Trade-off between root nitrogen acquisition and shoot nitrogen utilization across 13 co-occurring pasture grass species

Although plant nitrogen (N) strategies may play an important role for community structure and ecosystem functioning, there is not a clear understanding of the link between N acquisition by roots and N utilization by shoots. Particularly, it is unclear how the co-variations between size- and physiology-related traits determine N acquisition and N utilization at the plant scale.2. We used 13 co-occurring temperate pasture grasses to study inter-specific variations in above-ground N yield and in root N acquisition and shoot N utilization traits. N acquisition traits concerned root influx capacities for and , root mass and specific root area in ingrowth cores. N utilization traits concerned leaf life pan, leaf N content, leaf N resorption, mean residence time of N and leaf N use efficiency.3.We found evidence for three trade-offs across species concerning root N acquisition: (i) root mass increased when specific root area declined; (ii) an increase in root area was observed when total N influx capacity decreased; and (iii) root influx capacity increased when capacity declined.4.High total root uptake capacity gave rise to high leaf N content and was associated across species to low leaf N use efficiency. Tall grasses were characterized by high shoot N yield, high root biomass and high leaf N use efficiency. Physiology-related traits and size-related traits were generally found independent.5. Our study demonstrates how size and N uptake related root traits are associated to major axes of plant specialization ((i) plant size and (ii) conservation vs. exploitation of N) which were previously identified based on shoot traits. Contrasted N strategies were segregated across species according to four combinations along these two axes. Key-words:ammonium, nitrogen use efficiency, nitrate, plant functional trait, plant N strategies, root ingrowth core, root uptake.