Traditional paradigms of nutrient cycling and limitation posit that nitrogen enters terrestrial ecosystems solely from the atmosphere; yet, sedimentary and metamorphic rocks contain more reactive nitrogen (N) than any other reservoir on earth. We tested the hypothesis that bedrock contributes N to temperate coniferous forests that are underlain by N-rich parent material. Using a paired sampling design in the Klamath Mountains of northern California, we measured total N content, N stable isotope ratios (δ15N), and strontium isotope ratios (87Sr/86Sr) of soils, bedrock, and five conifer species. Our study sites are similar in climate, elevation, and dominant tree species but differ markedly in underlying parent material (N-rich mica schist vs. N-poor plutonic diorite). The N-rich mica schist derives from organic rich marine sediments (i.e., decomposed phytoplankton biomass) dating to 120 Ma. We used δ15N as an integrative tracer of nitrogen cycling and 87Sr/86Sr to trace bedrock weathering inputs to soils and plants. The δ15N and N content of samples were measured by CF-IRMS, and sample 87Sr/86Sr values were determined by multicollector ICP-MS following standard digestion and ion exchange methods.
Results/Conclusions
The mica schist rocks contained 680 ppm N (350 – 950 ppm N), while the diorite samples contained 55 ppm N (42 – 72 ppm N). Nitrogen in foliage of plant species was substantially elevated in the high geo-N site relative to low geo-N site; this enrichment also held for total soil N pools. The δ15N of plant foliage followed a similar pattern across sites, averaging 2.4‰ and -3.8‰ in the high geo-N vs. low geo-N sites respectively – indicative of accelerated rates of N cycling in the high geo-N site. 87Sr/86Sr values from mica (high geo-N) and diorite (low geo-N) bedrock samples were ~0.7143 and ~0.7035 respectively, with plants exhibiting similar ratios to the dominant bedrock at each site. Taken together, the positive δ15N, extremely high N contents of soils and plants, and similarities in Sr isotopes among plants and rocks reveal that rock weathering contributes significant amounts of N in sites underlain by N-rich parent material. We conclude that bedrock nitrogen can be a primary source of N to N-limited terrestrial systems, thereby influencing productivity and CO2 uptake at ecosystem scales.
