Evolutionary tradeoffs can select against nitrogen fixation and thereby maintain nitrogen limitation.

Symbiotic nitrogen (N) fixing trees are absent from old-growth temperate and boreal ecosystems, even though many of these are N-limited. To explore mechanisms that could select against N fixation in N-limited, old-growth ecosystems, we developed a simple resource-based evolutionary model of N fixation. When there are no costs of N fixation, increasing amounts of N fixation will be selected for until N no longer limits production. However, tradeoffs between N fixation and plant mortality or turnover, plant uptake of available soil N, or N use efficiency (NUE) can select against N fixation in N-limited ecosystems and can thereby maintain N limitation indefinitely (provided that there are losses of plant-unavailable N). Three key traits influence the threshold that determines how large these tradeoffs must be to select against N fixation. A low NUE, high mortality (or turnover) rate and low losses of plant-unavailable N all increase the likelihood that N fixation will be selected against, and a preliminary examination of published data on these parameters shows that these mechanisms, particularly the tradeoff with NUE, are quite feasible in some systems. Although these results are promising, a better characterization of these parameters in multiple ecosystems is necessary to determine whether these mechanisms explain the lack of symbiotic N fixers-and thus the maintenance of N limitation-in old-growth forests.