Nitrogen Mineralization and Streamwater Chemistry, Rock Creek Watershed, Denali National Park, Alaska, U.S.A.

Abstract

Change in upstream-downstream streamwater chemistry and terrestrial nitrification and nitrogen mineralization were studied on a south-facing, gauged watershed vegetated by spruce-aspen at low elevations, alder at mid- elevations, and dry-mesic tundra at upper elevations. The objectives were (1) to compare vegetation productivity and nutrient cycling rates as indicated by nitrification and nitrogen mineralization, and (2) see if variation in these processes might be reflected in streamwater. Replicated 0.1-ha plots, one facing east, south, and west within each vegetation type, served as study sites. The watershed air temperature gradient was 0.8°C 100 m^–1 elevation. Shallow (5 cm) soil temperatures for spruce, alder and tundra sites were 6.3, 5.5, and 7.1°C, respectively, while deep (10 cm) soil temperatures were 5.3, 4.0, and 7.1°C, respectively. Beneath spruce at the beginning of the monitoring period, the difference between shallow and deep average daily soil temperatures was > 2° (7.8 to 5.5°C). There was a similar difference beneath alder (7.5 to 4.0°C). This soil temperature difference with depth disappeared by early September. East-aspect plots were more moist, had the coolest soil temperature, largest N reservoirs, and most rapid nitrification and N mineralization rates. Upstream-downstream chemistry showed decreases in specific conductance (r² = 0.19, p = 0.03), H⁺ (r² = 0.37, p < 0.0001), and SO₄^2– (r² = 0.23, p < 0.01); and increases in NO₃^– (r² = 0.71, p < 0.0001), total N (r² =0.62, p < 0.0001), and dissolved organic carbon (DOC) (r² = 0.33, p < 0.0001) concentration. While these differences were significant, they were small in magnitude. The sharpest increases in NO₃⁻ and total N occurred in streamwater just downstream from alder. Nitrification rates increased throughout the season beneath alder, but N mineralization rates were highest in July when streamwater NO₃^– concentration peaked.