Experimental evidence for over-fixation in temperate nitrogen fixing legume species

Background/Question/Methods

Nitrogen (N) fixing trees are much more abundant in tropical compared to temperate forests.  One theoretical explanation for this counterintuitive pattern—the “differential regulation hypothesis”—depends on variation in the degree to which N fixers in the two biomes regulate the amount of N they fix.  Theory suggests that N fixers that rapidly regulate N fixation based on N demand (“facultative” fixers) should have abundance patterns resembling tropical forests, whereas N fixers that do not regulate N fixation (“obligate” fixers) should have abundance patterns resembling temperate forests.  At present there are some field data suggesting that temperate N fixers are obligate and tropical N fixers are facultative, but there is only a small amount of evidence, it is observational rather than experimental, and it is confounded by taxonomic differences (temperate data come from actinorhizal N fixers, tropical data from rhizobial N fixers).

To begin disentangling these patterns, we asked the question “are temperate N fixers obligate or facultative?”  We addressed this question by subjecting seven temperate rhizobial species to two experimental treatments in a greenhouse: (1) A wide gradient of N supply to capture N-limited and non-N-limited conditions, and (2) inoculation (or not) with species-specific N-fixing symbionts.

Results/Conclusions

Our N gradient (nine treatments, each replicated thrice) yielded a strong ability to detect facultative versus obligate N fixation in four species and a moderate detection ability in three.  “Strong” detection ability means three or more treatments where the inoculated N fixer was not N limited, whereas “moderate” means two such treatments.  For each species we tested three statistical models: obligate (no change in N fixation across the N gradient), facultative (downregulation to zero when N ceased to limit growth), and intermediate (some downregulation, but not as much as a facultative fixer).  In some cases the statistical support was similar for two models (ΔAIC < 1), so we report both.  Of the strong detection cases, two (Medicago polymorpha and Vicia sativa) were obligate, one (Astragalus douglasii) was obligate-intermediate, and another (Trifolium hirtum) was intermediate.  Of the moderate detection cases, two (Vicia americana, Lotus scoparius) were obligate and one (Genista monspessulana) was facultative-intermediate.

These results indicate that most temperate species for which we have data fix N when they are not N limited.  All of these species were rhizobial, suggesting that the difference in N fixation strategy might depend on environmental factors rather than inherent differences in actinorhizal versus rhizobial taxa.