Phosphorus dynamics in the rhizosphere of perennial ryegrass (Lolium perenne L.) and radiata pine (Pinus radiata D. Don.)

A short-term glasshouse experiment was carried out with two grassland soils to investigate phosphorus (P) availability and associated processes in the rhizosphere of perennial ryegrass (Lolium perenne L.) and radiata pine (Pinus radiata D. Don.) using a thin slicing technique. Results showed that readily extracted forms of soil inorganic P were depleted by both ryegrass and radiata pine, although depletion zones extended further adjacent to radiata pine (5 mm) compared with ryegrass (3 mm). Accumulation of bicarbonate extractable organic P occurred in rhizosphere soil under both ryegrass and radiata pine, which was related to increased levels of microbial biomass. Enhanced depletion of sodium hydroxide extractable organic P apparent in the rhizosphere of radiata pine compared with ryegrass was related to the presence of greater concentrations of water-soluble organic C, microbial biomass and higher alkaline phosphatase and phosphodiesterase enzyme activities. These results confirmed that microbial and enzyme activities played an important role in the mineralization of soil organic P, particularly under radita pine.