Rb Doyle

Clomazone dissipation in four Tasmanian topsoils

Abstract Clomazone dissipation was examined in four soils in field experiments. Field half-lives were 6 to 59 d (average of four field sites was 35 d) for ferrosol (clay loam), kurosol (loamy sand), sodosol (silt loam), and vertosol (light clay). The Hoerl equation provided a better fit to the measured field concentration at all four sites than did a first-order equation. The order of clomazone dissipation rate was ferrosol > sodosol > kurosol > vertosol. Clomazone desorption varied with soil type, with apparent Kd values of 3.6, 1.7, 1.8, and 2.4 for ferrosol, sodosol, kurosol, and vertosol, respectively. Clomazone residues became more strongly sorbed with time, as indicated by desorption hysteresis, but detectable concentrations were present in all soils 1 yr after application. The data indicate that the potential for carryover injury to crops is greatest in the kurosol and least in the ferrosol. Nomenclature: Clomazone.

Determining the frequency, depth and velocity of preferential flow by high frequency soil moisture monitoring

Preferential flow in agricultural soils has been demonstrated to result in agrochemical mobilisation to shallow ground water. Land managers and environmental regulators need simple cost effective techniques for identifying soil - land use combinations in which preferential flow occurs. Existing techniques for identifying preferential flow have a range of limitations including; often being destructive, non in situ, small sampling volumes, or are subject to artificial boundary conditions. This study demonstrated that high frequency soil moisture monitoring using a multi-sensory capacitance probe mounted within a vertically rammed access tube, was able to determine the occurrence, depth, and wetting front velocity of preferential flow events following rainfall. Occurrence of preferential flow was not related to either rainfall intensity or rainfall amount, rather preferential flow occurred when antecedent soil moisture content was below 226 mm soil moisture storage (0-70 cm). Results indicate that high temporal frequency soil moisture monitoring may be used to identify soil type - land use combinations in which the presence of preferential flow increases the risk of shallow groundwater contamination by rapid transport of agrochemicals through the soil profile. However use of high frequency based soil moisture monitoring to determine agrochemical mobilisation risk may be limited by, inability to determine the volume of preferential flow, difficulty observing macropore flow at high antecedent soil moisture content, and creation of artificial voids during installation of access tubes in stony soils.

Subsurface Lateral Flow in Texture-Contrast (Duplex) Soils and Catchments with Shallow Bedrock

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.

Impact of short-rotation Acacia hybrid plantations on soil properties of degraded lands in Central Vietnam

Acacia hybrid (A. mangiumA. auriculiformis) is the main species planted for short-rotation forestry in Vietnam. In this study, the effect of these plantations on some key properties of degraded gravelly soils in Central Vietnam was assessed. Soil samples were collected from second- or third-rotation plantations representative of five age classes (0.5-5 years old), and in adjacent abandoned lands as controls. Compared with abandoned land, stock of total soil carbon (C) was significantly higher at ages 0.5, 1.5, 2.5 and 5 years (18.4-19.5 v. 13.0Mgha -1 ), total nitrogen (N) at 0.5 and 1.5 years (1.5-1.7 v. 1.0Mgha -1 ), exchangeable calcium at 0.5, 1.5 and 2.5 years (215-294 v. 42Mgha -1 ), magnesium at 0.5, 1.5, 2.5 and 3.5 years (39-48 v. 19Mgha -1 ), and sodium at all ages (46-59 v.5Mgha -1 ). Electrical conductivity was significantly higher at all ages (58.5-69.4 v. 32.7 mScm -1 ). Differences in extractable phosphorus and exchangeable potassium were not significantly different between plantations and abandoned land. Bulk density was significantly lower in plantations than abandoned land at all ages (1.36-1.42 v. 1.55Mgha -1 ), pHCaCl2 at 0.5 and 5 years (3.78-3.84 v. 3.98), and pHH2O at 5 years (4.30 v. 4.52). Because the soils were gravelly, differences in concentration of total C and nutrients between abandoned land and plantations were not the same as those for stocks after correction for gravel content and bulk density. Within a rotation, most soil properties did not change significantly with plantation age, although they appeared to decrease during the first 3 years; total C then recovered to initial levels, but total N and exchangeable cations remained lower. Some soil properties were strongly related to gravel content and elevation, but not to growth rate. We conclude that consecutive plantings of short-rotation Acacia hybrid on degraded and abandoned land can lead to changes in some soil properties. Additional keywords: degraded land, gravelly soil, nitrogen fixation, nutrient stock, soil amelioration.

