PHOSPHORUS DEPLETION IN THE RHIZOSPHERE AS INFLUENCED BY SOIL-MOISTURE

To study the influence of soil moisture on phosphorus (P) depletion in the rhizosphere, maize (Zea mays cv. Trak) was pre-grown in vermiculite filled-PVC tubes for 9 days and then the plants with the tubes were transplanted into soil columns maintained at two soil moisture levels (theta) of 0.14 and 0.20 cm3 cm-3 for 10 days. The soil columns were separated at 1 cm depth by a nylon screen of 53 mum inner mesh size, into 1 cm soil layer above and 3 cm soil column below screen. A root mat developed over the screen, but root hairs only could penetrate it. Regardless of the soil moisture level in the columns, and adequate and equal water and nutrients supply was maintained via wicks from an external nutrient solution to the plant roots in vermiculite. After 10 days, the soil columns were separated from the root mats, quickly frozen in liquid nitrogen and sliced into thin layers (0.2 mm) using a refrigerated microtome to give soil samples at defined distances from the root mats for analyses. Lower soil moisture (theta = 0.14) resulted in narrower and steeper depletion profile of 0.5 M NaHCO3 extractable P (NaHCO3-P(i)) as compared to higher soil moisture (theta = 0.20). Depletion of P in soil solution in the immediate vicinity of root mats did not differ much but the extension of the depletion zones was 0.10 cm at theta = 0.14 and 0.20 cm at theta = 0.20. The depletion up to 0.05 cm with theta = 0.14 and up to 0.7 cm with theta = 0.20 was uniform, and may be attributed to the depletion in the root hair zone. Beyond the root hair zones, the theory of diffusion and mass flow was able to explain the observed differences in shape and extent of the P depletion profiles at the two soil moisture levels.