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Abstract

The release kinetics of phosphate (inorganic P [Pi]) at constant proton pressure in a pHstat experiment may be used as a proxy for P mobilization by rhizosphere acidification. pHstat experiments are challenging for calcareous soils because of the strong carbonate buffering. Our objectives were (i) to modify an existing pHstat method for calcareous soils and (ii) to determine plant species richness, plant functional group richness, and identity effects on pool sizes and rate constants (i.e., the fast- and slow-reacting Pools A and B, respectively, and the associated release constants ka and kb). The study was conducted in “The Jena Experiment” comprising grassland mixtures with different functional group composition and species richness. In 27 samples with inorganic C concentrations <10 g kg–1, a constant pH value of 3 after 2 h was reached by removing all released cations with ion-exchange membranes until all carbonates were destroyed. Thereafter, P release kinetics followed a biphasic course: the fast-reacting Pool A contained 86% of the bioavailable Pi extractable with NaHCO3 plus NaOH. The slow-reacting P pool additionally comprised P from dissolution of pedogenic oxides and more stable Ca-phosphates containing 17 to 40% of HCl-extractable Pi. Legumes decreased both pools (Pool A: 40.61 ± 3.83 with legumes vs. 65.24 ± 5.88 mg kg–1 Pi without legumes; Pool B: 36.88 ± 1.89 vs. 48.85 ± 1.81 mg kg–1 Pi) because of their increased P demand and associated ability to access hardly available P fractions. In conclusion, pHstat experiments are suitable for studying P dynamics in soil and reveal an aboveground plant composition feedback on soil P dynamics.