Land use and management influences on surface soil organic carbon in Tasmania

The effects of environmental parameters, land-use history, and management practices on soil organic carbon (SOC) concentrations, nitrogen, and bulk density were determined in agricultural soils of four soil types in Tasmania. The sites sampled were Dermosols, Vertosols, Ferrosols, and a group of texture-contrast soils (Chromosol and Sodosol) each with a 10-year management history ranging from permanent perennial pasture to continuous cropping. Rainfall, Soil Order, and land use were all strong explanatory variables for differences in SOC, soil carbon stock, total nitrogen, and bulk density. Cropping sites had 29–35% less SOC in surface soils (0–0.1 m) than pasture sites as well as greater bulk densities. Clay-rich soils contained the greatest carbon stocks to 0.3 m depth under pasture, with Ferrosols containing a mean of 158 Mg C ha–1, Vertosols 112 Mg C ha–1, and Dermosols 107 Mg C ha–1. Texture-contrast soils with sandier textured topsoils under pasture had a mean of 69 Mg C ha–1. The range of values in soil carbon stocks indicates considerable uncertainty in baseline values for use in soil carbon accounting. Farmers can influence SOC more by their choice of land use than their day-to-day soil management. Although the influence of management is not as great as other inherent site variables, farmers can still select practices for their ability to retain more SOC.

Plant growth and soil responses to soil applied organic materials in Tasmania, Australia

Biosolids, poppy mulch (PM), and poppy seed waste (PSW) were applied to soils in barley and wheat field trials in two areas of Tasmania to determine crop and soil responses and the potential for these materials to substitute for inorganic fertiliser. Lime-amended biosolids (LAB) was applied at one, two, and five times the nitrogen-limiting biosolids application rate (NLBAR). Anaerobically digested biosolids (ADB) was applied at the NLBAR, and PM and PSW were applied at industry-recommended rates. The results indicated that ADB, LAB, PM, and PSW can substitute for inorganic fertiliser in meeting plant nutrient requirements but may be releasing more plant-available N than guideline assumptions. However, organic amendments are less easily managed and applied than inorganic fertiliser, and release of plant-available N from organic amendments may be too rapid for plant uptake to prevent leaching losses. LAB can also leave elevated residual extractable phosphorus in soil after two cereal crops, and PM and LAB can increase soil pH. Microbial biomass showed an inconsistent, and in some cases unexpected, response to organic amendments after 1 year and no significant relationship to changes in soil carbon.

Persistent improvements in the structure and hydraulic conductivity of a Ferrosol due to liming

Changes in the soil structure and hydraulic conductivity of an Acidic Red Ferrosol were measured in a long-term (1968-2003) fertiliser experiment on pasture in north-western Tasmania, Australia. Studies were initiated following observations of both softer soil surface and cracking on plots that had received 15 t/ha of ground agricultural limestone. Liming decreased penetration resistance and increased hydraulic conductivity. These structural improvements were associated with increased mean dry aggregate size, a small increase in wet aggregate stability, higher exchangeable calcium levels, and increased plant growth, but a 9% decrease in total soil organic carbon in the surface 50 mm. This decrease in organic carbon was not associated with deterioration in soil structure, as may have been anticipated. This was probably because total organic C was still 82 g/kg on unlimed plots. Decreases in soil penetration resistance due to liming increased the likelihood of pugging from livestock but may improve ease of tillage. This research demonstrates that liming can improve the structure of a well-aggregated Ferrosol as well as its previously reported effects of increasing soil pH and yields of pasture and barley despite decreasing organic C.

