TOPSOIL PROPERTIES UNDER TUSSOCK GRASSLAND AND ADJOINING PINE FOREST IN OTAGO, NEW-ZEALAND

Properties of topsoils collected from beneath a 10-year-old Pinus radiata stand were compared with those of soil collected from beneath adjacent unplanted Chionochloa rigida grassland, at a paired catchment study site in the Lammerlaw Range, Otago, South Island, New Zealand. In late summer, the soil moisture content was lower in the planted catchment than in the unplanted grassland, and the air-filled porosity of the poorly drained Waipori silt loam soil was greater in the planted catchment. Soil bulk density and total porosity were similar in the two catchments. Soil pH and concentrations of total and organic phosphorus (P) and extractable cations were lower in the planted catchment, whereas levels of mineral and mineralisable nitrogen (N) and sulphate-sulphur (SO4-S) were higher in the planted catchment. Inorganic and Bray-2 extractable P levels were similar in the two catchments. Physical and chemical properties of soils collected from beneath tree crowns (0.5 m from the tree base) in the planted catchment were similar to those collected from between tree rows (1.75 m from the tree base). The differences in chemical properties observed between catchments are attributed to increased nutrient uptake by the young pines, and to increased mineralisation of soil organic matter, which may have been promoted by improved soil aeration under the pines. S concentrations in the foliage of tussocks persisting between tree rows in the planted catchment were 3 times those in the unplanted catchment, whereas concentrations of N, P, and Mg were almost double, indicating that the availability of these nutrients to tussocks was substantially higher in the planted catchment.