Organic acid mediated P mobilization in the rhizosphere and uptake by maize roots

The exudation of organic acids into the rhizosphere by plant roots has been hypothesized to be one potential mechanism by which plants can enhance the mobilization of poorly soluble nutrients in the soil. The experiments undertaken in this study were aimed at determining whether the organic acids, citrate and oxalate, could enhance the uptake of 33P from a calcareous soil with a high P fixation capacity (Typic rendoll). Soil-filled rhizosphere microcosms were constructed which allowed the growth of a single maize root axis through a KH233PO4-labelled patch of soil. After passage of the root through the 33P-labelled soil, organic acids or distilled water (control) were added to the patch at concentrations of 1 and 10 mM over a subsequent 4-day period. While oxalate resulted in an approximately two-fold enhancement in shoot 33P accumulation, citrate did not result in a significant enhancement of 33P uptake above controls to which only distilled water were added. No synergistic effect on shoot 33P accumulation was observed when both oxalate and citrate were added to the soil simultaneously. We hypothesize that the observed differences in shoot 33P accumulation by the two organic acids were due primarily to the differences in their biodegradation rate and P mobilization reactions. This study demonstrates that in vivo, organic acids can cause a significant enhancement of plant P uptake, however, the magnitude of the P mobilization response is likely to be highly context dependent.