Phosphate aquisition by red clover and black mustard on a humic podzol

Recent investigations have shown that phosphate (P) mobilization by root exudates is an important feature of genotypes to acquire P even in soils of low-P availability. We, therefore, investigated P mobilization processes in the rhizosphere of red clover (Trifolium pratense L.) and black mustard (Brassica nigra L.) on a humic podzol. As measured by the Kuchenbuch-Jungk method (Kuchenbuch and Jungk, 1982), both species accumulated similar quantities of citrate (12 micromole/g soil) in the rhizosphere in about 1 mm distance from the soil-root interface. Despite of similar concentrations of P-mobilizing citrate in the rhizosphere of both species, red clover took up nearly the two-fold of P compared to black mustard. Differences in rhizosphere pH were determined between both species. Black mustard did not acidify the rhizosphere, whereas red clover decreased the pH in the rhizosphere from 5.8 to about 4.0 (in 0.01M CaCl2). The simultaneous acidification and excretion of citrate compared to citrate excretion alone had consequences for P mobilization processes in the rhizosphere. Phosphate mobilization from the soil solid phase was higher at higher pH. Thus, the citrate-induced P desorption was not the limiting step in P acquisition by red clover and black mustard. Calculations of P distribution in the soil solution between free ortho-P and humic-associated P showed that at higher pH most of the P was associated with dissolved humic substances, whereas at pH < 5, most of the P was present as free ortho-P. These P species can readily be taken up by the roots whereas humic-associated P must probably be desorbed from the humic surface before uptake. Phosphate species calculations, therefore, explained the higher P uptake of red clover compared to black mustard. Aluminum species distribution calculations in the soil solution further show that even at pH < 5.0 in the soil solution, citrate strongly complex Al and thereby reduce the activity of monomeric Al species. The excretion of citrate can, therefore, counteract the root induced acidification of the rhizosphere with respect to Al toxicity.