Plant invasion alters soil phosphorus cycling on tropical coral islands: Insights from Wollastonia biflora and Chromolaena odorata invasions

Plants may modify soil biogeochemical properties that facilitate their invasion of new sites which may be mediated by soil microorganisms. We compared microbially-mediated soil phosphorus (P) cycling among sites with different degrees of invasion by Wollastonia biflora and Chromolaena odorata on two coral islands. Wollastonia biflora and C. odorata outcompeted all native species and invested more P in leaves (45–136% more than the native species). We found a decrease in the proportion of HCl–P and organic P, and a corresponding increase in the proportion of labile inorganic P (Pi) of the soil total P concentrations following plant invasion. Leaf P concentrations in invasive species were positively correlated with the proportion of labile Pi, suggesting that HCl–P and organic P were the major P sources for invasive plants, and that plant invasion increased soil P-cycling rates as a result of root carboxylate release (evidenced by greater leaf manganese concentrations) and enhanced soil alkaline phosphatase activity. The latter was confirmed by an increased soil phoD gene abundance after plant invasion. Soil P availability was elevated after plant invasion which was associated with increases in the release of root carboxylates and microbial activities that are involved in mobilization of HCl–P and mineralization of organic P, respectively, especially in the wet season. This, in turn, strongly increased leaf P concentrations of invasive species to sustain rapid plant growth, and presumably depriving competitors of P. These novel insights into soil P cycling through the enrichment of a specific soil microbial community and root carboxylate release in invaded ecosystems enhance our understanding of plant invasion mechanisms on coral islands.