Soil pH effects on phosphorus mobilization in the rhizosphere of Lupinus angustifolius

Increasing the pH of acid soils is a well-recognized means of improving their fertility; however, the effects that plants impose on rhizosphere soils in response to this change are not well understood. This research sought to investigate changes in phosphorus (P) availability in the rhizosphere of blue lupin (Lupinus angustifolius) in response to an increase in pH. Blue lupin plants were grown in rhizoboxes using two contrasting acid soils at their native pH (5.3 and 4.7) and treated with lime to increase their pH to 6.3 in a replicated trial. Measurements of localized acid phosphomonoesterase activity and P flux were made next to lateral root segments using a combination of zymography and diffusive gradient in thin films (DGT). Rhizosphere and bulk soils were tested for pH, organic anions, exchangeable aluminium (Al), labile P and phosphomonoesterase activity. Root morphological traits, root and shoot yields and shoot nutrient concentrations were also recorded. Profiles of DGT-P fluxes across lateral roots showed mobilization of P in the soil with the higher organic P content, and depletion in the other. The extent of acid phosphomonoesterase activity in the rhizosphere decreased with soil pH increase. Shoot P uptake was strongly correlated with fine root length and total organic anions in the rhizosphere. The proportion of thin roots decreased at pH 6.3 compared to the native pH, whereas that of thick roots increased. Soil pH increase to 6.3 using lime negatively affects the P acquisition by blue lupin due to the reduction of organic anion exudation, fine root length and the extent of acid phosphomonoesterase activity in the rhizosphere.