Phosphorus fractions depletion in the rhizosphere of young and adult maize and oilseed rape plants

In alkaline soils, where most of the P is acid soluble, we hypothesize that species that acidify the rhizosphere such as oilseed rape, are more efficient to use soil P than for example maize. In field experiments, adult maize plants extracted more P per unit of root than young plants. Here we hypothesize that older plants access P fractions that younger plants were not able to. Thus, the aim of this research was to study the P fractions used by maize and oilseed rape growing in an acid sandy and a neutral loamy soil and how plant age might affect it. A special pot system was developed in which P uptake by the plants came from roots that grew freely in soil. To obtain rhizosphere soil, a portion of the roots was concentrated to form a root mat on a planar soil surface covered by a fine nylon mesh. Cutting the soil on the other side of the mesh into slices gave rhizosphere soil at different distances from the root surface. We examined the P fractions used by maize and oilseed rape at three growth stages by measuring the depletion of three inorganic (P-i) and two organic (P-o) P fractions in the rhizosphere. Oilseed rape did not affect the P-o fractions in any of the two soils, and maize only in the acid soil. Both species in both soils depleted only the alkali soluble P-i fraction. The degree of depletion was between 12-26 %. The acid soluble P-i was not depleted by neither oilseed rape nor maize. Plant age had no effect on P fraction depleted or on the degree of depletion.