25 Years of Whole-Watershed Experimental N Additions in a Forested Maine Watershed
Ivan Fernandez1, Kaizad Patel2, Marie-Cecile Gruselle3, Stephen A. Norton4, Sarah J. Nelson5 and Aaron Weiskittel6
Chronic elevated nitrogen (N) deposition has altered the N status of temperate forest ecosystems in North America, with implications for forest productivity, nutrient transformations, ecosystem nutrient retention, and surface water chemistry. The Bear Brook Watershed in Maine (BBWM) is a paired whole watershed manipulation experiment focused on the effects of N and sulfur (S) deposition on ecosystem function. N was added bimonthly as (NH4)2SO4 to one watershed (West Bear) from 1989 to 2016. Research at the site has studied the evolution of ecosystem response to both treatments and ambient changes in the chemical and physical climate over time, Here, we synthesize results from three decades of research on streams, soils, and vegetation for both the reference and treated watersheds. While N leaching and export increased almost immediately in West Bear when treatments began, labile soil N (ammonium, NH4+-N and nitrate, NO3--N) did not increase in West Bear until the fifth year of treatment. Labile N became increasingly available in West Bear over time, and after 25 years of treatment, West Bear soils had 10X more extractable NH4+-N and 100X more extractable NO3--N than the reference watershed soils. Stream exports of N from the reference watershed declined by 95% from 1990 to 2016 to approximately 0.05 kg ha-1 yr-1, consistent with declines in ambient N deposition. In comparison, exports from West Bear were approximately 5 kg ha-1 yr-1 and remained constant. Forest vegetation contained 1.3X the N in the treated watershed compared to the reference, attributable to increased forest growth and tissue N concentrations as a result of the treatments. Despite multi-decadal N additions to the treated watershed, forest vegetation still was mining soil N stocks to meet annual increment demand. The study demonstrates how long-term ecosystem research is essential to define changing N dynamics on decadal timescales and how these patterns of N dynamics continue to evolve over time.
1University of Maine, ivanjf@maine.edu
2University of Maine, School of Forest Resources, kaizad.patel@maine.edu
3University of Maine, School of Forest Resources, mariececile.gruselle@maine.edu
4University of Maine, School of Earth and Climate Sciences, norton@maine.edu
5University of Maine, School of Forest Resources and Ecology and Environmental Sciences Program, sarah.j.nelson@maine.edu
6University of Maine, School of Forest Resources, aaron.weiskittel@maine.edu