Declining foliar and litter δ¹5N diverge from soil, epiphyte and input δ¹5N along a 120,000 yr temperate rainforest chronosequence.

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 δ¹5N 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 δ¹5N 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 δ¹5N 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 δ¹5N. Comparing input and whole ecosystem δ¹5N 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.