Soil P status and phosphomonoesterase activity of recently burnt and unburnt soil following laboratory incubation

The effects of laboratory incubation on P form distribution and phosphomonoesterase activity in a forest soil recently affected by a wildfire (B) were compared with its effects on these properties of an unburnt soil from an adjacent plot (UB). Samples of surface (0–5 cm) and subsurface (5–10 cm) layers of both soils were incubated at 75% field capacity and 28°C for 11 wk. After 0, 1, 2, 4, 6 and 11 weeks, samples were withdrawn and fractionated and the P contents of fractions from corresponding layers were compared. The 0–5 cm layer was most severely affected by the fire, which caused oxidation of organic P (Po) and a marked decrease in the content of residual P (Pr). The major process occurring during the incubation of this layer was microbial immobilization of inorganic P (Pi) in organic forms. By contrast, the 5–10 cm layer behaved similarly to the UB soil during incubation: essentially, Pr was transformed into rapid turnover Po (NaHCO3-extractable Po+NaOH-extractable Po), and Pi was occluded. After destruction of the enzyme due to burning, both layers of the B soil exhibited a very low initial rate of phosphomonoesterase activity. The failure of the enzyme activity to increase during incubation coincided with high labile Pi concentrations, suggesting that microbial synthesis of new phosphomonoesterase enzyme was repressed.