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

Dry sclerophyll forest dominated by Jarrah (Eucalyptus marginata Donn ex Sm .) covers about 1.6 Mha of south-western Australia and is, relative to othe r eucalypt forests, low in many nutrients, especially N and P. If fire is e xcluded from these forests, Banksia grandis Willd. (Proteaceae) often grows in dense thickets as the dominant understorey. Jarrah has an extensive sur face (0-20 cm) system of fine lateral roots with ectomycorrhizal associatio ns, while B. grandis produces a mat of cluster (proteoid) roots (0-20 cm) i n the late winter and early spring. We measured seasonal changes in acid ph osphatase 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 mu mol p-NP g(-1) h(-1) in the moist winter and spring, but declined to less than 10 mu mol p-NP g(-1) h(-1) during the dry summer. Microbial P varied from l ess than 10 mug g(-1) in the late summer to more than 50 mug g(-1) during t he wetter times of the year. Ergosterol also increased about four-fold as s oil moisture content increased (P less than or equal to 0.05). Acid phospha tase was significantly and positively related to both ergosterol and microb ial P in both Jarrah + Banksia and Jarrah soils, but the intercepts and slo pes of the regressions varied seasonally and were not significantly differe nt (P less than or equal to 0.05) between species composition. About 50% of the variation in phosphatase activity in Jarrah soils was explained by var iation in fungal biomass (ergosterol), regardless of season, while in Jarra h + 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 p hosphatase activity in soil, but their relative importance is dependent on plant species composition and season. We propose that the relative importan ce of fungal biomass as a determinant of phosphatase activity in the soil d ecreases as the seasonal flush of cluster roots dominate in the Jarrah + Ba nksia soils, while in Jarrah soils the phosphatase flush is primarily due t o increased activity of the fungal biomass. Seasonal and spatial heterogene ity in microbial P, ergosterol and in the source of acid phosphatase in the soil are therefore a consequence of plant species composition and root typ e and will influence nutrient availability at the ecosystem scale. (C) 2000 Elsevier Science Ltd. All rights reserved.