Rethinking nitrogen availability in dryland: how arid vegetation overcomes nutrient scarcity

Nitrogen (N), after water, is considered the key factor limiting net primary production in drylands. However, whether vegetation is N-limited depends on the balance between N supply and biological demand, a relationship that remains unclear in drylands. Here, we established a standardized field survey across 25 countries, including 326 plots, to assess how plant N limitation responds to aridity in global drylands. We found that while N availability decreased with aridity, soil and plant δ¹⁵N—an indicator of the balance between N supply and biological demand—unexpectedly increased in arid regions (aridity > 0.8), suggesting that plants in these regions may not have N-limitation as common views. Variations in soil N forms, functional genes, and fungal data provide further evidence that dryland vegetation has evolved a unique strategy for N uptake and utilization to overcome soil N availability declines. Data support the hypothesis that, with increasing aridity, plants favor the uptake of ammonium, a more toxic but metabolically efficient N source, and reduce their dependence on mycorrhizal associations, relying instead on direct root uptake for more efficient N allocation. Our work also highlights the impact of grazing on the development of this strategy, particularly in grasslands. These results clarify dryland plant N-use patterns and challenge the view that plants become more N-limited with increasing aridity, a perspective that should be considered when evaluating global change and human stress on drylands.