Nitrogen addition and drought affect nitrogen uptake patterns and biomass production of four urban greening tree species in North China

Nitrogen (N) is an essential nutrient element limiting plant growth and production, and plant N uptake capacity varies with environmental change. Recently, global climate changes such as N deposition and drought have important impacts on the terrestrial ecosystems, especially for urban greening trees. However, it's still unclear how N deposition and drought affect plant N uptake and biomass production and the underlying relationship between them. Therefore, we conducted a 15N isotope labeling experiment on four common tree species of urban green spaces in North China, including Pinus tabulaeformnis, Fraxinus chinensis, Juniperus chinensis, and Rhus typhina in pots. Three N addition treatments (0, 3.5, and 10.5 gN m −2 year −1; “no”, “low”, and “high” N treatments, respectively) and two water addition treatments (300 and 600 mm year−1; “drought” and “normal water”, respectively) were set up in a greenhouse. Our results showed that N and drought significantly affected tree biomass production and N uptake rates, and the relationship between them depended on the species specificity. Trees could transform their N uptake preference to adapt to the changing environment, from ammonium to nitrate or vice versa, which was also reflected in total biomass. Furthermore, the variation of N uptake patterns was also related to distinct functional traits, including aboveground (specific leaf area and leaf dry matter content) or belowground (specific root length, specific root area, and root tissue density) traits. There was a transformation of plant resource acquisitive strategy in a high N and drought environment. In general, there were tight connections among N uptake rates, functional traits, and biomass production of each target species. This finding comes up with a new strategy that tree species can modify their functional traits and plasticity of the N uptake forms for survival and growth in the context of high N deposition and drought. [Display omitted] •N and drought affected the uptake of NH4+ and NO3− among the four tree species.•Plants adapt to the high N and drought environment by altering N uptake preference.•N uptake changes are also reflected in functional traits and resource acquisition.•N uptake and functional traits explain changes in biomass with N deposition and drought.