The change in vegetation cover from rainforest with a C3 photosynthetic pathway to grasses with C4 pathways was used to follow input rates and turnover of organic matter in a krasnozem over an 83 year period. The measurement of 613c values on soils from three depths (0.0-7.5, 7.5-15.0, 60.0-80.0 cm) indicated that charcoal was a serious contaminant in the light fractions (<1.6 Mg mW3) of all samples and should be removed. Of the two grasses studied (Paspalum dilatatum and Pennisetum clandestinum), the latter gave more input of organic matter into the 7.5-15.0 cm horizon. In the other horizons, both grasses performed equally. Organic matter within microaggregates (<0.2 mm) proved to contain up to 32% more old carbon than the remaining soil after 83 years. Turnover times for organic matter in the >1.6 Mg m-3 fraction from the three depths were calculated as 60, 75 and 276 years respectively, compared with 75, 108 and 348 years for the organic matter within microaggregates from the same horizons. It is concluded that the presence of microaggregates is an important factor in stabilizing organic matter in this soil type. Some difficulties with the technique are also discussed.

The relative importance of texture, structure, organic matter and clay mineralogy to the nature of the soil moisture characteristic is examined for an extensive group of Australian soils using numerical classification and diagnostic methods.The presence of pedality, particle size composition and grade of structure were the soil properties most consistently associated with differences between the groups of soils with similar moisture characteristics. By association, field texture was shown to be a useful property. Although the presence of pedality and grade of structure were important, the shape and size of ped had only weak associations with differences in the soil moisture retention.Montmorillonite, iron oxide, vermiculite and quartz were the minerals in the clay size fraction which appeared to be important if they were present. In contrast, the presence of illite did not show any strong associations with a particular position or form of the moisture characteristic.The soil moisture characteristic was successfully modelled as a power function. It appears that being able to group and classify the soil moisture characteristic and then to provide a description of these groups both in terms of soil properties and model parameters is a valuable means of developing simple predictive models for field soils. The error of our predictions for 44 horizons based on this simple approach appears to be only marginally larger than that encountered in conventional laboratory methods, and in view of soil heterogeneity it is argued that following further development these predictions may be adequate in many hydrological and agricultural applications.

Summary A low‐resolution nuclear magnetic resonance analyser was used to measure soil water. The physical principles of the method are outlined to enable the non‐linearity of the response to water to be discussed. Two sands, five soils, and vermiculite were tested in the Laboratory. The instrument readings for all were depressed below the values that might have been expected from absolute water contents. In sands the depression increases with water content, that is, as pores of progressively larger size fill with water. In clay soils an initial depression is attributable to the rigidity with which the first small amounts of water are adsorbed on to the clay lattice: with more water the depression, as in sand, reflects the size distribution of the macropores.‘Relaxation’, a condition needed to ensure accuracy, can be induced by adding iron compounds and salts, but there is evidence that where these substances are present naturally in the soil they may cause different degrees of relaxation from soil to soil. When many soil samples of known type, structural state, and chemical status have to be analysed, a good calibration curve can be obtained and used to give water contents to a fraction of a per cent. A moisture characteristic was measured on a sand to assess the ability of the instrument to scan moisture profiles in long columns.

The redistribution of rainfall on silverleaf ironbark (Eucalyptus melanophloia) trees in a semiarid environment at Narayen Research Station has been studied using detailed measurements on four trees.Interception accounts for about 11% of the annual rainfall with only 0.6% as stemflow; canopy storage capacity is estimated to be 2 mm and an estimate of the rain required to initiate stemflow is 7 mm.With the exception of nitrogen, the amounts of major elements returned to the soil surface beneath the tree canopy via throughfall and stemflo w is of the same order as that added by litterfall.

The hydrological effect of a change in land use, where trees were killed and improved pasture was established, was examined in an open grassy woodland on duplex soil derived from granite. Two pairs of small catchments at the Narayen Research Station, Queensland, were instrumented to measure rainfall, runoff, soil water and meteorological variables. The treated catchments stored up to 26 mm more soil water in the 0-1 m depth than those in their original condition. Evapotranspiration over a period, calculated from rainfall, runoff and soil water storage change, was similar for both treated and untreated catchments. This result was attributed to compensating factors following death of the trees which removed interception of rain and solar radiation, caused an increase in wind velocity at ground level, and allowed enhanced grass growth in the areas previously under tree canopies. A water balance model did not provide a satisfactory calibration for the detection of runoff changes resulting from the treatment. The ratios of the annual runoff from catchment pairs, although variable, did not show drastic changes as a result of treatment. So, provided a good grass cover was maintained, it seems unlikely that the treatment would greatly alter runoff. The chloride balance in the undisturbed woodland under the present climate suggests that any changes due to treatment are unlikely, but indicates that in these soils soluble ions are readily lost from the system.