermore, N addition increased the soil N and phosphorus contents of both two trees and improved the root nutrient uptake capacity, resulting in the increase of root-shoot ratio. We found that optimal partitioning theory could better explain that trees would invest more resources in roots in the poor-resource area. Conclusion Trees changed their root structure and increased root biomass allocation to adapt to the high N deposition environment. Our findings highlight the importance of plant functional traits in driving the responses of biomass allocation to environmental factors for urban greening-dominated tree species.