Microbiotic crusts are assemblages of non-vascular plants (mosses, liverworts, algae, lichens, fungi, bacteria and cyanobacteria) which form intimate associations with surface soils. They play a major role in infiltration processes through changes to soil physico-chemical properties, and through their influence on soil surface roughness. Whilst some research suggests that they may restrict infiltration, Australian experience is that they are generally associated with enhanced infiltration. Unlike physical soil crusts, microbiotic crusts stabilize the soil against water and wind erosion, increasing landscape stability, particularly in areas of low vascular plant cover. Microbiotic crusts are thus useful indicators of soil surface condition, and cyanobacteria in the crusts fix nitrogen which may be utilized by developing vascular plant seedlings. Little is known, however, about how they interact with vascular plants and soil invertebrates. Their role in rangeland ecosystems has received renewed attention over the past few years with an increasing interest in ecologically sustainable development of arid and semi-arid grazing systems.In this review we discuss the characteristics and distribution of microbiotic crusts in the rangelands of Australia, their roles in soil and eco!ogical processes and the impacts of fire and grazing. Finally we propose a new system for classifying crusts into functional groups and identify areas requiring further investigation.

Soil salinity (high levels of water-soluble salt) and sodicity (high levels of exchangeable sodium), called collectively salt-affected soils, affect approximately 932 million ha of land globally. Saline and sodic landscapes are subjected to modified hydrologic processes which can impact upon soil chemistry, carbon and nutrient cycling, and organic matter decomposition. The soil organic carbon (SOC) pool is the largest terrestrial carbon pool, with the level of SOC an important measure of a soil's health. Because the SOC pool is dependent on inputs from vegetation, the effects of salinity and sodicity on plant health adversely impacts upon SOC stocks in salt-affected areas, generally leading to less SOC. Saline and sodic soils are subjected to a number of opposing processes which affect the soil microbial biomass and microbial activity, changing CO 2 fluxes and the nature and delivery of nutrients to vegetation. Sodic soils compound SOC loss by increasing dispersion of aggregates, which increases SOC mineralisation, and increasing bulk density which restricts access to substrate for mineralisation. Saline conditions can increase the decomposability of soil organic matter but also restrict access to substrates due to flocculation of aggregates as a result of high concentrations of soluble salts. Saline and sodic soils usually contain carbonates, which complicates the carbon (C) dynamics. This paper reviews soil processes that commonly occur in saline and sodic soils, and their effect on C stocks and fluxes to identify the key issues involved in the decomposition of soil organic matter and soil aggregation processes which need to be addressed to fully understand C dynamics in salt-affected soils.

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A scheme is proposed, together with a procedure suitable for routine laboratory use, for the prediction of dispersive behaviour of surface layers of red-brown earths and their classification into one of six classes. Each class is defined on the basis of predictive relationships established between dispersion (spontaneous and mechanical), sodium adsorption ratio (SAR) and total cation concentration (TCC). These relationships were established experimentally using 138 samples representing both surface and subsurface layers from 69 red-brown earth profiles. Preliminary studies including samples from red clay and black earth profiles indicated that the proposed scheme is not suitable for these soils. Neither can it be used for soils containing free lime. The procedure proposed enables the prediction of the probable dispersive behaviour of the surface layer of red-brown earths, including exposed subsoils. It provides a rational basis for the formulation of appropriate management strategies for the manipulation of the surface structure of individual red-brown earths used for dryland or irrigated agriculture. Application of the proposed scheme to the estimation of the minimum level of residual gypsum required to maintain aggregate stability via the electrolyte effect is discussed, with special reference to low-sodic soils (i.e. with a SAR below 3, e.g. Classes 2a and 3c).

