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

Abstract

Summary 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.

Response of photosynthetic carbon gain to ecosystem retrogression of vascular plants and mosses in the boreal forest

“…Foliar 15 N and 13 C values generally increased similarly for all species as retrogression proceeded, consistent with previous studies on these islands (Hyodo and Wardle 2009) and retrogressive chronosequences in Hawaii (Vitousek et al 1989) and California (Brenner et al 2001), but not New Zealand (Menge et al 2011). The increasing d 15 N signature on smaller islands has been attributed to increased biological N-fixation and greater dependency of plants on organic N (Brenner et al 2001).…”

δ15N constraints on long-term nitrogen balances in temperate forests

“…Total ecosystem d 15 N was initialized at 4.75% at the beginning of simulations, and after only 50 years of red alder N 2 -fixation, total ecosystem d 15 N declined to 2.40% and N capital nearly doubled from *8,600 to 14,400 kg N ha -1 . This rapid decadal-scale shift in ecosystem N and d 15 N highlights the need for dynamic simulation of N input-output balances that are far from equilibrium, and contrasts with conditions where N inputs and losses are relatively small and close to balanced (Houlton et al 2006;Menge et al 2011). As more N was fixed and diluted into an increasingly large ecosystem N pool, simulated ecosystem d 15 N exhibited smaller shifts, consistent with field observations showing that red alder has less impact on soil d 15 N signatures in N-rich sites (Binkley et al 1985).…”

Associative nitrogen fixation in nodules of the conifer Lepidothamnus fonkii (Podocarpaceae) inhabiting ombrotrophic bogs in southern Patagonia

“…Observed ranges of natural abundance of 15 N in other Podocarpaceae , e.g. δ 15 N of −8 to −3‰ for P. hallii and P. urbanii 4950, suggest that such associated nitrogen fixation is certainly not a general feature of Podocarpaceae , however, or at least its contribution may not always be significant. For L. fonkii , specifically the following findings support an effective and specific association: (i) high N 2 fixation rates, confirmed by both 15 N 2 uptake and active acetylene reduction (ii) molecular evidence of nitrogenase gene expression predominated by Beijerinckiaceae compared to a diverse diazotroph community in the surrounding rhizopheric peat soil (Fig.…”