Soil microorganisms and root exudation mediate rhizosphere acidification of the gypsum specialist Ononis tridentata Devesa & G. López

Abstract

Background and Aims Plants living on gypsum are adapted to uptake nutrients in extremely poor alkaline soils. Under such extreme conditions, processes affecting the chemical conditions of the rhizosphere may be crucial for plant survival and growth. Rhizosphere acidification in plants living on gypsum soils has never been reported before and the effect of root exudation and microbial fungi on the rhizosphere pH remains undescribed. Methods In this study we cultivated seeds of the gypsum specialist Ononis tridentata in rhizoboxes with natural gypsum soil and with fungi-sterile gypsum soil, and monitored changes in the rhizosphere pH with planar optodes coupled to a calibrated image recording system. Soil microbial life and root exudation were characterised. Results The acidification was steep in both treatments, more intense in the root tip. The higher presence of fungi led to lower pH values in the natural gypsum soil treatment. In the fungi-sterile treatment, however, rhizosphere acidification was more extensive across the root surface. Several organic acids and alcohols were exuded by plant roots, with a significantly higher concentration of some compounds in fungi-sterile roots, potentially due to the reduced fungal activity. However, the exudation of lactic acid, a compound related to rhizosphere bacteria, was higher in plants grown in the natural treatment. Conclusion Root exudation seemed to be a fundamental process to acidify the rhizosphere in gypsum soil, and fungal microbiota participated in the process without showing a dependency on plant growth. The direct visualization of pH changes at the rhizosphere helped to describe an important mechanism of plant life on gypsum.