Biotic Nitrogen Enrichment Regulates Calcium Sources to Forests

Abstract:

Calcium is an essential nutrient in forest ecosystems that is susceptible to leaching loss and depletion. Calcium depletion can affect plant and animal productivity, soil acid buffering capacity, and fluxes of carbon and water. 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 calcium sources and resupply is unknown. Here we use strontium isotopes (⁸⁷Sr/⁸⁶Sr) as a proxy for calcium to investigate how soil nitrogen enrichment from biological nitrogen fixation interacts with bedrock calcium to regulate both short-term available supplies and the long-term sources of calcium in montane conifer forests. Our study examines 22 sites in western Oregon, spanning a 20-fold range of bedrock calcium on sedimentary and basaltic lithologies.

In contrast to previous studies emphasizing abiotic control of weathering as a determinant of long-term ecosystem calcium dynamics and sources (via bedrock fertility, climate, or topographic/tectonic controls) we find instead that that biotic nitrogen enrichment of soil can strongly regulate calcium sources and supplies in forest ecosystems. For forests on calcium-rich basaltic bedrock, increasing nitrogen enrichment causes calcium sources to shift from rock-weathering to atmospheric dominance, with minimal influence from other major soil forming factors, despite regionally high rates of tectonic uplift and erosion that can rejuvenate weathering supply of soil minerals. For forests on calcium-poor sedimentary bedrock, we find that atmospheric inputs dominate regardless of degree of nitrogen enrichment. Short-term measures of soil and ecosystem calcium fertility are decoupled from calcium source sustainability, with fundamental implications for understanding nitrogen impacts, both in natural ecosystems and in the context of global change. Our finding that long-term nitrogen enrichment increases forest reliance on atmospheric calcium helps explain reports of greater ecological calcium limitation in an increasingly nitrogen-rich world.