Phosphatase Activity and Phosphorus Availability in Tropical Rain Forest Soils.

Measurements of soil phosphatase activity may provide an index of the phosphorus status of tropical forests, but interpretation is limited by a lack of empirical data. We used a sensitive fluorogenic procedure to measure phosphatase activity in a series of contrasting soils under lowland tropical forest spanning a strong rainfall gradient in the Republic of Panama. There was marked seasonal variation at four sites in response to soil moisture, with lower activity during the dry season. For wet season samples, acid phosphomonoesterase activity ranged between 14 and 97 nmol product g^-1 min^-1. Phosphodiesterase activity was between five- and ten-fold lower, while alkaline phosphatase activity was significant only in soils with pH > 5.8. Strong negative correlations between phosphatase activity and various measures of extractable phosphate indicated a clear link to soil phosphorus status. This was confirmed at a site of intermediate rainfall, where a decade of phosphorus fertilization reduced phosphatase activity by two-thirds and caused soil organic phosphorus to accumulate by approximately 5% per year compared to controls. In contrast, nitrogen fertilization increased phosphomonoesterase activity, but did not lead to the depletion of soil organic phosphorus. Unlike measures of extractable soil nutrients, phosphatase activity was relatively insensitive to storage time, with only small changes during several weeks storage of field-moist soil at 4°C or 22°C. Phosphatase activity therefore represents a robust measure of phosphorus availability in tropical forest soils that varies seasonally and is sensitive to perturbations in nutrient cycling.