Carbon allocation and soil microbes explain plant response to N deposition

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 two alternative mechanisms that could explain species responses to N enrichment: 1) carbon limitation at high N, and 2) 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 carbon limitation and transfer using a 13-C tracer experiment combined with soil microbial community composition analysis using PLFA.  We hypothesize that 1) the loser species allocates less carbon to storage organs in fertilized plots, and 2) the loser species allocates more carbon to soil fungi in fertilized plots.

We found opposite effects of N 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.  This suggests that Geum may be carbon limited and not have enough carbon to survive through the winter or for allocation to preformed leaves.  Carbon allocation to soil microbial communities differed in the winner vs. the loser species, but not in the way we predicted. 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, gram-negative bacteria were 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.