Plant‐induced Changes in the Rhizosphere of Rape (Brassica Napus Var. Emerald) Seedlings

SUMMARY Rape plants that were grown in thin layers of 32 P‐labelled Begbroke sandy loam (initial pH 6.1) attained such high root densities (> 90 cm cm −3 ) that after c. 14 days, all the soil volume became rhizosphere soil. At this time, the rhizosphere pH began to decrease and the quantity of plant available P in the rhizosphere soil, as measured by the L value, exceeded the isotopically exchangeable soil P ( E value), determined on samples of incubated but unplanted soil. The P concentration of the rhizosphere soil solution dropped to a minimum of 0.6 μM at 14 days, and remained low, but the phosphatase activity of the rhizosphere soil increased to 10 times that of the control soil after 35 days. In an identical experiment, after 20 and 41 days of growth, organic P (P o ) and inorganic P (P i ) in the control and rhizosphere soils were fractionated into the following forms: that extractable by an anion‐exchange resin, 0.5 M NaHCO 3 (pH 85), 0.1 M NaOH, 1 m NaOH, 1 m HCl and residual P. Fractionation of the soil P indicated that during the first 21 days, when the rhizosphere pH did not decrease, only resin P i 0.5 m NaHCO 3 P i and 0.1 m NaOH P i were significantly depleted by plant uptake. After 41 days, however, when the rhizosphere pH had fallen to 5.3, in addition to depletion of resin P i 0.5 m NaHCO 3 P i and 0.1 m NaOH P i there was substantial depletion of acid‐soluble forms of soil P i and residual P. It is suggested that acidification of the rape rhizosphere (Parts I and II) led to the dissolution of acid‐soluble forms of Pi that were not exchangeable with 32 P. In these experiments, there was no evidence for significant hydrolysis of soil P o . Indeed, there was a slight increase in P o in the rhizosphere soil due to the accumulation of fine root debris and microbially immobilized P i . A possible combination of processes leading to the solubilization of soil P in the rape rhizosphere is proposed.