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 ( 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, similar to 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 center dot ha-1 center dot 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 center dot ha-1 center dot 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.