Stream and snow patch transect sampling: At 10 stream locations and 5 different snow patches, transect sampling was conducted to determine soil parameters under moss vs. without moss presence. At stream locations, three replicate parallel transects were sampled. Between four and six samples were taken along each transect: in the dry soil beyond the hyporheic margin (Position 1), in the wetted area of the hyporheic zone (Position 2), at the edge of the flow of water in the stream from beneath a moss patch (Position 3a) and a nearby location not below moss (Position 3b), and from the center of the stream beneath a moss patch (Position 4a) and a nearby location not below moss (Position 4b). Because streams differ in morphology (e.g. stream width and size of wetted hyporheic zone), the length of the transects differs with each stream, ranging from 5-33 m in total length, as does the distance between sampling positions. At snow patch locations, three replicate parallel transects were sampled downslope of the snow patch, running parallel to the base of the snow patch.

Transect A was within 30 cm of the base of the snow patch, B was 1.5-2 m below A, and C was 2-3 m below B.

Transects were each 10 m in length, with five evenly-spaced points sampled along each. Moss samples occur randomly among those sampling points, and no moss was found below snow patches 1 and 4. At each sampling position, a soil moisture probe was inserted into the soil to measure field levels of soil water content. Any existing moss was sampled using a clean plastic spoon and put into a clean bag. Below the moss, soil samples were taken at two depths (0-2 cm and 2-7 cm, approx. 500 g each) using a clean plastic scoop and placed in a separate sterile bag. Both moss and soil were transported to the laboratory in a cooler, where they were stored at 4°C until processing. Moss tissues and subsamples of soil were frozen at -20°C and shipped to the United States for nutrient analysis. A variety of soil properties were measured on each soil sample. First, soils were hand-sorted to remove particles larger than approx. 2 mm diameter (or sieved, when soils were dry enough). Gravimetric SWC was estimated by drying approximately 25 g of soil at 105°C for 24 hrs and calculated as the percentage of dry soil. Soil pH was measured using a 2:1 water:soil dilution with a standard pH electrode. Electrical conductivity (EC) was measured using an EC meter on a 5:1 DI water:soil dilution by weight and reported as µS/cm.

Measurements were calibrated by measuring the conductivity of a standard solution of 0.01 M KCl and correcting values for the influence of solution temperature (Barrett et al. 2004). For measurements of extractable phosphate (PO4-P), 10 ± 0.5 g soil was extracted in 50 ml 0.5 M NaHCO3 at pH 8.5. Samples were filtered to remove soil, then 3 ml of 6 N HCl were added to each. Samples were allowed to degas prior to being frozen until run on a Lachat Autoanalyzer. For extractable inorganic N (NO3 + NO2-N and NH4-N), 20 ± 0.5 g soil was extracted in 50 ml 2 M KCl, filtered, then frozen until run on a Lachat Autoanalyzer. Total and inorganic C and N were measured on soils ground using a sapphire mortar and pestle that were either left unacidified or acidified with HCl respectively. Samples were analyzed on a Carlo Erba Elemental Analyzer.

Moss stoichiometry sampling and processing: Moss was sampled from multiple locations within and near the McMurdo Dry Valleys over the course of several field seasons. Samples were found across the three lake basins of Taylor Valley, as well as at several other locations, including a nearby valley (Garwood Valley) and Ross Island (n ear McMurdo Station at Observation Hill, as well as adjacent too two of the penguin rookeries at Cape Royds and Cape Crozier). When moss turfs were located, a moss sample of approx. 3 cm in diameter was collected (to a depth that included the entire core of moss carpet, usually ~1 cm) using a clean plastic spoon and placed in a sterile whirl-pack bag. The soil immediately beneath the moss was collected at the same time to approximately 7 cm (~150 g) using a clean plastic scoop and placed in a separate sterile whirl-pack bag. Both moss and soil were transported to the laboratory in a cooler, where they were both stored at 4°C until processing. When possible, stream and groundwater samples were also taken. Stream samples were collected directly from the nearby streamflow by submerging a plastic syringe, then ejecting the water into a clean, acid-washed plastic bottle through a sterile 0.45 μm nylon syringe filter. Groundwater samples were taken using a mini piezometer comprised of perforated plastic tubing inserted into the ground below the moss using a hand-vacuum pump to remove water. The groundwater was collected in clean Nalgene bottles, then vacuum filtered into a new plastic bottles. Filtered water samples were frozen until analyzed in the Crary Laboratory at McMurdo Station.

Inorganic N (NO3 + NO2-N and NH4-N) and P (PO4-P) were measured on a Lachat Autoanalyzer. Soil samples were analyzed for gravimetric SWC, mineral N and P, and total and organic C and N, as described in the previous section. Moss samples, including those from the transects, were measured for nutrient content. Given the amount of sedimentation, moss samples were washed free of as much soil as possible using tap water under a dissecting stereomicroscope. Moss samples were then dried at 60°C before being ground to a fine powder using a mortar and pestle.

Total C and N were measured on a subsample of moss on an elemental analyzer. Total P, as well as other cations micronutrients (K, Ca, Na, Zn, Mg, Mn, and Fe), were measured using a dry ash acid digestion method in which a moss subsample was ashed in a muffle oven that was gradually brought to 475°C over 1.5 hours, held at 475°C for 4 hours, then dropped to 105°C until digested. Moss ash was then weighed for calculation of ash-free dry mass (AFDM), then digested in 5 ml of 35% HNO3. Samples were then centrifuged at 25250 x g for 10 min, and the supernatant diluted to 5% HNO3 for measurement using inductively coupled plasma optical emissions spectroscopy (ICP-OES).