title: Ecosystem processes and nitrogen export in northern U.S. watersheds.
abstract: There is much interest in the relationship of atmospheric nitrogen (N) inputs to ecosystem outputs as an indicator of possible "nitrogen saturation" by human activity. Longer-term, ecosystem-level mass balance studies suggest that the relationship is not clear and that other ecosystem processes may dominate variation in N outputs. We have been studying small, forested watershed ecosystems in five northern watersheds for periods up to 35 years. Here I summarize the research on ecosystem processes and the N budget. During the past 2 decades, average wet-precipitation N inputs ranged from about 0.1 to 6 kg N ha(-1) year(-1) among sites. In general, sites with the lowest N inputs had the highest output-to-input ratios. In the Alaska watersheds, streamwater N output exceeded inputs by 70 to 250%. The ratio of mean monthly headwater nitrate (NO3-) concentration to precipitation NO3- concentration declined with increased precipitation concentration. A series of ecosystem processes have been studied and related to N outputs. The most important appear to be seasonal change in hydrologic flowpath, soil freezing, seasonal forest-floor inorganic N pools resulting from over-winter mineralization beneath the snowpack, spatial variation in watershed forest-floor inorganic N pools, the degree to which snowmelt percolates soils, and gross soil N mineralization rates.
authors.affiliation: U.S. Geological Survey, Ft. Collins, CO 80526, USA. robert_stottlemyer@usgs.gov
authors.forename: R
authors.initials: R
authors.lastname: Stottlemyer
mesh_terms: D000413:Alaska; D000838:Anions / Q000378:metabolism; D002980:Climate; D003120:Colorado; D017753:Ecosystem; D004784:Environmental Monitoring / Q000379:methods; D019015:Geologic Sediments / Q000737:chemistry; D006801:Humans; D007477:Ions / Q000378:metabolism; D008824:Michigan; D009566:Nitrates / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D011891:Rain; D045483:Rivers / Q000737:chemistry; D012621:Seasons; D012987:Soil / Q000032:analysis; D014197:Trees; D014867:Water / Q000737:chemistry / Q000378:metabolism; D014872:Water Movements
chemical_list: D000838:Anions; D007477:Ions; D009566:Nitrates; D000644:Quaternary Ammonium Compounds; D012987:Soil; D014867:Water; D009584:Nitrogen