Plant nitrogen uptake preference and drivers in natural ecosystems at the global scale.

Elucidating plant nitrogen (N) acquisition is crucial for understanding plant N strategies and ecosystem productivity. However, the variation in plant N uptake preference and its controlling factors on a global scale remain unclear. We conducted a global synthesis to explore plant N preference patterns and driving factors. Globally, the average contributions of ammonium (NH ), nitrate (NO ), and glycine N to the total plant N uptake were 41.6 ± 1.1%, 32.8 ± 1.2%, and 25.6 ± 0.9%, respectively. However, plant N uptake preferences differed significantly among climatic regions and vegetation types. Soil NH was the most preferred N form by plants in (sub)tropical regions, whereas NO preference was significantly higher in high-latitude than low-latitude regions. Plant functional type was one of the most important factors driving NO preference, with significantly higher NO preference of nonwoody species than broadleaf-evergreen, conifer, and shrub species. Organic N preference was lowest in (sub)tropics and significantly lower than that in temperate and alpine regions. This study shows clear climatic patterns and different influencing factors of plant NH and NO preference, which can contribute to the accurate prediction of N constraints on ecosystem productivity and soil carbon dynamics.