Modification of rhizosphere pH by the symbiotic legume Aspalathus linearis growing in a sandy acidic soil

Abstract

Aspalathus linearis is a N₂-fixing legume used for tea production, and grows in highly acidic soils (pH 3–5.3) of the Cederberg mountains in South Africa. Field and glasshouse studies revealed significantly higher pH in rhizosphere than non-rhizosphere soils. However, when six non-legume species were studied in adjacent fields, there were no differences in pH between rhizosphere and non-rhizosphere soils. The culture of A. linearis plants in sterile Leonard jars similarly showed a marked increase of 2.8 pH units in the nutrient solution bathing the roots of inoculated (nodulated) plants, compared to 1.5 pH units in uninoculated control. The uptake and reduction of NO₃^– by plants fed 2 mM NO₃^– also raised the rhizosphere pH by 3.5 units, a value comparable to that of the nodulated plants. The use of titrimetric methods showed that OH^– and HCO₃^– were the components of alkalinity in the nutrient solution bathing roots of A. linearis, and were directly responsible for the increase in rhizosphere pH. These findings suggest that the ability to raise rhizosphere pH is an adaptative feature of this legume symbiosis that overcomes the adverse effects of low pH in enhancing nutrient acquisition and reducing trace element toxicity.