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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.