Acid and alkaline phosphatase dynamics and their relationship to soil microclimate in a semiarid woodland

The seasonal dynamics of acid and alkaline phosphatase activity (μg p-nitrophenol released g−1 soil h−1), soil water potential and temperature, and the relationship of phosphatase activity to plant and soil microbial processes underneath Juniperus monosperma canopies and Hilaria jamesii-dominated intercanopy areas were studied in a northern Arizona pinyon–juniper ecosystem. Alkaline phosphatase activity was significantly higher in soils under junipers (126.5±3.9 μg p-nitrophenol g−1 soil h−1) than in intercanopy soils (106.6±4.0 μg p-nitrophenol g−1 soil h−1), and significantly exceeded acid phosphatase activity by a factor of 6. Seasonal high phosphatase activities were up to 2.4 times greater than seasonal lows. Activities were maximal in summer and winter. Juniper soils were cooler than intercanopy soils except during the coldest months of the year, when they were up to 2.7°C warmer. Intercanopy soils were up to 6.2°C warmer than juniper soils, and had the highest (30.0±0.3°C) and the lowest average temperatures (2.3±0.2°C). Soil microclimate explained as much as 20% of the variation in acid and alkaline phosphatase. Temperature and water potential together were better predictors of phosphatase activity than either one alone. The soil water potential class −0.1 MPa≥ψ>−0.5 MPa was the most frequent best predictor of phosphatase activity, especially alkaline phosphatase. The winter peak in alkaline phosphatase activity is attributed to a buildup of phosphatase released into the soil from dying soil organisms, and the desorption and reactivation of previously accumulated phosphatase.