Coupled Nitrogen-Base Cation Cycles
Collaborators:	Emily Sinkhorn, Kermit Cromack (OSU), Thomas Bullen (USGS-WRD), Jana Compton (US EPA)
Funding:	NSF Ecosystems

Nitrogen (N) is a master variable that controls the cycling of calcium (Ca) and influences long-term forest productivity and stability. Increases in N availability promote nitrification and nitrate leaching from soils, which in turn accelerate the availability, uptake, weathering, and loss of Ca from soils to streams. Over long time scales these interactions may deplete easily weatherable Ca from soils. However, changes in the sources and retention of Ca under long-term N enrichment remain difficult to predict from the flux-based approaches typically applied in short-term N addition studies. Despite inputs of N from atmospheric deposition, the greatest accumulations of soil N on Earth occur in forest ecosystems of the Pacific Northwest (> 30,000 kg N ha-1) due to a mosaic of long-term disturbance, biological N fixation by red alder, and replacement to conifers. Douglas-fir forests of this region range from N-limited to N-saturated along wide gradients of soil N, and provide a unique opportunity to examine long-term interactions between N and Ca cycles under various levels of N supply. In addition, recent growth declines in Douglas-fir at high N sites have been linked to soil Ca depletion, yet information on the long-term sustainability of Ca sources for these forests is sorely lacking.

The two overarching questions posed with this research are: (1) What are the relative importance of atmospheric vs. weathering sources of Ca to forests growing across wide gradients in soil N; and (2) How does site N status influence the retention and cycling of new Ca inputs? Complementary natural abundance and enriched tracer stable isotope studies of N, Ca, and strontium (Sr) are being used to probe biogeochemical couplings between N and Ca cycles across existing long-term N-fertility gradients. Both patterns and underlying mechanisms of N and Ca retention in plant-soil-mycorrhizal systems are being investigated across sites with similar vegetation, soil parent material, climate, and other factors. The impact of this research will be to facilitate the sound management and restoration of Pacific Northwest Douglas-fir forests, as well as to refine broader-scale predictions of how temperate forests will function in an increasingly N-rich world.

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