Soil 15N Enrichment in a Whole Watershed 15N Tracer Experiment in New England Forests.

Changes in nutrient stoichiometry and nitrogen (N) cycling are important consequences of chronic N inputs in temperate forest ecosystems. The Bear Brook Watershed in Maine (BBWM) is a long-term experimental paired watershed experiment studying the effects of acidification and N enrichment on forest biogeochemistry. The West Bear (WB) watershed has been subjected to long-term ammonium sulfate additions (25.2 kg N ha^-1 y^-1 and 28.8 kg S ha^-1 y^-1) and the East Bear (EB) watershed serves as the reference watershed. Each watershed includes 10 permanent sampling plots. Within these plots were sub-plots amended with phosphorus (P) fertilizer (NaH₂PO₄.H₂O, 100 kg P ha^-1) and control sub-plots (no Phosphorus) installed on June 1, 2012. On June 5, 2012, both watersheds were enriched with 0.4 kg ha^-1 of 98-atom % (¹⁵NH₄)₂SO₄ using backpack sprayers. Immediately following the treatment began a schedule of repeated sampling to follow the redistribution of the ¹⁵N tracer through the ecosystem. Here we report forest floor and mineral soil ¹⁵N enrichments (using the δ notation, ‰) in the “phosphorus added” and the control sub-plots over the 2012 and 2013 growing seasons. Forest floor (Oe and Oa layers) mean ¹⁵N enrichments in the control sub-plots were significantly higher than prior to the tracer addition in both watersheds and ranged between 3.3 and 7.3 ‰ in the reference watershed (EB) and between 5.8 and 14.4 ‰ in the N treated watershed (WB). In the “phosphorus added” sub-plots, the mean ¹⁵N enrichments varied from 3.2 to 12.7 ‰ in EB and from 5.2 to 18.3 ‰ in WB across the two growing seasons. The forest floor ¹⁵N enrichments were significantly higher in WB sub-plots than in EB but did not differ between sub-plot types, indicating no P treatment effect. This finding shows a long-term N manipulation effect for both sub-plot types. Enrichments were also time dependent – forest floor ¹⁵N enrichments in WB control and “phosphorus added” sub-plots were higher in the second growing season than the first one after the tracer application, whereas this was not observed in EB, the reference watershed. Moreover, the mineral soil ¹⁵N enrichments were significantly higher than prior to the tracer addition in both watersheds and sub-plot types. In the control sub-plots, mean ¹⁵N enrichments ranged between 6.9 and 8.9‰ in EB and between 7.7 and 9.7‰ in WB. In the “phosphorus added” sub-plots, the mean ¹⁵N enrichments varied from 6.7 to 9.3‰ in EB and from 6.6 to 9.4‰ in WB. No differences in enrichments between watersheds or sub-plot types were found for the mineral soil. However, the mineral soil ¹⁵N enrichments generally increased within two weeks of tracer application and then decreased in the 2012 and 2013 growing seasons in both sub-plot types and watersheds. The preliminary conclusions from these results are that (i) a significant isotopic enrichment was achieved in both watersheds and sub-plot types that was a function of time, (ii) the P addition treatment did not lead to higher ¹⁵N enrichment either in the forest floor or the mineral B horizon over two growing seasons, and (iii) the watershed subjected to long-term N additions was cycling N faster as evidenced by higher ¹⁵N enrichments in the forest floor compared to the reference watershed in both sub-plot types.