Phosphate fertiliser alters carboxylates and bacterial communities in sweet potato (Ipomoea batatas (L.) Lam.) rhizosheaths

Aims: This study aimed to determine the structure of the bacterial community inhabiting the roots and rhizosheath of sweet potato cultivars, and how these bacterial communities respond to P addition and subsequent changes in carboxylate exudation by sweet potato roots. Methods: Five sweet potato cultivars were grown with and without P addition in a low P soil in a glasshouse: Beauregard and Northern Star (both international cultivars) and Whagi Besta, Maraso and Marasunda which originate from Papua New Guinea. Rhizosheath carboxylates were measured and the root/rhizosheath bacterial community analysed using 16S rRNA gene sequencing. Results: Carboxylate amount per g root weight was reduced (citrate) or increased (malate and oxalate) with addition of P. Carboxylate amount also positively correlated (citrate, oxalate and shikimate) or negatively correlated (malate) to bacterial alpha diversity when no P was added. P addition decreased bacterial diversity, altered the structure of the bacterial community (taxa and predicted function) and led to an enrichment of bacteria related to known P-solubilising bacteria (Burkholderia and Massilia) as well as to functions related to biosynthesis. Conclusion: This study provides a first indication of types and putative roles of root/rhizosheath bacteria in sweet potato cultivars originating from a wide range of environments including marginal and P-impoverished soils and high-P agricultural systems.