J. W. Cox

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.

Seasonal changes in hydrochemistry along a toposequence of texture-contrast soils

Ameliorative strategies are urgently required in some agricultural catchments in southern Australia to reduce the loss of potential contaminants to streams. However, a better understanding of where the contaminants are generated on hillslopes, their forms, and the pathways through which they are transported were required. Thus, seasonal changes in the quantities and forms of several chemical species were measured in both vertical and lateral flow pathways at 4 sites along a toposequence in the Mt Lofty Ranges, South Australia. Instrumentation was installed to measure and quantify overland flow and throughflow, and porous-wick samplers were installed at 2 depths to study the chemistry of leachate. Neutron moisture meter access tubes were installed to measure seasonal changes in soil water content with depth as this influences chemical concentrations and mobility. In years of average to below average annual rainfall, throughflow was the most important transport pathway for contaminants. However, it was expected that overland flow will be the dominant transport pathway when annual rainfall is above about 550 mm. Changes in water content of the texture-contrast soils was caused by seasonal rainfall causing periodic saturation, by waterlogging, groundwater, or both. This affected the type and form of contaminant. For example, Na and Cl concentrations were generally only large (800 and 1500 mg/L, respectively) on the lower slopes but in the wettest seasons their concentrations increased at depth on mid-slopes due to the influence of shallow saline groundwater. These chemicals then leached when groundwater levels subsided. The results suggest that ameliorative strategies to reduce agricultural contaminants should target the transport pathways specific to each chemical species, at the point (or points) in the landscape where they are generated.

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.

Crop rotation effect on wheat grain yield as mediated by changes in the degree of water and nitrogen co-limitation

Theoretically, growth of stressed plants is maximised when all resources are equally limiting. The concept of co-limitation could be used to integrate key factors affected by crop rotation. This paper tested the hypothesis that the effect of crop rotation on the yield of wheat is partially mediated by changes in the degree of co-limitation between nitrogen and water. Four rotations were established on a sodic, supracalcic, red chromosol in a Mediterranean-type environment of southern Australia. Rotations included wheat grown after (a) faba bean harvested for grain, (b) faba bean incorporated as green manure, (c) ryegrass pasture, or (d) medic pasture; barley was grown after wheat in all cases. The response of wheat to the rotations during 3 growing seasons was analysed in terms of nitrogen and water co-limitation, and the response of barley was taken as a measure of the persistence of rotation effects. Daily scalars quantifying water and nitrogen stress effects on tissue expansion were calculated with a crop simulation model. These scalars were integrated in a series of seasonal indices to quantify the intensity of water (SW ) and nitrogen stress (SN ), the aggregated intensity of water and nitrogen stress (SWN ), the degree of water and nitrogen co-limitation (CWN ), and the integrated effect of stress and co-limitation (SCWN 25 CWN/SWN ). The expectation is that grain yield should be inversely proportional to stress intensity and directly proportional to degree of co-limitation, thus proportional to SCWN . Combination of rotations and seasons generated a wide variation in the amount of water and inorganic nitrogen in the 1-m soil profile at the time of wheat sowing. Plant-available water ranged from 33 to 107 mm, and inorganic nitrogen from 47 to 253 kg N/ha. Larger amounts of nitrogen were found after green-manured faba bean, and smaller after grass pasture. There was a consistent effect of rotation on wheat yield and grain protein content, which persisted in subsequent barley crops. Measured grain yield of wheat crops ranged from 2.5 to 4.8 t/ha. It was unrelated to water or nitrogen stresses taken individually, inversely related to the aggregated stress index SWN , and directly related to the CWN index of co-limitation. The combination of stress and co-limitation in a single index SCWN accounted for 65% of the variation in measured crop yield. This is a substantial improvement with respect to the stress effect quantified with SWN , which accounted for 43% of yield variation. It is concluded that rotation effects mediated by changes in the relative availability of water and nitrogen can be partially accounted for by degree of resource co-limitation.

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 Role of Shallow Drains in Removing 'Excess' Water from Texture-contrast Soils

Abstract Variability of flow in shallow drains installed in a grazing catchment in South Australia was partly caused by the enormous variability each winter in the saturated thickness of the texture-contrast soils and their hydraulic conductivity. High variability has been found in other trials of shallow drains but the causes were not well understood. This study found that perched watertables are not always found on top of the B horizon. The depth and thickness of saturation varies with landscape position (between texture-contrast soil types) and also varies seasonally at one landscape position. Because drains are on a gradient, they pass through a variety of texture-contrast soils. These soils may have different soil chemistry, physical properties and depth of saturation. The presence of sodic subsoils with low hydraulic conductivity has a major impact on drainage volumes, particularly throughflow. In some years the drains intercepted deep, perched water or groundwater, which rose into the collection zone of the shallow drain. Flow from the drains in the study catchment was compared to that from other catchments with texture-contrast soils. Drain flows are highly variable and dependent on localised soil conditions, making flow prediction difficult. Despite this, a clear relationship was found between average annual catchment drain flow and annual rainfall. Drains installed across waterlogged paddocks with texture-contrast soils in southern Australia will average 10 mm flow (2.5% of annual rainfall) when annual rainfall is 400 mm, increasing to 100 mm (14%) when rainfall is 700 mm.