Carbon allocation to soil microbes and storage differs in a winner and loser species during nitrogen deposition

Background/Question/Methods

Nitrogen enrichment causes changes in species composition, with a few species increasing in dominance and many species declining.  Despite this consistent pattern across many systems, it is still largely unknown why these different responses occur.  It is often assumed that competition drives species decline with N enrichment, however, in our system in the alpine tundra, an abundant species declines with N addition even when the dominant competitor is removed.  We propose three alternative mechanisms that could explain species responses to N enrichment: 1) microbial community changes, 2) carbon limitation at high N, and 3) fungi becoming parasitic in the loser species at high N.  We test this for a winner (Deschampsia cespitosa) and loser (Geum rossii) species in control and long-term fertilization plots.  We measure soil microbial community composition using PLFA, and we measure carbon limitation and transfer using a 13-C tracer experiment.  We hypothesize that 1) nitrogen enrichment alters the soil microbial community more in soil under the loser species, 2) the loser species allocates less carbon to storage organs in fertilized plots, and 3) the loser species allocates more carbon to soil fungi in fertilized plots.

Results/Conclusions

We found that N fertilization affected soil microbial communities in both Geum and Deschampsia soil, and that Geum and Deschampsia soil microbes were distinct from each other. We found opposite effects of fertilization on plant carbon allocation in Geum and Deschampsia.  Geum allocates less carbon to belowground structures (rhizomes and small roots) in fertilized plots compared to control plots, whereas Deschampsia allocates more carbon belowground in fertilized plots.  For Deschampsia, despite differing soil microbial communities in fertilized vs. control plots, similar microbial groups received carbon regardless of fertilization treatment. For Geum, carbon allocation patterns to soil community members differed in fertilization treatments: contrary to our hypothesis, Geum in fertilized plots did not allocate more carbon to soil fungi, in fact they tended to allocate less, suggesting that parasitic fungi are not the cause of Geum decline.  However, one group of gram-negative bacteria was heavily labeled only in the Geum fertilized plots, which suggests it might be pathogen that negatively affects Geum.  Overall, differing responses to N fertilization in terms of carbon storage and carbon transfer to soil microbial communities in a winner and a loser species may explain their differing responses to N enrichment.