Nigel K. Fleming

Mobility of phosphorus through intact soil cores collected from the Adelaide Hills, South Australia

Intact cores were collected from a variety of soils in the Adelaide Hills, South Australia, and tested for phosphorus retention and mobility (P in drainage) under various rainfall intensities (5, 25, and 50 mm/h). Phosphorus mobility was high in soils with significant macropore structure. However, all soils exhibited some degree of preferential flow of P, including the heavy-textured soils with high P adsorption that were not P saturated. A phosphorus adsorption index based only on the chemical properties of the soil did not accurately predict the mobility of P through soils with macroporosity. A phosphorus mobility index was developed encompassing both soil chemical and physical parameters. Results showed the sandy soils, and the loams over clays with high macroporosity that are located in the more elevated parts of the Adelaide hills, are most susceptible to P leaching. Management to reduce P loss to groundwater, streams, or surface water storages must aim to increase the residence time of P within soils and thereby allow mineral and organic fractions time to sorb P. Phosphorus loss through wet soils was significantly less than P loss through dry soils with high macroporosity. Application of P fertiliser to soils with high macroporosity may need to be delayed until later in the growing season than is currently practised.

Stratification, forms, and mobility of phosphorus in the topsoil of a Chromosol used for dairying

The forms and stratification of soil phosphorus (P) and their relationship to mobile forms of P were investigated in soils collected from a subcatchment used for grazing of dairy cattle in the Adelaide Hills, South Australia. Phosphorus in the soils was highly stratified. The concentration of calcium chloride extractable P in the 0–0.01 m increment was, on average, 5.7 times greater than in the 0.05–0.10 m increment. Organic P (% of total P) in the top 0.01 m was significantly (P 50%) of dissolved unreactive P (DUP), whereas runoff from high P soils (low Po) had low proportions of DUP (<10%). Ultrafiltration of runoff samples revealed that 94 and 65% of the dissolved reactive P and DUP, respectively, was subcolloidal (i.e. <1 nm). These results highlight the relationship between soil fertility, the forms of soil P, and the concentrations and forms of P mobilised in runoff. Such relationships need to be considered in further studies of P mobilisation and the subsequent development of strategies designed to reduce runoff P concentrations.

Small-scale, high-intensity rainfall simulation under-estimates natural runoff P concentrations from pastures on hill-slopes

Rainfall simulation is a widely used technique for studying the processes, and quantifying the mobilisation, of phosphorus (P) from soil/pasture systems into surface runoff. There are conflicting reports in the literature of the effects of rainfall simulation on runoff P concentrations and forms of P compared to those under natural rainfall runoff conditions. Furthermore, there is a dearth of information on how rainfall simulation studies relate to hill-slope and landscape scale processes and measures. In this study we compare P mobilisation by examining P forms and concentrations in runoff from small-scale, high-intensity (SH, 1.5 m2, 80 mm/h) rainfall simulation and large-scale, low-intensity (LL, 1250 m2, 8 mm/h) simulations that have previously been shown to approximate natural runoff on hill-slopes. We also examined the effect of soil P status on this comparison. The SH methodology resulted in lower (average 56%) runoff P concentrations than those measured under the LL methodology. The interaction method × soil P status was highly significant (P < 0.001). There was no significant effect of method (SH v. LL) and soil P status on P forms (%).The hydrological characteristics were very different between the 2 methods, runoff rates being c. 42 and 3 mm/h for the SH and LL methods, respectively. We hypothesise that the lower runoff P concentrations from the SH method are the result of a combination of (i) the P mobilisation being a rate-limited process, and (ii) the relatively high runoff rates and short runoff path-lengths of the SH method allowing for relatively incomplete attainment of equilibrium between P in the soil/pasture system and runoff. We conclude that small-scale, high-intensity rainfall simulation provides a useful tool for studying treatment effects and processes of mobilisation in pastures, but concentration and load data should not be inferred for natural conditions at larger scales without a clear understanding of the effects of the rainfall simulation methodology on the results for the system being studied.

The effect of vegetation and soil texture on the nature of organics in runoff from a catchment supplying water for domestic consumption.

The influence of vegetation and soil texture on the concentration and character of dissolved organic matter (DOM) present in runoff from the surface and sub-surface of zero order catchments of the Myponga Reservoir-catchment (South Australia) was investigated to determine the impacts of catchment characteristics and land management practices on the quality of waters used for domestic supply. Catchments selected have distinct vegetative cover (grass, native vegetation or pine) and contrasting texture of the surface soil horizon (sand or clay loam/clay). Water samples were collected from three slope positions (upper, middle, and lower) at soil depths of ~30 cm and ~60 cm in addition to overland flows. Filtered (0.45 μm) water samples were analyzed for dissolved organic carbon (DOC) and UV-visible absorbance and by F-EEM and HPSEC with UV and fluorescence detection to characterize the DOM. Surface and sub-surface runoff from catchments with clay soils and native vegetation or grass had lower DOC concentrations and lower relative abundances of aromatic, humic-like and high molecular weight organics than runoff from sandy soils with these vegetative types. Sub-surface flows from two catchments with Pinus radiata had similar DOC concentrations and DOM character, regardless of marked variation in surface soil texture. Runoff from catchments under native vegetation and grass on clay soils resulted in lower DOC concentrations and hence would be expected to have lower coagulant demand in conventional treatment for potable water supply than runoff from corresponding sandy soil catchments. However, organics in runoff from clay catchments would be more difficult to remove by coagulation. Surface waters from the native vegetation and grass catchments were generally found to have higher relative abundance of organic compounds amenable to removal by coagulation compared with sub-surface waters. Biophysical and land management practices combine to have a marked influence on the quality of source water used for domestic supply.

Characterization of the transport of sediment and nutrients in the Mt Lofty Ranges watershed, South Australia

Local runoff from the catchments in the Mount Lofty Ranges watershed provides a major source (up to 60%) of drinking water for Adelaide, South Australia. In this study major land uses in the Mount Lofty Ranges were monitored for off-site transport of sediment and nutrients over approximately 40 months. Runoff from three case-study sites (apple, cherry and grape cultivation) was monitored using automatic samplers for collection of water samples on a flow-weighted basis. Selected nutrients and the total suspended sediment (TSS) were measured in unfiltered and filtered water samples to provide information about pollutant characterization and the mode of off-site transport. Flow meters were installed at each site to measure runoff and enabled concentration loads to be calculated.