PLANT UPTAKE OF INORGANIC AND ORGANIC NITROGEN: NEIGHBOR IDENTITY MATTERS

The importance of interspecific competition as a cause of resource partitioning among species has been widely assumed but rarely tested. Using neighbor removals in combination with 15 N tracer additions in the field, we examined variation among three alpine species in the uptake of 15 N‐NH 4 + , 15 N‐NO 3 − , and 15 N‐ 13 C‐[2]‐glycine in intact neighborhoods, when paired with a specific neighbor, and when all neighbors were removed. Species varied in the capacity to take up 15 N‐labeled NH 4 + , NO 3 − , and glycine in intact neighborhoods and in interspecific pairs. When interspecific neighbor pairs were compared with no neighbor controls, neighbors reduced 15 N uptake in target species by as much as 50%, indicating competition for N. Furthermore, neighbor identity influenced the capacity of species to take up different forms of N. Thus, competition within interspecific neighbor pairs often caused reduced uptake of a particular form of N, as well as shifts to uptake of an alternative form of N. Such shifts in resource use as a result of competition are an implicit assumption in studies of resource partitioning but have rarely been documented. Our study suggests that plasticity in the uptake of different forms of N may be a mechanism by which co‐occurring plants reduce competition for N.