Phylogenetic and functional diversity consistently increase engineered ecosystem functioning under nitrogen enrichment: The example of green roofs

Recent studies have explored changes in biodiversity and ecosystem function relationships in natural ecosystems under global climatic changes. However, little is known about whether these patterns and regulatory mechanisms can be applied to engineered ecosystems. On a green roof ecosystem, one of the most rapidly developing engineered ecosystems in recent years, we manipulated species richness and N addition, coupled with phylogenetic and functional analyses, to explore the determinants of green roof ecosystem function. Our results suggested that low N enrichment strengthened the positive relationships between species richness and ecosystem function. Species richness increased ecosystem function mainly by increasing phylogenetic diversity and functional dispersion of aboveground and belowground traits (niche complementarity effect). N enrichment directly increased ecosystem function, while it decreased ecosystem function through its negative effects on community weighted means of aboveground and belowground traits (mass ratio effect). Additionally, aboveground and belowground traits affected ecosystem function through different ways, and belowground traits played more direct roles than aboveground traits. Our findings emphasize that increasing the phylogenetic diversity (PD) and functional dispersion (FD) of traits could consistently improve ecosystem performance, irrespective of N enrichment. This study provides new insights into the relationships between biodiversity and ecosystem function for engineered ecosystems under global climatic changes, which can help practitioners optimize the design and management of engineered ecosystems.