Carbon – Nitrogen Cycling along a Precipitation Gradient in Olympic National Park
Collaborators:	Edward Schreiner (USGS-FRESC), Bob McKane (US EPA), Bob Edmonds (U. WA), Dave Peterson (USFS), Steve Acker (NPS)
Funding:	USGS Global Change Research Program

Precipitation and nitrogen are key drivers of carbon sequestration in temperate forests. Interactions between these factors can however lead to complex feedbacks between plants and soils that result in unanticipated shifts in carbon dynamics and greenhouse gas emissions. Carbon stores (Mg/ha) of forests in the Pacific Northwest exceed those of any other biome, anywhere on Earth, and are highly sensitive to current and future anticipated changes in precipitation. Global circulation models have high uncertainty in how precipitation will change in the Pacific Northwest over the next century, so that increases or decreases in future precipitation are possible. The long-term response of these forests to changing precipitation and other factors remains poorly characterized, in part because current ecosystem simulation models do not consider critical “leaks” in the nitrogen cycle that can constrain carbon sequestration, accumulation, and turnover by as much as 30-70%. These models also fail to consider effects of climate change on emissions of potent nitrogen-based greenhouse gases from soils to the atmosphere.

This project combines field studies, manipulative experiments, and simulation modeling to examine critical linkages between water, carbon, and nitrogen dynamics in forests of the Pacific Northwest. We are particularly interested in how precipitation controls leaks of dissolved organic nitrogen (DON) from soils, and how these losses constrain plant-soil feedbacks, carbon sequestration, and greenhouse gas emissions. We use an exceptionally wide precipitation gradient of Douglas-fir forests in Olympic National Park as “model ecosystems” for this research. In situ studies, as well as transplants of litter and soils across sites, identify mechanisms and trajectories of ecosystem response to increasing precipitation. This data will be used to modify the General Ecosystem Model for predicting how regional increases in precipitation will drive short- vs. long-term changes in carbon sequestration and greenhouse gas emissions. Simulation results will be actively shared with NPS, BLM, and USFS resource managers to assist in developing management plans that address wildlife habitat, resource utilization, and the provision of essential ecosystem services in the face of climate change. The findings and concepts emerging from this work will also advance fundamental understanding of climate-biogeochemistry interactions, with wide applicability to virtually all terrestrial ecosystems.

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