Mobilization and turnover of soil phosphorus in the rhizosphere

Recent progress in methods enables a better understanding of the turnover of P in the rhizosphere. Examples of this progress are the separation of soil layers differing in proximity to the roots, improved methods for extraction and fractionation of soil P, application of ³²P isotope dilution analysis to follow P fluxes between various fractions and direct determination of microbially bound P and of root phosphatases.

• These methods were combined to investigate the following aspects
• –labile P pools, the P fluxes between these pools and their contribution to the P supply to growing maize roots
• –the role of microbial biomass in these interactions and the partition of mobilized P between plants and microorganisms
• –modifications of sorption and transport of P in the rhizosphere
• –plant availability of native and added organic phosphates, and the relative significance of root and soil phosphatases.

There is a significant transformation of P in the rhizosphere with a corresponding redistribution among fractions of different plant availability. About 9% of the inorganic ³²P added to soil were incorporated within 2 weeks into microbial and organic fractions. The transfer of P from non-exchangeable forms exceeded the depletion of the exchangeable P by a factor of 5. About 53% of the mobilized P originated from inorganic, the remaining 47% from organic fractions. Of the mobilized P 80% was taken up by the plants and 20% was found in the microbial biomass. Up to 90% of the P in the rhizosphere soil solution was organic with a maximum just outside the root zone. Soluble inositol hexaphosphate modified the sorption of inorganic P, thus shifting its equilibrium solution concentration. The phosphatase activity of the roots is considerable. Both root phosphatase activity and the utilization of inositol hexaphosphate depend on the P supply and nutritional status of plants with regard to P. It is concluded that the rhizosphere is a key site of P transformation with a significant mobilization of P from the non-exchangeable inorganic and organic fractions. Organic P fractions not only play a significant role as a P source but also modify important soil parameters related to the sorption and transport of P in the rhizosphere.