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15N enrichment of the forest floor after a whole scale 15N tracer addition at a long-term forested paired watershed site in Maine, USA

Marie-Cécile Gruselle1, Ivan Fernandez2, Kevin Simon3, Stephen Norton4
1 School of Forest Resources, University of Maine
2 School of Forest Resources and Climate Change Institute, University of Maine
3 School of the Environment, The University of Auckland, Auckland, New Zealand
4 School of Earth and Climate Sciences and Climate Change Institute, University of Maine

O 1.13 in Long term trends in the functioning of ecosystems

15.07.2014, 11:40-12:00, H18

The Bear Brook Watershed in Maine, USA, is an ecosystem-scale whole watershed manipulation experiment started in November 1989. The overarching goals of the study are to understand long-term multi-decadal effects of chronic additions of nitrogen (N) and sulfur (S) as ammonium sulfate [(NH4)2SO4] (rates: 25.2 kg N ha-1 y-1 and 28.2 kg S ha-1 y-1) on watershed biogeochemistry. The treated watershed is West Bear (WB, 10.3 ha) and the watershed, East Bear (EB, 11.0 ha), serves as a reference. To understand differences in N dynamics between watersheds in the third decade of experimentation, we are conducting a whole ecosystem pulse-chase 15N tracer experiment. On June 5, 2012, backpack sprayers were used to enrich both the treated and reference watersheds with 0.4 kg ha-1 of 98-atom % (15NH4)2SO4. This began an ongoing period of sampling to follow the redistribution of the tracer into ecosystem components from tree foliage to mineral sub-soil to streams. Here we report 15N enrichments of the forest floor across two growing seasons after the tracer application. Forest floor components were the top layer of the forest floor (Oi layer) and the fine (< 6 mm) and coarse (> 6 mm) fractions of the underlying organic horizon (Oe+Oa layers). These were sampled 1, 14, 34, 62, 136, 406, 468 days after the tracer application in 10 permanent plots per watershed. The mean 15N enrichments (± S.E.) over two growing seasons, in decreasing order of enrichment in EB and WB, respectively, were 127.5 ‰ (± 9.0) and 120.3 ‰ (± 11.7) in the Oi layer, 18.1 ‰ (± 2.4) and 25.6 ‰ (± 3.2) in the coarse organic fraction, and  6.0 ‰ (± 0.6) and 10.4 ‰ (± 1.3) in the fine organic horizon (Oe+Oa layers). Significantly higher enrichments were found in the treated watershed than in the reference watershed for the coarse and fine fractions of the organic horizon. Furthermore, values were elevated compared to 15N natural abundances measured for the same ecosystem components prior to the tracer application. Besides total concentrations, 2M KCl extractable 15N-NH4 of the fine organic horizon was measured as an index of labile N. Our analysis showed that 15N-NH4 in extracts was 39.3 ‰ (± 5.1) and 45.2 ‰ (± 8.5) for the period 1 to 62 days for EB and WB, respectively. This was ~10 times more enriched than the 15N-NH4 natural abundances of 3.4 ‰ and 6.3 ‰ in EB and WB, respectively. The initial conclusions from these results are that (a) a clear isotopic enrichment was achieved in both watersheds, and (b) the watershed subject to long-term N additions was cycling N faster as evidenced by higher 15N enrichments compared to the reference watershed. Further whole ecosystem isotopic analyses will be used to quantify differences in 15N enrichments between watersheds over time.



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last modified 2014-04-02