Relationships between subtropical, semiarid forest of Acacia harpophylla (Brigalow), microrelief, and chemical properties of associated gilgai soil

Abstract

In a study of a brigalow (Acacia harpophylla F. Muell.) ecosystem the underlying soil material (0-90 cm) was intensively sampled in addition to complete harvesting of vegetative material. Microrelief contours were plotted and random sampling of soils within each of three microtopographic classes carried out.

There was a tendency for larger trees to be confined to the higher portions of the microrelief. Differences in soil properties in various microtopographic positions appear to be related to canopy interception and leaf drip of these larger trees. Thus the apparent uniformity of soil salinity isopleths in corresponding layers at different topographic positions, in spite of run-off from higher portions of the microrelief, was attributed to the counteracting effect of canopy interception resulting in the concentration of incident rainfall on the higher parts. This effect was also considered to contribute to the higher soil exchangeable potassium values found on the upper portions of the microrelief. The higher values for soil organic carbon and nitrogen in the upper portions of the microrelief were attributed to the greater concentration of roots in these areas.

Relationships between the chemical content of the various components of the ecosystem were examined. The relatively low phosphorus content of the soils and of the forest suggests the adaptation of brigalow to soils of low phosphorus status. The high uptake of calcium is of especial interest in view of the acidity and salinity of the soil. It seems probable that the trend of increasing acidity with depth in these soils is associated with the accumulation of calcium by brigalow and its release during litter decomposition, the low buffering capacity of the soil, and the increasing salinity of the soil with depth.

The factors involved in the persistence of brigalow and its high dry matter production under the hot, dry climatic and saline soil conditions in this area are worthy of further study.