PARENT SESSION
Poster Session #75: Nutrient Cycling II.
Friday, August 9. Presentation from 8:00 AM to 9:30 AM. Exhibit Hall B & C, TCC

Phosphorus availability limits nitrogen fixation across an alpine tundra soil age gradient.

Holland, Keri^*,1, Townsend, Alan¹, Snyder, Carolyn², Bowman, William¹, ¹ University of Colorado, Boulder, CO² Amherst College, Amherst, MA

ABSTRACT- Theoretical models predict that nitrogen fixation will be favored in younger ecosystems where nitrogen (N) limitation is stronger and phosphorus (P) availability is relatively greater. As N accumulates in the ecosystem and P becomes more limiting, N inputs from biotic fixation may decline, as it is potentially a more energetically expensive way for plants to acquire nitrogen, and nitrogen-fixing plants have higher phosphorus requirements. This theoretical model appears to hold true in the Colorado alpine tundra. We demonstrated a gradient in total and available phosphorus in sites ranging from younger, recently glaciated soils in the Green Lakes watershed next to Niwot Ridge to successively older soils on the Ridge itself. Some plant communities on the oldest, most P-poor soils in the saddle region of Niwot have been shown to be co-limited by P. We showed that frequency of a common nitrogen-fixing alpine plant, Trifolium dasyphyllum, is significantly higher in relatively P-rich soils in the watershed and just upslope from the saddle than on the saddle itself, 0.44, 0.37, and 0.28 respectively. Measurements of isotopic variation in foliar ¹⁵N of Trifolium dasyphyllum suggest that in addition to the higher abundance of Trifolium in P-rich sites, rates of biotic N-fixation are also higher here, where Trifolium is also more abundant.

KEY WORDS: nitrogen fixation, alpine, phosphorus, age gradient