Nitrogen availability in soil controls uptake of different nitrogen forms by plants

Nitrogen (N) uptake by plant roots from soil is the largest flux within the terrestrial N cycle. Despite its significance, a comprehensive analysis of plant uptake for inorganic and organic N forms across grasslands is lacking. Here we measured in-situ plant uptake of 13 inorganic and organic N forms by dominant species along a 3,000 km transect spanning temperate and alpine grasslands. To generalize our experimental findings, we synthesized data on N uptake from 60 studies encompassing 148 plant species worldwide. Our analysis revealed that alpine grasslands had faster NH4+ uptake than temperate grasslands. Most plants preferred NO3– (65%) over NH4+ (24%), and then over amino acids (11%). The uptake preferences and uptake rates were modulated by soil N availability that was defined by climate, soil properties, and intrinsic characteristics of the N form. These findings pave the way towards more fully understanding of N cycling in terrestrial ecosystems, provide novel insights into the N form-specific mechanisms of plant N uptake, and highlight ecological consequences of chemical niche differentiation to reduce competition in co-existing plant species.