Soil Temperature, Moisture, and Carbon and Nitrogen Mineralization at a Taiga-Tundra Ecotone, Noatak National Preserve, Northwestern Alaska

Northwestern Alaska has been warming (0.3°C/yr) since the early 1990s. Ecotonal (treeline) Arctic ecosystems are expected to exhibit the effects of climate change earliest. High-latitude terrestrial ecosystems contain from 30 to 45 percent of the global organic-C pool. Soil warming may enhance release of CO2 and CH4, soil N mineralization, and the production and export of dissolved organic C and N to the aquatic ecosystem. In 1990, we began our research on climate-change effects in the small (800-ha area) Asik watershed, Noatak National Preserve, northwestern Alaska. We report results here from an intensive study conducted during 1997–98 on the relation between soil temperature, moisture, and C and N mineralization rates across a treeline taiga-tundra ecotone within the watershed. Soil C and N contents and C/N ratio were greater (p<0.05) beneath tundra. The depth of the soil active layer (annual thaw depth) increased (p<0.001) more than 40 cm during the growing season. Thaw depths reached most of the deeper layers of surface organic matter. Despite this warming, most plots did not show a clear relation between soil temperature and soil respiration (CO2 efflux). Peak soil temperatures were below the threshold needed for a respiration response to temperature, and the thawing resulted in saturated deeper soils keeping temperatures low. Soil C respiration rates did increase through the summer. Soil inorganic-N pools were larger early in summer (p<0.001) and beneath tundra (p<0.05). Net N mineralization rates were higher (p<0.01) in spruce but were positive only during early summer. Rates were negative for tundra and the taiga-tundra transition zone. In midsummer, soil microbial N consumption exceeded gross N mineralization rates except beneath tundra. Rates of gross ammonification (p<0.01) and microbial N consumption (p=0.05) increased with moisture content in the surface organic matter (Oa soil horizon). Soil-water organic and inorganic chemistry did not reflect the patterns in stream water, suggesting that the processes observed within these plots may not dominate at the watershed level. Stream-water NO3 concentrations at the mouth of the watershed increased with discharge (p<0.01, r=0.52), and autumn inorganic-N flux from the watershed increased tenfold from summer lows.