Comparative analysis of nitrogen biogeochemistry in high elevation ecosystems.

摘要

Owing to sparse vegetation and soils, un-reactive rocks, and short growing seasons, high-elevation ecosystems may be particularly susceptible to impacts from atmospheric nitrogen (N) deposition. Based on intensive monitoring of nitrogen fluxes to and from watersheds of the Sierra Nevada, California, hydrological, biological and climatic controls on nitrogen (N) cycling were investigated to determine how these montane ecosystems will react to greater N loads.; At the Emerald Lake watershed annual yield of N varied by a factor of 8 and was a linear function of runoff. The timing of snowmelt runoff had a large effect on catchment inorganic N dynamics: nitrate (NO₃ −) pulses were greater and inorganic nitrogen (DIN) retention lower in years with deep, late-melting snowpacks. About 50 to 70% of the nitrate exported during snowmelt was derived from catchment soils and talus; the remainder was snowpack nitrate. The nitrate export pulse occurred 2 to 6 weeks after the start of snowmelt and was fundamentally different from export patterns of other solutes suggesting that: (1) labile nitrate was produced and released only after intense soil-flushing has occurred, and (2) a N-sink was operating in catchment soils during the early stages of snowmelt. Soil cover was the major determinant of N retention and loss in 28 high-elevation catchments of Sierra Nevada and Rocky Mountains.; At Emerald Lake (Sierra Nevada) increased P-loading has resulted in lower growing-season nitrate concentrations and a shift from predominant P-limitation to frequent N-limitation of phytoplankton productivity. Concurrent with these changes was a more than 4-fold increase in lake particulate carbon concentrations indicating that the lake has undergone mild eutrophication during the last two decades. Comparisons between lakes sampled as part of synoptic surveys in 1985 and 1999 reveals that nitrate has declined and total phosphorus has increased in a regionally extensive set of lakes; these conditions are indicative of N-limited phytoplankton populations. Thus, trophic changes detected at Emerald Lake may be part of along-term, regional pattern.