Evidence for Bedrock Nitrogen Fertility in Temperate Conifer Forests of Northern California.

Sedimentary and metasedimentary rocks contain more reactive nitrogen (N) than all other reservoirs combined, but are not considered sources of N in traditional models of nutrient cycling. N inputs are thought to be derived solely form the atmosphere, yet weathering of N-rich bedrock has been demonstrated to increase N export to streams and aquifers and to induce aluminum toxicity in soils. Here we test the hypothesis that geologic N is a source of N fertility to forest ecosystems underlain by N-rich mica schist in the Klamath Mountains, northern California. Using a paired sampling design, we measured nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope values and total elemental composition of  rocks, soils, and conifer tree foliage in sites that had similar climate, topography, and dominant tree species, but differed in parent material (N-rich schist vs. N-poor diorite). Schist rocks contained 680 ppm N, while the diorite rocks contained 55 ppm N. The mica phase of the schist rock contained upwards of 2000 ppm N, representing a 1:8 substitution of NH₄⁺ for K⁺ on the interlayer. δ¹⁵N values on schist site were 3.3‰ (rock), 3.6‰ (soil), and 3.1‰ (foliage) compared to 21.7‰ (rock),  1.9‰ (soil),  -1.89‰ (foliage) in the diorite site.  N content of foliage (per needle basis) was elevated 50% on average in the schist vs diorite site, and total N content of soils was 0.25% and 0.15% for the schist and diorite sites respectively. δ¹³C values for soil and foliage did not vary between sites. Significant differences in foliar δ¹⁵N values, and total N of foliar and soil samples between the sites suggest that N availability is much higher in the schist site. Additionally, δ¹⁵N values of rocks and soils in the schist site look nearly identical to foliage, suggesting that plants are directly taking up N weathered from rocks.