Consequences of rhizosphere acidification on delivery and uptake kinetics of soil phosphorus

Experiments were undertaken to quantify the mechanisms by which acidity might affect phosphorus movement and uptake by black locust (Robinia pseudoacacia L.). Phosphorus diffusion rates in six forest soils were determined by desorption-isotherm buffer power assays. Changes in diffusion as a function of acidity were measured using pH-adjusted desorption solutions. Depending on soil type and phosphorus source solubility, up to a fivefold increase in phosphorus diffusion rate was observed with an increased acidity of one pH unit. Phosphorus uptake rates of mycorrhizal and non-mycorrhizal black locust at pH 4 and pH 7 were determined from (32)P-labeled nutrient-depletion experiments. In mycorrhizal plants, the rate of phosphorus uptake at pH 4.0 was twice that observed at pH 7.0. Non-mycorrhizal plant uptake rates were similar at both pH 4.0 and pH 7.0. The contribution of soil nutrient supply mechanisms to plant nutrient content increased with rhizosphere acidification. Uptake kinetics were also affected by rhizosphere acidification, but to a lesser extent.