District ammonium-to-nitrate ratios change soil N dynamics and shape inverse patterns of resource acquisition strategy and biomass production of four urban greening trees.

Ammonium nitrogen (NH) and nitrate nitrogen (NO), the primary soil accessible nitrogen (N) forms for most plants, can affect plant ecophysiology and biomass production in different ways. Plants typically exhibit varying capacities for uptake and assimilation of the two N forms, leading to differences in the ecological strategies and niches within ecosystem. Recently, variations in atmospheric NH/NO deposition have severely threatened plant growth and ecosystem functions, especially in urban green spaces. Therefore, a pot experiment was carried out to explore the response of four common urban greening tree species (Pinus tabulaeformnis, Juniperus chinensis, Fraxinus chinensis, Sophora japonica) in North China, to five NH/NO addition treatments. Our results indicated that trees could adapt to varying soil N environments by modifying their resource acquisition strategies and biomass production, where the response patterns depended on the species specificity. High NH/NO addition increased soil urease activity and NH/NO ratios planted to coniferous trees, enhancing the plant fast traits highly coordinated across different organs, which promoted the plant growth. While broadleaved tree species exhibited stronger plasticity under NO-rich conditions. Reduced NH/NO ratios increased soil NO and available phosphorus availability, improving their resource acquisition capacity and root nitrate reductase activity, which favored NO utilization and biomass production. Overall, this study highlights the importance of plant resource acquisition strategy in driving the responses of biomass production to soil N dynamic changes and puts forward a new growth strategy for urban greening tree species in the plant-soil feedback system.