Foliar 15N natural abundance indicates phosphorus limitation of bog species

Foliar delta N-15, %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 delta N-15 correlations with P. The two heath shrubs, Leptospermum scoparium (Myrtaceae) and Dracophyllum scoparium (Epacridaceae), showed considerable isotopic variation (-2.03 to -15.55 parts per thousand, and -0.39 to -12.06 parts per thousand, respectively) across the bogs, with foliar delta N-15 strongly and positively correlated with P concentrations in foliage and peat, and negatively correlated with foliar N:P ratios. For L. scoparium, the isotopic gradient was not linked to ectomycorrhizal (ECM) fractionation as ECMs occurred only on higher nutrient marginal peats where N-15 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 delta N-15 levels consistently around 0 parts per thousand (means of -0.12 parts per thousand and +0.15 parts per thousand respectively), and S. ferrugineus, which co-habited with E. minus, was more depleted (mean -4.97 parts per thousand). 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. Leptospermum 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 probably exacerbated by markedly reduced mycorrhizal colonisations. The coupling of delta N-15 depletion and %P in heath shrubs suggests that N fractionation is promoted by P limitation. In contrast, the constancy in delta N-15 of the restiad species through the N and P gradients suggests that these are not suffering from P limitation.