Relationships between plant cover, runoff and erosion of a massive red earth were investigated for a runoff zone of an intergrove area in a semi-arid wooded rangeland in eastern Australia. The measurements were carried out in small experimental paddocks with different stocking rates of sheep and kangaroos. A trailer-mounted rainfall simulator was used to apply rainfall at a time averaged rate of 30 mm h-' to obtain runoff rates and sediment concentrations.There was a significant negative relationship (2 = 0.58; P < 0.01) between final runoff rate and plant cover. It is probable that the plants increase infiltration and decrease runoff by (i) funnelling water down their stems and (ii) providing macropores a t the base of the plant through which water can rapidly enter the soil.However, there was no significant effect of plant cover on sediment concentration. Probable reasons for this are: (i) even though plant cover will absorb raindrop energy and decrease the erosive stress on the soil, the nature of the plants investigated is such that they may not be 100% effective in protecting the soil beneath them, and (ii) the distribution of contact cover provided by the base of the plants is highly patchy and thus relatively inefficient at reducing sediment concentration.At zero cover final runoff rates from paddocks with a high and low stocking rate were similar, i.e. 23.4 and 22.3 mm h-I respectively. However, at zero cover, the sediment concentration from the high stocking rate paddock was significantly (P < 0.01) greater than that from the low stocking rate paddock. Greater hoof activity and lower organic matter (and hence lower structural stability) of the 0-20 mm layer in the high stocking rate paddock caused the soil surface to be more susceptible to erosion.These results show that grazing by removing perennial grasses and pulverizing the surface soil can have a major impact on local water balances and erosion rates respectively within the intergrove areas. The implications of these results for the long-term stability of semi-arid mulga woodlands is briefly discussed.

This paper reviews knowledge of nitrogen and phosphorus generation from land use and export to waterways, including studies relevant to Australia. It provides a link between current and future modelling requirements, and the context for incorporation of this knowledge into catchment models for use by catchment managers. Selected catchment models used by catchment managers are reviewed, and factors limiting their application are addressed. The review highlights the importance of dissolved N and P for overland flow and groundwater pathways, for sheep, beef and dairy grazing land use. Consequently, the effectiveness of riparian buffers to remove N and P may not be adequate. Consideration of the effects of rainfall and hydrology, dissolved P and N losses from pastures and event-based catchment-scale loads are therefore important factors that should be incorporated into catchment models. The review shows that it is likely that nutrient losses under Australian dairying conditions have many similarities to worldwide studies. Catchment models need to represent the importance of event-based loads, intensively farmed land use, management and forms of nutrients. Otherwise there is a likelihood of either underestimating nutrient losses, or potentially overestimating the effectiveness of riparian buffers.

The effects of fire on the cryptogam cover and physical and micromorphological properties of a massive red earth soil were studied in a semi-arid eucalypt woodland, heavily invaded by shrubs, near Coolabah, N.S.W. Fire reduced the cryptogam cover and concomitantly increased the depositional material produced by erosion and the area of bare surface. Annual fires for 7 years completely destroyed the cryptogamic crusts, but they recovered slowly in the absence of fire to reach the same cover as unburnt areas after about 4 years. A single fire also caused a major decline in aggregate stability of the 0-1 cm horizon, possibly because of alteration of organic cementing materials which consist of gels secreted by algae. Micromorphological observations of surface crusts showed that, as the frequency of fire increased, there was more depositional material produced by erosion coupled with the presence of thin laminated deposits. There was also less surface irregularity, fewer algal gels and less evidence of soil mixing by soil fauna. There was a significant negative relationship between the saturated infiltration rate and the number of fires (r2 = 0.63, P = 0.05). However, there was no effect of fire treatment on the unsaturated infiltration rate measured at a supply pressure of -40 mm, at which pores >0.75 mm diameter are excluded from water flow. In our burned plots, the rate of recolonization by cryptogams was relatively fast and, with approximately 4 years recovery, cryptogam cover reached the level of unburned controls. This cryptogam cover is critical in maintaining the physical properties of the soil. It is concluded, therefore, that irregular fires in this land system will not result in a permanent decline in the physical properties of the soil.