Effect of mycorrhizae and pH change at the root-soil interface on phosphorus uptake by sorghum using a rhizocylinder technique

Sorghum (Sorghum bicolor) was grown for 40 days in a rhizocylinder (a growth container which permitted access to rhizosphere and nonrhizosphere soil), in two soils of low P status. Soils were fertilized with different rates of ammonium and nitrate and supplemented with 40mg phosphorus (P) kg-1 and inoculated with either Glomus mosseae (Nicol. and Gerd.) or nonmycorrhizal root inoculum. N-serve (2mg kg-1) was added to prevent nitrification. At harvest, soil from around the roots was collected at distances of 0-5, 5-10, and 10-20mm from the root core which was 35mm diameter. Sorghum plants, with and without mycorrhiza, grew larger with NH4+ than with NO3- application. After measuring soil pH, suspensions of the same sample were titrated against 0.01 M HCl or 0.01 M NaOH until soil pH reached the nonplanted pH level. The acid or base requirement for each sample was calculated as mmol H+ or OH- kg-1 soil. The magnitude of liberated acid or base depended on the form and rate of nitrogen and soil type. When the plant root was either uninfected or infected with mycorrhiza, soil pH changes extended up to 5mm from the root core surface. In both soils, ammonium as an N source resulted in lower soil pH than nitrate. Mycorrhizal (VAM) inoculation did not enhance this difference. In mycorrhizal inoculated soil, P depletion extended up to 20mm from the root surface. In non-VAM inoculated soil P depletion extended up to 10mm from the root surface and remained unchanged at greater distances. In the mycorrhizal inoculated soils, the contribution of the 0-5mm soil zone to P uptake was greater than the core soil, which reflects the hyphal contribution to P supply. Nitrogen (N) applications that caused acidification increased P uptake because of increased demand; there is no direct evidence that the increased uptake was due to acidity increasing the solubility of P although this may have been a minor effect