Biological nitrogen enrichment shifts long-term calcium sources from bedrock weathering to atmospheric inputs
Calcium is an essential nutrient in forest ecosystems that is susceptible to leaching loss and depletion. Intense calcium depletion can ultimately affect plant and animal productivity, soil acid buffering capacity, carbon storage, and other ecosystem processes. Excess nitrogen supply and associated soil acidification are often implicated in short-term calcium loss from soils, but the long-term role of nitrogen enrichment on ecosystem calcium depletion and resupply remains unclear. Using strontium isotopes (87Sr/86Sr) as a proxy for Ca, we evaluated how variation in long-term nitrogen enrichment from biological fixation in western Oregon forests shapes atmospheric deposition vs. bedrock weathering supply of calcium across 22 sites spanning a 20-fold range of bedrock calcium on sedimentary and basaltic lithologies.
In contrast to previous studies emphasizing abiotic control of long-term calcium dynamics through rock and soil weathering, our preliminary results show that biotic nitrogen enrichment interacts with bedrock calcium content to elicit strong control over ecosystem calcium sources. For forests on calcium-rich basaltic bedrock, soil nitrogen enrichment causes calcium sources to shift from rock weathering to atmospheric dominance, with minimal influence from other major soil forming factors. For forests on calcium-poor sedimentary bedrock, we find that atmospheric inputs dominate calcium sources regardless of the degree of nitrogen enrichment, and despite high rates of tectonic uplift and erosion that can rejuvenate soil mineral weathering. Calcium content of plants and soils declined at high nitrogen enrichment regardless of bedrock type, illustrating that short-term measures of calcium availability are poor indicators of long-term calcium supply. Our finding that long-term nitrogen enrichment increases forest reliance of atmospheric calcium helps explain reports of greater calcium limitation in an increasingly nitrogen rich world.