- Ecosystem nitrogen (N) supply strongly influences the availability and cycling of other essential nutrients in temperate forests, especially calcium (Ca). Short-term additions of N that exceed ecosystem demands often increase dissolved nitrate fluxes and decrease soil pH, which can stimulate soil Ca loss. However, the long-term effects of high N supply on ecosystem Ca availability are more difficult to determine, and may depend on the Ca content of bedrock and mineral soils. To address this, we examined major and trace element concentrations and ⁸⁷Sr/⁸⁶Sr ratios that trace Ca sources in precipitation, foliage, soil pools, and bedrock at 24 forested sites in the Oregon Coast Range having a wide, natural range of soil N (0.16 - 0.97 % N, 0-10 cm) on contrasting basaltic and sedimentary bedrock.
Using a suite of 17 site properties, we also evaluated whether soil N variation across sites was related to the five major state-factors of soil and ecosystem development: climate, organisms, topography, parent material, and time. We found that as soil N increased across sites, its ¹⁵N/¹⁴N ratio declined towards atmospheric values, suggesting that soil N variation reflects a biotic legacy of symbiotic N fixation inputs. In contrast, soil N variation was unrelated to 17 other metrics of soil forming factors that represented climate (mean annual precipitation, mean annual temperature, and distance from the coast), topography (slope, soil depth, and abundance of coarse rock fragments), parent material (within bedrock type bulk and 1 M HNO₃ leachable rock Ca chemistry), and proxies of soil age (Hurst's redness rating, effective cation exchange capacity, Ca in non-exchangeable soil residues, chemical index of alteration, weathering index of Parker, Ca in coarse soil fragments, and soil Ca loss relative to bedrock). These analyses highlight symbiotic N-fixing red alder as a keystone organismal state-factor that produces a wide range of soil N accumulation in coastal Oregon forests.
Strontium isotopes (⁸⁷Sr/⁸⁶Sr) and other geochemical analyses indicate that long-term Ca sources in foliage and exchangeable soil pools in Oregon Coast Range forests depend on an interactive effect between N availability and bedrock. Basaltic rocks contained nearly 20-times more Ca than sedimentary rocks across our sites, and this difference was reflected in Sr-isotope partitioning of base cation sources. Atmospheric sources dominated plant and soil pools in forests overlying Ca-poor sedimentary rock, regardless of variation in soil N, indicating extremely limited capacity of weathering to support forest Ca demands. In contrast, forests overlying basaltic rock obtained as much as 80% of Ca from rock weathering in low N sites, yet relied to a greater extent on atmospheric Ca as soil N increased, with less than 10% of Ca from rock weathering at sites with the highest soil N. Surprisingly, differences in fresh rock Ca content and base cation sources between sedimentary and basaltic sites was not reflected in ecosystem Ca availability, and instead increasing soil N caused similar declines in foliar and exchangeable Ca across both rock types. This illustrates that nutrient pool sizes do not necessarily reflect long-term nutrient supply, and highlights how coupled biogeochemical cycles within ecosystems can regulate nutrient loss and supply to biota. Broadly, our results highlight how interactions between biological and geologic factors can influence base cation sources in forest ecosystems. The sustainability of base cation supplies to forests may therefore depend greatly on variation in bedrock weathering at low N sites, yet converge to depend on atmospheric inputs in sites that receive high N loading from biological fixation or anthropogenic deposition.