Organic acid concentration thresholds to mobilize phosphorus (P) in dryland soils

Available phosphorus (P) concentrations are low in dryland soils due to high pH values linked to the presence of pedogenic carbonates. Thus, dissolution and mobilization of P compounds are important controls on P availability for plants, microbes, and biocrusts. One process that has been hypothesized as a way for dryland organisms to acquire P is the exudation of organic acids that can release bound P compounds. To explore this process, we assayed the critical thresholds of organic acid (citrate, malate, and oxalate) concentrations required to mobilize P in a range of dryland soils. Our results showed that: (1) Concentrations of oxalate or citrate, on the order of 1000 µmol/L are needed to effectively mobilize PO₄ ^3- in all landforms and microsites we examined. (2) The in situ organic acid concentrations in bulk soil core samples were < 100 µmol/L, both under plant canopies and in interspaces, suggesting they are below the needed threshold to mobilize P. However, hot spots such as the rhizosphere, though difficult to quantify, may still be locations where the concentration of organic acids approach the threshold. (3) Oxalate was the most effective in releasing PO₄ ^3-, likely as a result of removing aqueous Ca via calcium oxalate precipitation. Overall, our results show that for P acquisition through organic acid production to be effective in dryland soils, a relatively high threshold of organic acid concentration that substantially exceeds bulk soil concentrations must be reached, suggesting that if it occurs, it is restricted to localized microsites within the soil matrix.

The online version contains supplementary material available at 10.1007/s10533-025-01298-5.