Contents lists available at ScienceDirect Applied Soil Ecology journal homepage: www.elsevier.com/locate/apsoil Seabird guano and phosphorus fractionation in a rhizosphere with earthworms Hongtao Zhong a,1 , Young-Nam Kim a,2 , Carol Smith b , Brett Robinson b , Nicholas Dickinson a, ⁎ a Department of Ecology, Lincoln University, Christchurch, Lincoln 7647, New Zealand b Department of Soil and Physical Sciences, Lincoln University, Christchurch, Lincoln 7647, New Zealand ARTICLE INFO Keywords: Phosphorus Earthworm Guano Flax Fractionation ABSTRACT Soil phosphorus (P) is a critical nutrient for plant growth and is an important determinant of vegetation de- velopment 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, CaCl 2 -P, citrate-P and HCl-P were significantly modified by earthworm-guano interactions, altering the P status of the original forest soil. Furthermore, inter- actions between the rhizosphere and earthworms stimulated transformation of soil P and guano P; the pro- portional 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, che- mical 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. 1. Introduction After nitrogen (N), phosphorus (P) is the second most important nutrient for plant growth but differs to N by being one of the least available plant-essential nutrients in soil. Soil P exists in various che- mical forms, either inorganic (typically 35–70% of total P) or organic (20–80% of total P); the proportional importance varies between soil types and with land management regime (McLaren and Cameron, 1996). Phosphorus is taken up by plants from soil solution in the form of orthophosphate, mainly as H 2 PO 4 - in acid soil and as HPO 4 2- in alkaline soil (Shen et al., 2011). It is the transformation and dynamics of chemical forms of P that determine its bioavailability in soils, itself driven by multiple physico-chemical and biological factors (Brady and Weil, 2008; Condron and Newman, 2011). Bioavailability of inorganic P is particularly limited in highly weathered soils of old ecosystems (Peltzer et al., 2010), such as in the primeval coastal forests of New Zealand in the present study (Turner et al., 2014). Bird guano contains variable concentrations of P, typically ranging from 0.12 to 16% DW (Otero et al., 2015). Large coastal populations of seabirds in New Zealand were decimated by relatively recent human settlement, but may have previously provided a significant source of this element to coastal forest soils (Hawke and Newman, 2004; Mulder and Keall, 2001; Roberts et al., 2007). The fate of guano-P deposited on soil is known to change the ratio of organic and inorganic P fractions (Ziółek and Melke, 2014). Soil bicarbonate-extractable soil P (potential plant available P) appears to be replenished by the depletion of non- labile inorganic P forms potentially sourced from deposited and later transformed guano-P (Hawke and Condron, 2014). With regard to P mobility and bioavailability in soil, biological processes generally play a significant role in determining its chemical form and mobility; for example, through earthworm feeding, digestion, excretion, burrowing and casting which can substantially modify the physical, chemical and biological properties of soil (Bertrand et al., 2015; van Groenigen et al., 2014). However, earlier studies of relevance to the current work have mostly focused on P availability to plants (e.g. Chapuis-Lardy et al., 2011; Le Bayon and Milleret, 2009), rather than on soil P fractionation and dynamics in earthworm-inhabited rhizospheres with external P inputs. http://dx.doi.org/10.1016/j.apsoil.2017.08.006 Received 10 April 2017; Received in revised form 18 July 2017; Accepted 10 August 2017 ⁎ Corresponding author. 1 Present address: School of Biological Sciences, The University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia. 2 Present address: Ohio Agricultural Research and Development Center, The Ohio State University, 1680 Madison Avenue Wooster, OH 44691, USA. E-mail address: nicholas.dickinson@lincoln.ac.nz (N. Dickinson).