Gross P Mineralization and Microbial P Uptake in Forage Field Soils Along a Soil Test P Gradient.

Soil P tests are a measure of plant available inorganic orthophosphate (P_i) but fail to account for P_i that may become available through biologically-driven processes of organic phosphorus (P_o) mineralization. This P source may be especially important in soils with low available P_i.  In this experiment, gross P_o mineralization and microbial P cycling rates were assessed using isotopic techniques in 4 calcareous soils having varying levels of Olsen soil test P.  The soils were collected from dairy farm forage fields in Ontario, Canada.  In an incubation experiment, rapid microbial ³³P uptake was found for the soils with the lowest available P_i. Amounts of ³³P incorporated into the hexanol-labile P pool (microbial biomass) ranged from 7 to 44% after 8 days and were significantly negatively correlated with water soluble P_i according to a power type relationship.  This suggests greatly accelerated P cycling in the microbial biomass pool at solution P values < 0.1 mg P kg^-1 soil, or about 4 mg P kg^-1 Olsen P, as a result of biologically-induced P uptake.  Daily gross P_o mineralisation rates ranged from 0.37 to 2.87 mg P kg^-1 d^-1 and contributed 11 to 58% of the isotopically-exchangeable P in 8 days.  Based on these findings, P_o mineralisation has the potential to make a significant contribution to forage P nutrition.  Contrary to our hypothesis, gross P_o mineralisation was not found to be negatively correlated with available P_i levels. It appears that gross P_o mineralisation was underestimated for the soils with the lowest levels of available P_i due to rapid ³³P uptake and subsequent remineralisation of P_o by the microbial biomass pool.  Methodological considerations for using the isotopic dilution method in high P-fixing calcareous soils are discussed.