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Abstract:Antecedent soil moisture significantly influenced the hydraulic conductivity of the A1, A2e and B21 horizons in a series of strong texture-contrast soils. Tension infiltration at six supply potentials demonstrated that in the A1 horizon, hydraulic conductivity was significantly lower in the 'wet' treatment than in the 'dry' treatment. However in the A2e horizon, micropore and mesopore hydraulic conductivity was lower in the 'dry' treatment than the 'wet' treatment, which was attributed to the precipitation of soluble amorphous silica. In the B21 horizon, desiccation of vertic clays resulted in the formation of shrinkage cracks which significantly increased near-saturated hydraulic conductivity and prevented the development of subsurface lateral flow in the 'dry' treatment. In the 'wet' treatment, the difference between the hydraulic conductivity of the A1 and B21 horizons was reduced; however, lateral flow still occurred in the A1 horizon due to difficulty displacing existing soil water further down the soil profile. Results demonstrate the need to account for temporal variation in soil porosity and hydraulic conductivity in soilwater model conceptualisation and parameterisation.

Development-perched watertables and subsurface lateral flows in texture-contrast soils (duplex) are commonly believed to occur as a consequence of the hydraulic discontinuity between the A and B soil horizons. However, in catchments containing shallow bedrock, subsurface lateral flows result from a combination of preferential flow from the soil surface to the soil—bedrock interface, undulations in the bedrock topography, lateral flow through macropore networks at the soil—bedrock interface, and the influence of antecedent soil moisture on macropore connectivity. Review of literature indicates that some of these processes may also be involved in the development of subsurface lateral flow in texture contrast soils. However, the extent to which these mechanisms can be applied to texture contrast soils requires further field studies. Improved process understanding is required for modelling subsurface lateral flows in order to improve the management of waterlogging, drainage, salinity, and offsite agrochemicals movement.

Abstract:Seasonal variation in potential water repellence has not been widely reported in the literature, and little is known of the processes that cause changes in potential water repellence. In this study, the severity and stability of potential water repellence varied seasonally from being weakly hydrophobic in July 2009 (water drop penetration time, 0.19 min; water entry potential, 0.0 cm) to severely hydrophobic (water drop penetration time, 54 min; water entry potential, 14.3 cm) in May 2009. Seasonal variation in the stability of potential water repellence was significantly correlated with cumulative rainfall, air temperature and soil water deficit, which indicated that the accumulation of water-repellent compounds, presumably polar waxes, resulted from microbial or plant inputs to the soil. Laboratory experiments demonstrated that saturating and mixing the soil resulted in a two to three order of magnitude reduction in the stability of potential water repellence, even after oven drying at 40 C and 60 C. Repeated leaching resulted in sequential reduction in both the stability and severity of water repellence. The significant correlation between soil water repellence and dissolved organic carbon content of the leachate, together with pedological evidence of organic staining of ped faces in the clay subsoil indicate that seasonal rainfall leached soluble water-repellent compounds from the topsoil. The reestablishment of water repellence after saturation and leaching required the input of new water-repellent compounds. These findings suggest that the use of surfactants before sowing may assist to leach water-repellent compounds from the topsoil, allowing improved infiltration and reduced runoff through the remainder of the cropping season.

Soil carbon (C) stocks were calculated for Tasmanian soil orders to 0.3 and 1.0 m depth from existing datasets. Tasmanian soils have C stocks of 49–117 Mg C/ha in the upper 0.3 m, with Ferrosols having the largest soil C stocks. Mean soil C stocks in agricultural soils were significantly lower under intensive cropping than under irrigated pasture. The range in soil C within soil orders indicates that it is critical to determine initial soil C stocks at individual sites and farms for C accounting and trading purposes, because the initial soil C content will determine if current or changed management practices are likely to result in soil C sequestration or emission. The distribution of C within the profile was significantly different between agricultural and forested land, with agricultural soils having two-thirds of their soil C in the upper 0.3 m, compared with half for forested soils. The difference in this proportion between agricultural and forested land was largest in Dermosols (0.72 v. 0.47). The total amount of soil C in a soil to 1.0 m depth may not change with a change in land use, but the distribution can and any change in soil C deeper in the profile might affect how soil C can be managed for sequestration. Tasmanian soil C stocks are significantly greater than those in mainland states of Australia, reflecting the lower mean annual temperature and higher precipitation in Tasmania, which result in less oxidation of soil organic matter.

The effect of water repellence and antecedent soil moisture on wetting front stability and infiltration rate are reported for a seasonally water repellent topsoil. The effect of water repellence on infiltration was determined by comparing the in situ infiltration of water to that of a 7M ethanol solution. Wetting front stability was measured during infiltration of water into repacked, wettable and water repellent soils, within a Hele‐Shaw chamber. Water repellence restricted in situ movement of water through large macropores (>500 μm), which decreased intrinsic permeability by 1 to 2 orders of magnitude. In repacked soils, water repellence caused the development of unstable wetting fronts and reduced infiltration from 240 mm h−1 to 101.7 mm h−1. Infiltration into wettable soils at moisture contents near field capacity was expected to result in rapid infiltration and stable wetting fronts. However in repacked soils, wetting front instability developed, and infiltration rates were 190% lower when air and/or water movement through the base of the chamber was restricted. Infiltration into in situ soil was also slower at high antecedent soil moisture. The hydraulic conductivity of the 7M ethanol solution decreased significantly from 112.3 mm h−1 in dry water repellent conditions, to 35.6 mm h−1 in wettable soils at high antecedent moisture contents. Consequently the previously reported development of wetting front instability and reduced infiltration into in situ wettable soils at high moisture contents were confirmed and attributed to difficulty displacing existing soil water during infiltration of new water.

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