Plant species affect acid phosphatase, ergosterol and microbial P in a Jarrah ( Eucalyptus marginata Donn ex Sm.) forest in south-western Australia

Abstract

Dry sclerophyll forest dominated by Jarrah ( Eucalyptus marginata Donn ex Sm.) covers about 1.6 Mha of south-western Australia and is, relative to other eucalypt forests, low in many nutrients, especially N and P. If fire is excluded from these forests, Banksia grandis Willd. (Proteaceae) often grows in dense thickets as the dominant understorey. Jarrah has an extensive surface (0–20 cm) system of fine lateral roots with ectomycorrhizal associations, while B. grandis produces a mat of cluster (proteoid) roots (0–20 cm) in the late winter and early spring. We measured seasonal changes in acid phosphatase activity and concentrations of ergosterol, microbial P and other P fractions (NaOH-extractable inorganic and organic P, Bray inorganic P) in soil from a forest where B. grandis was either present (Jarrah+Banksia) or absent (Jarrah). Acid phosphatase activity was between 30 and 40 μmol p-NP g −1 h −1 in the moist winter and spring, but declined to less than 10 μmol p-NP g −1 h −1 during the dry summer. Microbial P varied from less than 10 μg g −1 in the late summer to more than 50 μg g −1 during the wetter times of the year. Ergosterol also increased about four-fold as soil moisture content increased ( P≤0.05). Acid phosphatase was significantly and positively related to both ergosterol and microbial P in both Jarrah+Banksia and Jarrah soils, but the intercepts and slopes of the regressions varied seasonally and were not significantly different ( P≤0.05) between species composition. About 50% of the variation in phosphatase activity in Jarrah soils was explained by variation in fungal biomass (ergosterol), regardless of season, while in Jarrah+Banksia soils, 74% was explained by fungal biomass when soils were dry, decreasing to less than 10% as soil moisture increased. Linear regression models showed that ergosterol and microbial P are good predictors of acid phosphatase activity in soil, but their relative importance is dependent on plant species composition and season. We propose that the relative importance of fungal biomass as a determinant of phosphatase activity in the soil decreases as the seasonal flush of cluster roots dominate in the Jarrah+Banksia soils, while in Jarrah soils the phosphatase flush is primarily due to increased activity of the fungal biomass. Seasonal and spatial heterogeneity in microbial P, ergosterol and in the source of acid phosphatase in the soil are therefore a consequence of plant species composition and root type and will influence nutrient availability at the ecosystem scale.