Phosphatases are important in the mineralization of organically-bound phosphorous. AIthough phosphatases have been studied extensively in agricultural soils, there are few reports on Phosphatase activity in forest soils, and no information on the effect of forest management practices, such as thinning and N-fertilization, on Phosphomonoesterase (PME) activities. In the present study, the (PME) activity of forest soils from Shawnigan Lake and Jordan River (mg p-nitrophenol released g-1 dry soil h-1) was highest in the forest floor horizon (L-H) and decreased with depth to 60 cm in mineral soil horizons. Addition of orthophosphate and urea during the assay of PME demonstrated these chemicals competitively depressed PME activity. With Shawnigan Lake soil (L-H), the minimum rate (V max) of PME activity was 5.71 mg p-nitrophenol released g-1 dry soil h'-1. For Jordan River, V max was 3.33 mg p-nitrophenol released g-1 h-1. Incubation of L-H horizon of both soils, fumigated or non-fumigated, at 30°C for up to 40 days after rewetting resulted in an initial reduction in PME activity. Comparatively, the PME activity of Jordan River soil recovered faster than that of the Shawnigan Lake soil whose activity remained very low at the end of the incubation. In field plots where trees were thinned and urea and ammonium nitrate fertilizers had been applied 2 yr previously, assays for PME demonstrated that urea fertilization and thinning depressed PME activity while ammonium nitrate did not. This study demonstrated (1) PME activity depended on soil geographic location and depth, and (2) fertilization of forests with phosphates and urea appeared to reduce PME activity and thus may have the potential to depress the mineralization of organic-P.
