Marcus Hardie

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.

Effect of biochar on nutrient leaching in a young apple orchard

Nutrient leaching from agricultural soils is a worldwide problem that has been implicated in deleterious impacts on the environment. Application of biochar to soil has been proposed as a means to reduce nutrient leaching and improve fertilizer use efficiency. The potential for biochar to reduce nutrient leaching and increase fertilizer use efficiency was tested by applying 47 Mg ha hardwood biochar before replanting a commercial apple () orchard, in the Huon Valley, Tasmania. Passive wick flux meters were installed at the base of the A1 horizon at a depth of 25 cm to monitor leachate volume and the concentration of nutrients leached below the A1 soil horizon over a 38 mo period. Biochar application significantly increased the concentration of phosphorous in the leachate, while having no significant effect on nitrate or potassium concentration. The volume of leachate collected in the flux meters was significantly higher in the biochar treatment, which resulted in significantly higher amounts of potassium and phosphorous being leaching from the biochar treatment than the control. Biochar application had no significant effect on either the concentration or the flux of nitrate leached from the A1 horizon. Nonetheless, nutrient application was well in excess of tree requirements, such that between 53 to 78% of the applied nitrogen, 5 to 11% of the applied phosphate, and 69 to 112% of the applied potassium were leached below the A1 horizon.

Effects of application of poppy waste on spinach yields, soil properties, and soil carbon sequestration in southern Tasmania

Production of fresh market salad and lettuce in southern Tasmania has reduced soil organic carbon levels, resulting in the development of surface crusts, erosion, and poor water infiltration. Options for increasing soil organic carbon under this production system are limited by strict food safety protocols which prohibit the use of composts or ‘animal’-based waste products. Poppy waste was identified as a suitable seed-free, inexpensive source of non-animal-based organic carbon. Trials were established on a Chromosol to evaluate the effects of poppy waste incorporation on soil organic carbon and production of Bocane spinach (Spinach oleracea). Application of 50–200 m3/ha of poppy waste resulted in significant yield loss (up to 57%) of seedlings planted within 8 weeks following waste incorporation. It was speculated that yield loss resulted from nitrogen drawdown; however, soil analyses demonstrated that yield loss resulted from a combination of increased soil pH and soil salinity (EC). The 200 m3/ha treatment increased soil pHwater from 7.2 before application to 8.5 and 7.7, at 4 and 22 weeks after application. Soil EC1 : 5 increased from 0.15 dS/m before application to 0.45 dS/m at 2 weeks after application, before returning to 0.15 dS/m at 22 weeks. Application of poppy waste at 200 m3/ha significantly increased soil organic carbon from 1.24% to 1.57%; however, applications at lower rates were not significant. The carbon sequestration efficiency from poppy waste to soil organic carbon was calculated to be approximately 0.20.

Benchmarking nitrous oxide emissions in deciduous tree cropping systems

Nitrous oxide (N2O) emissions contribute 6% of the global warming effect and are derived from the activity of soil-based microorganisms involved in nitrification and denitrification processes. There is a paucity of greenhouse gas emissions data for Australia’s horticulture industry. In this study we investigated N2O flux from two deciduous fruit tree crops, apples and cherries, in two predominant growing regions in eastern Australia, the Huon Valley in southern Tasmania (Lucaston – apples and Lower Longley – cherries), and high altitude northern New South Wales (Orange – apples and Young – cherries). Estimated from manual chamber measurements over a 12-month period, average daily emissions were very low ranging from 0.78gN2O-Nha–1day–1 in the apple orchard at Lucaston to 1.86gN2O-Nha–1day–1 in the cherry orchard in Lower Longley. Daily emissions were up to 50% higher in summer (maximum 5.27gN2O-Nha–1day–1 at Lower Longley) than winter (maximum 2.47gN2O-Nha–1day–1 at Young) across the four trial orchards. N2O emissions were ~40% greater in the inter-row than the tree line for each orchard. Daily flux rates were used as a loss estimate for annual emissions, which ranged from 298gN2O-Nha–1year–1 at Lucaston to 736gN2O-Nha–1year–1 at Lower Longley. Emissions were poorly correlated with soil temperature, volumetric water content, water filled porosity, gravimetric water content and matric potential – with inconsistent patterns between sites, within the tree line and inter-row and between seasons. Stepwise linear regression models for the Lucaston site accounted for less than 10% of the variance in N2O emissions, for which soil temperature was the strongest predictor. N2O emissions in deciduous tree crops were among the lowest recorded for Australian agriculture, most likely due to low rates of N fertiliser, cool temperate growing conditions and highly efficient drip irrigation systems. We recommend that optimising nutrient use efficiency with improved drainage and a reduction in soil compaction in the inter-row will facilitate further mitigation of N2O emissions.

