Seabird guano and phosphorus fractionation in a rhizosphere with earthworms

Soil phosphorus (P) is a critical nutrient for plant growth and is an important determinant of vegetation development and long-term ecosystem sustainability. We investigated the effects of rhizosphere-earthworm-guano interactions on soil P dynamics in a mesocosm involving two species of native New Zealand earthworms (Megascolecidae Sp.1 and Maoridrilus transalpinus) and introduced Eisenia fetida, in the context of inputs from seabird guano and the ecological restoration of a unique coastal sandplain forest. A fully factorial experimental design included a tall fibrous liliaceous perennial plant (New Zealand flax) growing in a low P forest soil, a guano-P amendment (with and without guano-P), and earthworm inoculation (with and without species of epigeics, endogeics and anecics). Soil dehydrogenase activity, CaCl₂-P, citrate-P and HCl-P were significantly modified by earthworm-guano interactions, altering the P status of the original forest soil. Furthermore, interactions between the rhizosphere and earthworms stimulated transformation of soil P and guano P; the proportional importance of soil microbial biomass P, organic-P and more soluble P fractions were substantially modified. These findings show that rhizosphere-earthworm interactions are likely to mediate the supply, chemical forms and plant-availability of P, and are likely to have an important role in successional processes and the trajectory of ecological restoration in coastal forests of New Zealand.