STABLE ISOTOPES ISSUE Beverley R. Clarkson Æ Louis A. Schipper Bernard Moyersoen Æ Warwick B. Silvester Foliar 15 N natural abundance indicates phosphorus limitation of bog species Received: 15 September 2004 / Accepted: 1 February 2005 / Published online: 11 May 2005 Ó Springer-Verlag 2005 Abstract Foliar d 15 N, %N and %P in the dominant woody and herbaceous species across nutrient gradients in New Zealand restiad (family Restionaceae) raised bogs revealed marked differences in plant d 15 N correla- tions with P. The two heath shrubs, Leptospermum scoparium (Myrtaceae) and Dracophyllum scoparium (Epacridaceae), showed considerable isotopic variation (2.03 to 15.55&, and 0.39 to 12.06&, respec- tively) across the bogs, with foliar d 15 N strongly and positively correlated with P concentrations in foliage and peat, and negatively correlated with foliar N:P ra- tios. For L. scoparium, the isotopic gradient was not linked to ectomycorrhizal (ECM) fractionation as ECMs occurred only on higher nutrient marginal peats where 15 N depletion was least. In strong contrast, restiad species (Empodisma minus Sporadanthus ferrugineus, S. traversii) showed little isotopic variation across the same nutrient gradients. Empodisma minus and S. traversii had d 15 N levels consistently around 0& (means of 0.12& and +0.15& respectively), and S. ferrugineus, which co- habited with E. minus, was more depleted (mean 4.97&). The isotopic differences between heath shrubs and restiads were similar in floristically dissimilar bogs and may be linked to contrasting nutrient demands, acquisition mechanisms, and root morphology. Lepto- spermum scoparium shrubs on low nutrient peats were stunted, with low tissue P concentrations, and high N:P ratios, suggesting they were P-limited, which was prob- ably exacerbated by markedly reduced mycorrhizal colonisations. The coupling of d 15 N depletion and %P in heath shrubs suggests that N fractionation is pro- moted by P limitation. In contrast, the constancy in d 15 N of the restiad species through the N and P gradi- ents suggests that these are not suffering from P limita- tion. Keywords Heath shrubs Æ Mycorrhiza Æ Nutrient gradients Æ Raised bogs Æ Restionaceae Introduction In mangals, tundra, bogs, and other nutrient deficient environments, the nitrogen isotopic signature of plants has been linked with nutrient availability and changes in plant N demand. Studies have shown plant d 15 N be- comes more depleted with increasing N availability and with increasing P limitation (Goerick et al. 1994; Schu- ltze et al. 1994; Montoya and McCarthy 1995; Fry et al. 2000; McKee et al. 2002). A plant fractionation model to explain isotopic differences between source and plant across N-limitation and P-limitation gradients in man- grove communities was proposed by Fry et al. (2000) and refined by McKee et al. (2002). The model predicts that where N is in excess of demand (i.e. a P-limited system) uptake of N will show discrimination against the heavier 15 N isotope because not all N is assimilated. Plant d 15 N depletion is also caused by mycorrhizal fractionation and varies according to mycorrhizal type. Ectomycorrhizal (ECM) and ericoid mycorrhizal (ERM) plants in tundra ecosystems showed greater d 15 N depletion than co-occurring non-mycorrhizal (NM) and arbuscular mycorrhizal (AM) plants (Michelsen et al. 1996, 1998). In New Zealand, nutrient gradients occur in restiad raised bogs (dominated by Restionaceae), where min- erotrophic margins have higher nutrient levels (total N, available N, total P, available P, total K) than ombro- Communicated by Jim Ehleringer B. R. Clarkson (&) Æ L. A. Schipper Landcare Research, Private Bag 3127, Hamilton, 2001 New Zealand E-mail: bev@landcareresearch.co.nz Fax: +64-7-8584964 B. Moyersoen 4 Place Ste Veronique, 4000 Liege, Belgium W. B. Silvester Centre for Biodiversity and Ecology Research, Department of Biological Sciences, The University of Waikato, Private Bag 3105, Hamilton, 2001 New Zealand Oecologia (2005) 144: 550–557 DOI 10.1007/s00442-005-0033-4