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 (NH₄ ⁺), nitrate (NO₃ ^-), 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 NH₄ ⁺ was the most preferred N form by plants in (sub)tropical regions, whereas NO₃ ^- preference was significantly higher in high-latitude than low-latitude regions. Plant functional type was one of the most important factors driving NO₃ ^- preference, with significantly higher NO₃ ^- 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 NH₄ ⁺ and NO₃ ^- preference, which can contribute to the accurate prediction of N constraints on ecosystem productivity and soil carbon dynamics.