Lime-Induced pH Elevation Influences Phosphorus Biochemical Processes and Dynamics in the Rhizosphere of Lupinus polyphyllus and Lupinus angustifolius

The biochemical drivers of phosphorus (P) availability and cycling are sensitive to changes in soil pH. However, reports of lime-induced pH modification effects on P availability are (1) inconsistent, (2) focused mainly on chemical changes, and (3) often limited to the bulk soil. Using lupin as an indicator species, we aimed to evaluate the effects of lime-induced soil pH change, from 5.3 to 6.0, on biochemical processes responsible for P mobilization and dynamics in the rhizosphere of two different lupin species. Indicator species, blue lupin (Lupinus angustifolius) and Russell lupin (Lupinus polyphyllus), were grown in a P-deficient acid grassland soil without P inputs for 11 weeks in a glasshouse. The rhizosphere soils were analyzed for enzyme activity, microbial P, and carboxylates. Both bulk and rhizosphere soils were analyzed for P fractions, exchangeable aluminum (Al), and pH. Plant yields and shoot P uptake were measured. Labile inorganic P (P-i) increased in the rhizospheres of both lupin species, likely due to P-i desorption combined with labile organic P (P-o) mineralization, induced by rhizosphere pH elevation after liming. Soil pH increase promoted microbial P immobilization and reduced phosphomonoesterase activity in the rhizosphere, leading to an accumulation of P-o mainly as moderately labile and stable P-o forms. Total carboxylate concentration (TCC) increased with soil pH increase. Variation in shoot P uptake was mostly explained by TCC. These results indicate that (1) lime application strongly affects P biochemical processes and dynamics in the rhizosphere of lupins. (2) Russell lupin utilizes less P than blue lupin and is unresponsive to liming.