Multi-decadal evolution of nitrogen dynamics at the Bear Brook watershed in Maine

Patel, Kaizad; Fernandez, Ivan; Nelson, Sarah; Gruselle, Marie-Cècile; Norton, Stephen; Weiskittel, Aaron

From the abstract: "Chronic elevated nitrogen (N) deposition has altered the N status of temperate forest ecosystems in North America, with significant 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, in particular, declining S and N deposition and warmer, wetter climate in the Northeast. Here, we synthesize results from three decades of research on streams, soils, and vegetation to describe the long-term effect of N deposition at BBWM. While N leaching and export increased almost immediately in West Bear, 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 (East Bear) soils. Stream exports of N from East Bear 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. Chronic N additions increased N availability and reduced percent N retention in the ecosystem. The study illustrates how long-term data are essential to capture the evolution of ecosystem response."