PHOSPHATE AQUISITION BY RED MUSTARD ON A HUMIC PODZOL

Recent investigations have shown that phosphate (P) mobilization by ro ot exudates is an important feature of genotypes to acquire P even in soils of low-P availability. We, therefore, investigated P mobilizatio n processes in the rhizosphere of red clover (Trifolium pratense L.) a nd black mustard (Brassica nigra L.) on a humic podzol. As measured by the Kuchenbuch-Jungk method (Kuchenbuch and Jungk, 1982), both specie s accumulated similar quantities of citrate (12 mu mol/g soil) in the rhizosphere in about 1 mm distance from the soil-root interface. Despi te of similar concentrations of P-mobilizing citrate in the rhizospher e of both species, red clover took up nearly the two-fold of P compare d to black mustard. Differences in rhizosphere pH were determined betw een both species. Black mustard did not acidify the rhizosphere, where as 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 ci trate compared to citrate excretion alone had consequences for P mobil ization 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 an d black mustard. Calculations of P distribution in the soil solution b etween free ortho-P and humic-associated P showed that at higher pH mo st 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 ca n readily be taken up by the roots whereas humic-associated P must pro bably be desorbed from the humic surface before uptake. Phosphate spec ies calculations, therefore, explained the higher P uptake of red clov er compared to black mustard. Aluminum species distribution calculatio ns 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, cou nteract the root induced acidification of the rhizosphere with respect to Al toxicity.