Dual-mycorrhizal plant functional responses following shifts in soil nutrient availability during ecosystem development

Abstract

The majority of terrestrial plants form root symbioses with arbuscular mycorrhizal (AM) fungi to enhance nutrient (particularly phosphorus, P) acquisition. However, some of these plant species also form dual symbioses involving ectomycorrhizal (ECM) fungi. The factors driving plants to form dual-mycorrhizal symbioses is poorly understood. It has been suggested that these plants show plasticity in root symbioses to optimize nutrient acquisition depending on the type and strength of soil nutrient limitation (e.g., N vs. P). Alternatively, the degree of investment or ?preference? in particular root symbioses might simply reflect differences in inoculum potential among soils of contrasting nutrient availability, reflecting adaptations of root symbionts to different edaphic conditions. To better understand the ecology of dual-mycorrhizal plants, we grew two co‐occurring plant species forming AM and ECM (Acacia rostellifera and Melaleuca systena) symbioses in soils of increasing age and contrasting nutrient availability from an Australian long‐term soil chronosequence. Specifically, we aimed to disentangle the relative importance of abiotic factors (e.g., soil nutrient availability) and biotic factors (e.g., soil inoculum potential) in determining root colonization patterns and functional outcomes of these multiple root symbioses. For both plant species, we found clear hump‐shaped plant growth patterns along the strong gradient in soil nutrient availability, with peak growth in intermediate‐aged soils, while high levels of mycorrhizal colonization by the ?preferred? root symbionts were maintained across all soils. We found large increases (540%) in foliar manganese concentrations with increasing soil age and declining P availability, suggesting that plants may be relying on the release of carboxylates to help acquire P in the most P‐impoverished soils. Finally, we found that soil abiotic properties, such as strong differences in soil nutrient availability, are generally more important than soil inoculum potential in explaining these shifts in our plant and root responses. Our study suggests that plants capable of forming dual-mycorrhiza root symbioses show plasticity in their nutrient‐acquisition strategies following shifts in soil nutrients during long‐term ecosystem development, yet maintain a preference for certain root symbionts despite changes in soil microbial inoculum.