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

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, 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 to 7.0 kg ha -1 yr -1 ). In addition, N depletion from rock minerals was greater than sodium, suggesting active biologically-mediated weathering of growth-limiting nutrients compared to non-essential 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.