Mobilisation of recalcitrant soil nutrient fractions supports foliar nitrogen to phosphorus homeostasis in a seabird soil

Abstract

Although the nutrient enrichment literature emphasises anthropogenic sources, seabirds deposit large quantities of marine detritus at breeding and roosting sites. Little is known of the chemical fractions and plant availability of seabird soil nutrients and their relationship to nutrient limitation patterns. Nutrients in mineral soil from a breeding colony of burrowing seabirds were progressively depleted by growing radiata pine (Pinus radiata D. Don.) and wheat (Triticum aestivum) separately in small pots over 4–10 months. Soil from destructively sampled pots was analysed using a version of the Hedley fractionation scheme; foliage was analysed for C, N and δ¹⁵N using isotope ratio mass spectrometry, and for P using microwave assisted digestion and ICP-OES. Foliar C:N and δ¹⁵N increased with plant mass for both species, but N:P remained constant within plants of each species. As total soil P was progressively depleted, concentrations of bicarbonate-extractable soil P were maintained. This occurred mainly by depletion of non-labile inorganic P forms, thus demonstrating potential mobilisation of all refractory P (as defined by our chemical fractionation method) into plants growing at the seabird site. The increasing foliar δ¹⁵N was consistent with the progressive mobilisation of more highly recycled forms of N. We infer a species-specific stoichiometric homeostasis for N and P in plants grown in seabird soil, facilitated by mobilisation of recalcitrant forms of soil N and P.