Declining foliar and litter delta 15N diverge from soil, epiphyte and input delta 15N along a 120 000 yr temperate rainforest chronosequence

P>Patterns in the natural abundance of nitrogen (N) isotopes (15N and 14N) can help in the understanding of ecosystem processes along environmental gradients, because some processes fractionate against the heavier isotope. We measured delta 15N in many components of the Franz Josef soil chronosequence in New Zealand to see how each component varied along the sequence and within sites, and to see what this variation can tell us about how ecosystem processes such as N losses change with soil age. We analyzed delta 15N in foliage from 18 woody species, abscised leaves from seven woody species, three soil horizons, bryophytes, lichens, bulk deposition, and nodules from the N-fixing tree Coriaria arborea (Coriariaceae). Foliar delta 15N varied significantly across plant species. Foliage and bulk litter became 15N-depleted as soil age increased. Soil N from organic and mineral horizons was significantly more 15N-enriched than bulk litter N at each site. Increasing precipitation also decreased foliar and soil delta 15N. Comparing input and whole ecosystem delta 15N revealed limited evidence for net fractionation during N losses. These trends are consistent with some combination of increasing fractionation during plant N uptake, mycorrhizal transfer, within-plant processing, and soil decomposition as soils age.