Diverging patterns at urban-rural forest grandients:biological nitrogen fixation responses to N addition

The process of rapid urbanization is a global phenomenon that exposes the natural landscape to high levels of nitrogen (N) deposition, leading to serious ecological consequences. How does soil biological nitrogen fixation (BNF), a crucial process for replenishing nitrogen in terrestrial ecosystems, respond to urban expansion and consequent N deposition? To address this, we established a transect from the city center to the countryside to capture the urban–rural gradient. Permanent sites were set up along the gradient to encompass urban, suburban, and remote rural forests. A 5-year large-scale field experiment of N addition was conducted to simulate N deposition, aiming to investigate the impact of N deposition on soil BNF along this urban-rural gradient. We found that soil BNF activity was drastically reduced (dropped by 94.2% to 96.5%) in both urban and suburban forests compared with the rural forest without additional N application. Nitrogen addition treatments had no effect on BNF activity in the urban forest, but significantly decreased BNF activity in the rural forest by over 50% with low N addition. Further analysis revealed that reductions in BNF activity were associated with changes in the composition of diazotrophic communities, favoring facultative diazotrophs that are detrimental to soil BNF. Soil acidification was primarily responsible for limiting soil BNF and associated microbes in the urban forest. Overall, our findings indicate that external N inputs primarily pose a threat to soil diazotrophic communities and their N fixation capacity in rural forests, whereas this adverse effect is not persisted in urban forests, thereby improving our understanding of soil behavior and biogeochemical cycles in forest landscapes.