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Direct quantification of long- term rock nitrogen inputs to temperate forest ecosystems

  • Author(s): Morford, SL
  • Houlton, BZ
  • Dahlgren, RA
  • et al.

Published Web Location

https://doi.org/10.1890/15-0501.1
Abstract

© 2016 by the Ecological Society of America. 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•ha -1 •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•ha -1 •yr -1 ). In addition, N depletion from rock minerals was greater than sodium, suggesting active biologically mediated weathering of growthlimiting 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.

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