Collaborative Research: Importance of heterotrophic and phototrophic N2 fixation in the McMurdo Dry Valleys on local, regional and landscape scales

During our two field seasons, we surveyed the Taylor Valley and parts of the Victoria Valley (year 1: 12 sites with transects wet to dry; 11 individual sites, 177 samples per parameter) and the Garwood Valley and sites nearby it (Nostoc Flats and Miers Valley)(year 2: 9 transect sites from wet to dry; 6 individual sites, 135 samples per parameter) with regards to our main scientific questions. For the survey part of the project in the Garwood/Marshall/Miers areas and the Taylor Valley that included collections for the full suite of nutrients (Carpenter lab), DNA/RNA (Cary lab), and rate measurements of nitrogen fixation, carbon fixation, bacterial productivity and sulfate reduction (Capone lab).

We also undertook experiments on the effects of light on the balance of nitrogen fixation between heterotrophs and autotrophs each year. We carried out one shading experiment in the Taylor Valley in year 1, and in the Garwood Valley in year 2. There were 6 treatments of shading levels (4%, 10%, 15%, 25%, 49%, and 100% (control)). For both series, zero time samples were taken for each rate measurement followed by a second sampling after one week.

Data generated by the Capone lab include:

Bacterial productivity by Thymidine (3H-Tdr) uptake in the upper 1 cm of mat or soil for both field seasons. Incubations were from 30min- 2h. Units: mol Thy/(cm3*h). Assays run in triplicate.

Sulfate reduction samples by uptake of 35SO4, generally over 18h incubations. Units: nmol SO4/(cm3*d). Samples were from the top 2 cm of the mat or soil. Assays run in triplicate.

Stable isotope tracer uptake samples for both years. 15N2 & 13CO2 uptake in 24h incubations of the upper 1 cm. In year 2, 3 sites were additionally sampled for nitrate and ammonium uptake. Units: nmol N/(cm3 * d). Assays run in triplicate.

Nitrogen fixation (acetylene reduction) data. Time courses for 6-8h using the top 1 cm. Units: nmol N/(cm3 * d). Assays run in triplicate.
These are all discrete rate measurements.
We also collected soil particulate matter samples at each site for POC, PON, 13C and 15N analysis.