Fate of 15 N Tracer Additions in a New England Watershed with a 25 Years History of Experimental N Addition.

Long-term ecosystem manipulation experiments with nitrogen (N) have shown that: (1) few forest ecosystems are presently N saturated, (2) N-enriched ecosystems retain less N than ambient ecosystems, and (3) organic and mineral soil horizons are the largest sinks for N retention. At the long-term Bear Brook Watershed in Maine, one watershed is fertilized bimonthly at the rate of 25.2 kg N ha^-1yr^-1 and 28.8 kg S ha^-1yr^-1 since November 1989, with an adjacent untreated watershed serving as the biogeochemical reference. Both forests are dominated by northern hardwoods and red spruce stands. In June 2012, a ¹⁵N pulse-chase experiment with 0.4 kg ha^{-1 15}N-ammonium sulfate was initiated to study the fate of ¹⁵N in the reference and the treated watersheds. We measured ¹⁵N in aboveground and belowground ecosystem components at 1, 14, 34, 62, 136, 406, 468, and 735 days post ¹⁵N addition. Overall, higher d¹⁵N values in foliage, litterfall, fine and coarse organic fractions of the forest floor were found in the treated watershed compared to the reference watershed, except for ground vegetation exhibiting the opposite trend. However, there were no clear temporal d¹⁵N patterns within and between watersheds for each ecosystem component studied, except a steady increase in tree foliage d¹⁵N in both watersheds over three growing seasons. The large ground vegetation tracer enrichment in the reference watershed was attributed to high biological N demand, whereas the overall tracer enrichment in the treated watershed may be explained by a combination of the effects of nitrification, denitrification, and abiotic N retention. Further conclusions about total ecosystem ¹⁵N recovery by watershed and forest type, and its controls will also be presented.