title: Direct quantification of long-term rock nitrogen inputs to temperate forest ecosystems.
abstract: Sedimentary and metasedimentary rocks contain large reservoirs of fixed nitrogen (N), but questions remain over the importance of rock N weathering inputs in terrestrial ecosystems. Here we provide direct evidence for rock N weathering (i.e., loss of N from rock) in three temperate forest sites residing on a N-rich parent material (820-1050 mg N kg(-1); mica schist) in the Klamath Mountains (northern California and southern Oregon), USA. Our method combines a mass balance model of element addition/ depletion with a procedure for quantifying fixed N in rock minerals, enabling quantification of rock N inputs to bioavailable reservoirs in soil and regolith. Across all sites, -37% to 48% of the initial bedrock N content has undergone long-term weathering in the soil. Combined with regional denudation estimates (sum of physical + chemical erosion), these weathering fractions translate to 1.6-10.7 kg x ha(-1) x yr(-1) of rock N input to these forest ecosystems. These N input fluxes are substantial in light of estimates for atmospheric sources in these sites (4.5-7.0 kg x ha(-1) x yr(-1)). In addition, N depletion from rock minerals was greater than sodium, suggesting active biologically mediated weathering of growth-limiting nutrients compared to nonessential elements. These results point to regional tectonics, biologically mediated weathering effects, and rock N chemistry in shaping the magnitude of rock N inputs to the forest ecosystems examined.
authors.forename: Scott L
authors.initials: SL
authors.lastname: Morford
authors.forename: Benjamin Z
authors.initials: BZ
authors.lastname: Houlton
authors.forename: Randy A
authors.initials: RA
authors.lastname: Dahlgren
mesh_terms: D064751:Ammonium Compounds / Q000737:chemistry; D004784:Environmental Monitoring / Q000379:methods; D065928:Forests; D055593:Geological Phenomena; D008954:Models, Biological; D009584:Nitrogen / Q000737:chemistry* / Q000378:metabolism*; D012987:Soil / Q000737:chemistry
chemical_list: D064751:Ammonium Compounds; D012987:Soil; D009584:Nitrogen