Effect of grazing on chemical and physical properties of an earthy sand in the Western Australian Mulga Zone

Abstract

An earthy sand supporting a mulga shrubland community in the arid winter rainfall zone in Western Australia is characterized in terms of its chemical and physical properties. In this study, changes in these properties with overgrazing were investigated.

Nutrient levels were low in relation to some soils supporting mulga communities elsewhere in Australia. Marked accumulations of total N, organic C and exchangeable Ca occurred in the hummocks of wind blown material surrounding surface obstructions. Improved water relationships are probably responsible for the abundance of ephemeral growth on such areas, and for their subsequent chemical enrichment. Changes in chemical properties with depth were evident for pH, total N, organic C, available P and exchangeable Mg with values decreasing from the 0-2 cm layer to the 2-10 cm layer in all cases.

Chemical changes associated with overgrazing were restricted to the 0-2 cm layer. Some trends towards lower levels of organic C, total N, and available P could be distinguished, particularly for organic C and total N in hummock surfaces, but chemical parameters generally did not provide a sensitive measure of grazing impact.

Sorptivity varied between the sandy and crusted phases of the soil mosaic and was reduced on sites in very poor condition. Sorptivity changes under grazing were apparently mediated partly by changes in the structural properties of the soil crust. There was no significant effect of overgrazing on either the bulk density of the surface (sub-crust) soil or on summer surface temperatures.