Mobilization of phosphate from phosphate rock and alumina–sorbed phosphate by the roots of ryegrass and clover as related to rhizosphere pH

Abstract

SummaryThe mobilization of phosphate by two pasture species was studied in an alumina sand with P added as phosphate rock (PR–P) and as phosphate sorbed on to alumina (A12O3–P). The experiments were performed with a cropping device which enables the collection of rhizosphere material at defined distances from the roots of ryegrass (Lolium rigidum L.) and clover (Trifolium subterraneum L.). Nitrogen was supplied as nitrate alone or in combination with ammonium, which stimulates proton excretion by plant roots. Whatever the phosphate source, rhizosphere pH decreased relative to control pots without plants when ammonium was supplied. The pH decrease was larger for ryegrass than clover and reached 1.5 pH units when phosphate was supplied as A12O3–P. When nitrogen was supplied as nitrate only, rhizosphere pH remained close to the near neutral pH of the control. Root–induced dissolution of PR ranged between 20% of applied PR–P for nitrate–fed ryegrass and 40% for ammonium–fed ryegrass. Most of the P dissolved from PR was sorbed on to alumina and accumulated as alkali soluble P in the rhizosphere of both species, whilst dissolved Ca from PR also accumulated. The excretion of protons by roots of clover and ryegrass was thus the driving force for the root–induced dissolution of PR. When P was supplied as A12O3–P, ryegrass grew more than clover and took up more phosphate whatever the nitrogen source. Depletion of alkali soluble P, that is phosphate sorbed on to alumina, occurred only for ryegrass and extended up to two millimetres from the roots. These differences between the two plant species suggest that the mobilization of A12O3–P was not simply related to rhizosphere pH. Instead, the superior ability of ryegrass to mobilize A12O3–P relative to clover might be related to the ability of its roots to cope with smaller solution P concentration (smaller external P requirement).