Plasticity in kinetics of organic and inorganic root uptake in conifer seedlings experiencing high nitrate input

Background/Question/Methods

Atmospheric nitrogen (N) deposition has serious consequences in many ecosystems of the world but the problem is particularly acute in coniferous forests. Plant supply of N in these systems often includes ammonium and nitrate but a large portion of N demand may be met by organic sources. There are reasons to believe that chronic N addition may downregulate root system transporters of these N sources primarily through changes in plant N demand. However, the direct impact cannot be ruled out. Here, we examined effects of chronic nitrate addition on uptake kinetics of ammonium; two amino acids (glycine and glutamine) and one amino sugar (glucosamine). We used one year old seedlings of four conifers species:  Abies fraseri (AF), Picea abies (PA), Picea pungens (PP), and Pseudotsuga menziesii (PM). During the two months of N manipulation, seedlings received all essential nutrients according to a ½ strength Hoagland recipe but nitrate was added at either 0.1 or 5 mM concentration daily.

Results/Conclusions

In all species, high nitrate exposure inhibited kinetics of ammonium uptake. Kinetics of amino acids and the amino sugar responded to nitrate addition in a species specific manner. High nitrate exposure resulted in a significant reduction in root uptake capacity of all organic N forms, in AF and PP, a significant decrease in AP and no significant changes in PM. We conclude that increased atmospheric N input results in significant changes in root N uptake kinetics but this plasticity is highly dependent on species identity and N form. The differential species responses shown here is a possible mechanism potentially explaining why in many ecosystems chronic N addition alters plant community composition.