Long-term controls on ecosystem calcium: Nitrogen accumulation versus bedrock weathering

Background/Question/Methods

Ecosystem nitrogen supply strongly influences the biogeochemical cycling and availability of other essential nutrients in temperate forests, especially calcium. Short-term additions of nitrogen often increase dissolved nitrate fluxes and decrease soil pH, which can stimulate soil calcium loss. However, the long-term effect of high nitrogen availability on ecosystem calcium supply is more difficult to determine, and may depend on calcium supply and weathering from different types of bedrock. Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) provide a reliable method to distinguish whether long-term sources of calcium to ecosystems originate from bedrock versus atmospheric sources.  We examined nutrient concentrations and ⁸⁷Sr/⁸⁶Sr ratios in tree needles and soil pools from 24 Douglas-fir forests growing on two distinct bedrock types across a wide natural gradient of soil nitrogen (0.16 – 0.88% N, 0-10 cm) in the Oregon Coast Range. Our goal was to evaluate the interactions between long-term nitrogen accumulation and bedrock type on calcium availability.

Results/Conclusions

Our preliminary data show that calcium concentrations in tree needles decreased by a factor of 2 with increasing soil nitrogen concentration, despite an enrichment of residual soil calcium in surface soil (0-10 cm) compared to mineral soil at depth (40-50 cm). Preliminary strontium isotope data for tree needles showed differing trends based on bedrock type; specifically, the ⁸⁷Sr/⁸⁶Sr of tree needles from basalt sites showed an increasing trend from 0.7057 to 0.7089 toward an atmospheric source (⁸⁷Sr/⁸⁶Sr = 0.7092) with increasing soil nitrogen. In contrast, no trend in strontium isotope ratios was observed for tree needles from sedimentary sites (⁸⁷Sr/⁸⁶Sr range from 0.7081 to 0.7107). Our results suggest that bedrock type controls the relative importance of different calcium sources (i.e., weathering vs. atmospheric) to forests, but that ecosystem nitrogen status controls overall calcium availability to plants.