Effects of light availability on crown structure, biomass production, light absorption and light-use efficiency of Hopea odorata planted within gaps in Acacia hybrid plantations

Background: Young saplings of Hopea odorata, a native dipterocarp in Vietnam, require shading to prevent photoinhibition but they must avoid competition that stagnates their growth. Aims: To develop a silvicultural regime by examining how the biomass production of H. odorata changes along a light gradient in gaps within Acacia hybrid plantations. Methods: Hopea odorata saplings were planted in 22-m-diameter circular gaps within a 3-year-old Acacia hybrid plantation and in 5-m and 7.5-m-wide strip gaps within a 2.5-year-old Acacia hybrid plantation. Results: In the circular gap, biomass growth increased nearly tenfold from the gap perimeter (GP) to about 9 m from the perimeter, and then decreased. This was associated with a fivefold increase in the absorption of photosynthetically active radiation (APAR) and a nearly twofold increase in light-use efficiency (LUE). The increasing APAR was primarily related to increasing H. odorata crown sizes and a reduction in shading from the Acacia hybrid. In the strip gaps, the APAR was similar to that at the GP in the circular gap, however LUE was about threefold higher. Conclusions: Hopea odorata expressed plasticity in crown structure in response to incident light. While it grows under a wide range of availability of photosynthetically active radiation, growth increases strongly to a certain level of PAR. The best conditions for H. odorata growth were near the centre of the circular gaps, where PAR was 60% of full sunlight, while the strip gaps, where PAR was 20%, were too narrow owing to shading from the Acacia hybrid.

The Distribution of Sodic Soils in Tasmania

It is estimated that sodic soils (ESP>6) occupy at least 23% of Tasmanias land area based on the present limited soil data set. Sodic soils are mostly restricted to lower rainfall areas (<800 mm/y) of eastern Tasmania, occurring primarily in the Launceston Tertiary Basin, the Derwent, Coal, Jordan and Huon River Valleys and on Flinders Island. In Tasmania, sodic soils have formed predominantly from Triassic and Permian mudstones and sandstones, Tertiary clays and unconsolidated Quaternary deposits. However, profiles with sodic features have also developed above granite, Jurassic dolerite and Tertiary basalt. Sodic soils most commonly occur on lowland plains, river terraces and in valley floors. In Tasmania, sodic soils are characterized morphologically by: (i) abrupt separation of a sandy, bleached A2 horizon from a moderately sodic (ESP 6-15) clay subsoil; (ii) coarse prismatic, columnar and/or angular blocky pedality in the subsoil, which may exhibit vertic properties; (iii) hardsetting sandy A2 horizons in some profile classes; (iv) fine sandy crack infills and clay-organic coatings on ped faces in the upper B2 horizon; and (v) thick, sticky and greasy fine clay argillans on ped faces, and clay infills in cracks and other voids in the lower B2 horizon that contribute to reduced porosity and permeability. Sodic soils in Tasmania have traditionally been utilized for pasture production with occasional cultivation for fodder crops and pasture renewal. Under a pastoral system, few sodicity problems have been recognized as such. However, in the last 10 years there has been increased cropping, particularly for poppies and more recently potatoes. Soil structure decline and drainage problems have become key factors limiting production. Management problems are mainly due to poor internal and external drainage, with poor structure in the A2 horizons which liquefy in winter and often set hard in summer. Salinity in associated drainage depressions is a problem gaining increasing recognition.

Novel 3D geometry and models of the lower regions of large trees for use in carbon accounting of primary forests

The largest uncertainty in human’s contribution to climate change from land use is the fate of carbon that was below ground in pre-modified forests. We produced high-resolution 3D models of the rarely measured zone near the base of large, mature trees by using photogrammetry. The models led to equations linking the easy-to-measure trunk diameter and ground slope to attributes such as tree buttress shape, humus mound, wood and hollow area, and root volume. The equations can be used for carbon accounting. The 3D models are irreplaceable, being for increasingly rare, large trees, and may be useful to other scientific endeavours.