Soil carbon sequestration in cool-temperate dryland pastures: mechanisms and management options

Permanent pastures, which include sown, native and naturalised pastures, account for 4.3 Mha (56%) of the national land use in Australia. Given their extent, pastures are of great interest with respect to their potential to influence national carbon (C) budgets and CO2 mitigation. Increasing soil organic C (SOC) mitigates greenhouse gases while providing other benefits such as pasture productivity, soil health and ecosystem services. Several management approaches have been recommended to increase C sequestration in pasture-based systems; however, results have proved variable and often contradictory between sites and years. Here, we present an overview of the processes and mechanisms responsible for C sequestration in permanent pastures. In addition, we discuss the merits of traditional and emerging pasture-management practices for increasing SOC in pastures, with a focus on dryland pasture systems of south-eastern Australia. We conclude by summarising the knowledge gaps and research priorities for soil C-sequestration research in dryland pastures. Our review confirms that soils under a range of pasture types have considerable potential for sequestration of atmospheric CO2 in Australia, and that the magnitude of this potential can be greatly modified by pasture-management practices. Although the shortage of long-term studies under Australian conditions limits our ability to predict the potential of various management approaches to sequester soil C, our review indicates that prevention of erosion through maintenance of groundcover and adoption of options that promote deep C sequestration are likely to confer broad-scale maintenance or increases in SOC in pasture soils over a decade or longer. We acknowledge that the evidence is limited; therefore, confidence in the recommended practices in different locations and climates is largely unknown.

Role of site in the mortality and production of Acacia mangium plantations in Indonesia

In Indonesia, Acacia mangium plantations exceed 1.6 Mha contributing approximately 3.5% of the country’s GDP. The viability of these plantations is increasingly threatened by fungal pathogens, insect pests, squirrels, monkeys, elephants and wind damage. Studies indicate that the problem is growing and in some areas, fungal pathogens such as Ganoderma and Ceratocystis species have contributed up to 50% tree mortality by the fourth rotation. Multiple statistical procedures were employed to examine the influence of soil and topographical properties on tree survival (trees ha−1), wood production (m3 ha−1), and mortality associated with Ganoderma root rot, Ceratocystis wilt and by wind. Soil family level was found to be a good indicator of tree mortality. Plots with fine-loamy Typic Kandiudult soils had the highest tree survival and mortality associated with species of Ganoderma and Ceratocystis, but had the lowest incidence of mortality by wind. The degree of association between soil and topographic variables with tree survival, wood production and the cause of mortality were poor and inconsistent. Tree survival was slightly higher on upslope areas away from valley bottoms, and drier mid-slopes, ridges and hilltops, and very low pH (<3.3) soils. Wood production was also slightly higher in drier, elevated locations, away from valley bottoms. Mortality by wind was slightly higher in moist, poorly drained, low-lying valley bottoms and topographically flat areas. Our ability to further pinpoint the influence of topography and soil attributes on wood production and cause of mortality was greatly compromised by the lack of site-specific soil data, and potential misclassification of the cause of mortality. This study could not reliably or consistently relate tree survival, wood production or the cause of mortality to any one, or combination of, soil and topographic variables.

Can soil crusting be reduced through application of gypsum, organic waste, and phosphoric acid?

Soil crusting is a form of land degradation in which the breakdown of aggregates results in the formation of a thin impermeable layer on the soil surface. An earlier pilot trial indicated that application of paper waste, gypsum, phosphoric acid, and covering the soil surface with wire mesh showed potential for reducing soil crusting. This study was established to evaluate the use of products for reducing the severity of soil crusting, while also testing different approaches for measuring the severity and likelihood of soil crust formation. Gypsum was applied at 0.25 and 0.50 kg m−2 (low rate [LG] and high rate [HG], respectively), paper waste was applied at 1, 2.5, and 7.5 kg m−2 (low [LPW], moderate [MPW], and high [HPW] rates, respectively), and phosphoric acid was applied at 80 and 160 mL m−2 (low rate [LP] and high rate [HP], respectively). Combinations of these products were made including (1) wire mesh (WM) and 0.50 kg m−2 gypsum (WM + HG); (2) 0.50 kg m−2 gypsum and 80 mL m−2 phosphoric acid (HG + LP); (3) 2.5 kg m−2 paper waste, 0.50 kg m−2 gypsum, and 80 mL m−2 phosphoric acid (MPW + HG + LP); and (4) 7.5 kg m−2 paper waste and 160 mL m−2 phosphoric acid (HPW + HP). The likelihood of crust formation was inferred from aggregate stability determined by rainfall simulation and wet sieving, while the severity of soil crusting was inferred from crust density, hydraulic conductivity, and penetration resistance. The four measures of crust severity/likelihood were highly correlated with each other (R2 = 0.57 to 0.80). The HPW + HP, MPW + HG + LP, and MPW treatments increased hydraulic conductivity by 72%, 66%, and 45%, respectively; increased aggregate stability determined by rainfall simulation by 28%, 37%, and 39%, respectively; reduced surface density by 10%, 7%, and 6%, respectively; and reduced penetration resistance by 33%, 37%, and 34% as average at all five sampling dates (days 8, 14, 28, 71, and 197). Moreover, the high rate of gypsum significantly reduced bulk density by 7% and penetration resistance by 26%, yet had no effect on any other measure of crusting. Phosphoric acid (HP) significantly increased aggregate stability determined by rainfall simulation by 29% (days 8, 14, 28, and 71), reduced bulk density by 6% (days 8 and 14), and increased hydraulic conductivity at day 8 by 110%. Reduced severity and or likelihood of crust formation following application of gypsum and paper waste were attributed to the increase in calcium cations (Ca+2) and soil organic carbon (C). The paper waste and gypsum were the most effective amendments over the duration of the trial, while phosphoric acid reduced the severity of crust formation in the 14 days after application. Recommendations are provided on the efficiency of different approaches for measuring soil crusting, in which penetration resistance is preferable because of its high correlation with other measurements and being the least time consuming.