Plant nitrogen (N) uptake is a critical ecosystem function, especially when worldwide terrestrial ecosystems are threatened by increasing anthropogenic N deposition. However, our understanding of the mechanisms by which community-level N uptake responds to N addition is limited. Here, we decomposed N uptake capacity (including plant 15N and N uptake) in a given community into two additive parts: fixed N uptake capacity (caused only by species turnover) and intraspecific variability, and predict how they contribute to the changes of community-level N uptake to N addition. We measured plant 15N and N uptake from ammonium or nitrate of 25 common species (approximately 65% of aboveground net primary production) in control plots, and community-level 15N and N uptake along N addition gradients (0, 5, 10, and 15 g N m-2 year-1), using short-term in-situ 15N labeling in a 7-year NH4NO3 addition experiment in a Tibetan alpine meadow.
Results/Conclusions
15Nitrogen uptake was defined as the proportion of N taken up from soil available N when the amount of 15N tracer labelled in each subplot was a constant (6.3 mg 15N). Nitrogen addition increased community-level N uptake from ammonium or nitrate, through positive intraspecific variability (i.e. increasing individuals’ N uptake), although with limited species turnover effects. Plants would take up more N from ammonium with decreasing N uptake efficiency, while plants would take up more N from nitrate with stable N uptake efficiency (i.e. 15N uptake). Our results provide a novel insight into the mechanism about how N addition affect plants N uptake, and show that intraspecific trait variability plays an important role in community N uptake. Furthermore, our results suggest that plants will have a higher potential uptake capability from nitrate than ammonium under future continuous N addition.