Research Methods:

• Moss and soil samples were collected from sites in the Sonoran Desert in and around Phoenix, AZ. When moss turfs were located, a moss sample of approx. 3 cm in diameter was collected using a clean plastic spoon and placed in a sterile whirl-pack bag. The soil immediately beneath the moss was collected to approximately 7 cm 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.
• 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 a Perkin-Elmer Elemental Analyzer. Total P was measured using a dry ash acid digestion method. 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 for 10 min, and the supernatant diluted to 5% HNO3 for measurement on an ICP-OES.
• Soil nutrient content was measured on each soil sample. First, soils were sieved to 2 mm diameter. Gravimetric soil water content (SWC) was estimated by drying approximately 20 g of soil at 105°C for 24 hrs and calculated as the percentage of dry soil. 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. The samples were decanted into centrifuge tubes and spun at for 10 min. The supernatant is then poured into a Nalgene bottle, through a 0.45 µm nylon filter to remove remaining sediment where necessary, and 3 ml of 6 N HCl were added to each sample. The 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, and then decanted into centrifuge tubes and spun 10 min. The supernatant was poured into a Nalgene bottle, again through a 0.45 µm filter where necessary, and 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 Perkin-Elmer Elemental Analyzer.