Microbially-mediated P fluxes in calcareous soils as a function of water-extractable phosphate

•Gross organic P mineralization and immobilization were determined in calcareous soils.•Microbial 33P uptake was greatest in soils with the lowest available inorganic P.•A faster rate of microbial P turnover when available inorganic P is low is indicated.•Microbial processes make a significant contribution to P cycling in forage field soils.

Soil phosphorus (P) tests are designed to indicate plant-available inorganic orthophosphate (Pi), but fail to account for Pi that may become available through organic phosphorus (Po) mineralization. This P source may be especially important in soils with low concentrations of solution and labile Pi. We assessed gross Po mineralization and immobilization using labeling with 33P in four calcareous Alfisols with varying concentrations of Olsen soil test P that were collected from forage fields of dairy farms in Ontario, Canada. Rapid microbial 33P uptake during incubation was found for the soils with the lowest available Pi as indicated by both Olsen soil test P and water-extractable Pi. The tracer incorporation into microbial P after 8 days ranged from 7 to 44% of applied 33P and was negatively related to water-extractable Pi following a power-type relationship. As concentrations of microbial P were similar in all soils, this suggests faster turnover of P in the microbial biomass at water-extractable Pi below 0.1 mg P kg−1 soil. Daily gross Po mineralization rates ranged from 0.2 to 2.8 mg P kg−1 soil d−1 and contributed 7–56% of the isotopically-exchangeable P in 8 days. Based on these findings, microbial processes have the potential to make a significant contribution to forage P nutrition.