title: Long-term trends in nitrogen isotope composition and nitrogen concentration in brazilian rainforest trees suggest changes in nitrogen cycle.
abstract: Direct or indirect anthropogenic effects on ecosystem nitrogen cycles are important components of global change. Recent research has shown that N isotopes in tree rings reflect changes in ecosystem nitrogen sources or cycles and can be used to study past changes. We analyzed trends in two tree species from a remote and pristine tropical rainforest in Brazil, using trees of different ages to distinguish between the effect of tree age and long-term trends. Because sapwood differed from heartwood in delta(15)N and N concentration and N can be translocated between living sapwood cells, long-term trends are best seen in dead heartwood. Heartwood delta(15)N in Spanish cedar (Cedrela odorata) and big-leaf mahogany (Swietenia macrophylla) increased with tree age, and N concentrations increased with age in Cedrela. Controlling for tree age, delta(15)N increased significantly during the past century even when analyzing only heartwood and after removing labile N compounds. In contrast to northern temperate and boreal forests where wood delta(15)N often decreased, the delta(15)N increase in a remote rainforest is unlikely to be a direct signal of changed N deposition. More plausibly, the change in N isotopic composition indicates a more open N cycle, i.e., higher N losses relative to internal N cycling in the forest, which could be the result of changed forest dynamics.
authors.affiliation: Institute of Botany, University of Natural Resources and Applied Life Sciences (BOKU), 1180 Vienna, Austria. peter.hietz@boku.ac.at
authors.forename: Peter
authors.initials: P
authors.lastname: Hietz
authors.forename: Oliver
authors.initials: O
authors.lastname: Dünisch
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Wanek
mesh_terms: D001938:Brazil; D004784:Environmental Monitoring; D009584:Nitrogen / Q000032:analysis* / Q000378:metabolism*; D009587:Nitrogen Isotopes / Q000032:analysis* / Q000737:chemistry; D014197:Trees / Q000737:chemistry*
chemical_list: D009587:Nitrogen Isotopes; D009584:Nitrogen