Competition, nitrogen partitioning, and plant coexistence

One widespread explanation for the maintenance of diversity is that competition drives species to partition resources and occupy distinct niches.  While there is evidence that plants vary in their capacity to assimilate inorganic and organic nitrogen (N), it remains unknown whether current competitive interactions affect the degree to which a plant specializes on particular forms of N.  We tested whether resource partitioning of chemical forms of N plays a role in maintaining plant coexistence in the alpine tundra. Specifically, we explored how interactions with three dominant species affect the growth and nitrogen uptake of a subdominant nitrate specialist, Mertensia lanceolata. We grew Mertensia with and without the three neighbor species for one year in field “competition arenas.” In the second year, we used a ¹⁵N tracer to examine whether species differed in their ability to acquire N and if uptake patterns and N form preference of the specialist varied in the presence of the other species.  To link above and belowground dynamics, we also examined ¹⁵N recovery in soil N pools and microbial biomass.  Plant identity and plant competition had large effects on soil N pools and microbial biomass.  Mertensia uptake and N preference varied depending on the identity of the neighbor: when the dominant shared the same N preference as Mertensia, competition drove a reduction in nitrate uptake by Mertensia. These results suggest that competition and N partitioning may play a large role in determining community structure and diversity in the alpine tundra.