Mobilization and turnover of soil phosphorus in the rhizosphere

Abstract

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.

Induction of an Extracellular Cyclic Nucleotide Phosphodiesterase as an Accessory Ribonucleolytic Activity during Phosphate Starvation of Cultured Tomato Cells

“…It has been suggested that, due to their compact secondary structure, both free DNA and RNA might become stabilized in soil following the release from lysing cells (Nannipieri et al, 1986). In view of the high turnover of organic phosphorus in the rhizosphere (Helal and Dressler, 1989), the importance of secretory nucleolytic enzymes for effective phosphorus acquisition of plants is evident. This is further demonstrated by normal growth and development of Arabidopsis plants on synthetic media containing purified nucleic acids as the only source of phosphorus (S. Abel, unpublished data).…”

Inositol phosphates in the environment

“…Indeed, soil IP 5ϩ6 contents have been strongly correlated with phosphate sorption capacity (McKercher & Anderson 1968b). However, Helal & Dressler (1989) noted that the addition of Na-phytate to soil displaced Fe as well as orthophosphate, indicating that the reaction was not a simple phytate/orthophosphate exchange, but involved the modification of soil properties related to sorption. For example, changes in electrochemical properties almost certainly contribute (Celi et al 1999).…”

Changes in the Content of Soil Phosphorus after its Application into Chernozem and Haplic Luvisol and the Effect on Yields of Barley Biomass

“…Approximately 40 % of organic soil P are found in the inositol P fractions whereas 7 % are bound in lipids and nucleic acids (Dalal, 1977). It is known that organic P is involved to a great extent in the dynamics and cycling of soil P (Helal and Sauerbeck, 1984;Helal and Dressler, 1989).…”