Vanessa N.L. Wong

Salinity and sodicity effects on respiration and microbial biomass of soil

An understanding of the effects of salinity and sodicity on soil carbon (C) stocks and fluxes is critical in environmental management, as the areal extents of salinity and sodicity are predicted to increase. The effects of salinity and sodicity on the soil microbial biomass (SMB) and soil respiration were assessed over 12weeks under controlled conditions by subjecting disturbed soil samples from a vegetated soil profile to leaching with one of six salt solutions; a combination of low-salinity (0.5dSm−1), mid-salinity (10dSm−1), or high-salinity (30dSm−1), with either low-sodicity (sodium adsorption ratio, SAR, 1), or high-sodicity (SAR 30) to give six treatments: control (low-salinity low-sodicity); low-salinity high-sodicity; mid-salinity low-sodicity; mid-salinity high-sodicity; high-salinity low-sodicity; and high-salinity high-sodicity. Soil respiration rate was highest (56–80mg CO2-C kg−1 soil) in the low-salinity treatments and lowest (1–5mg CO2-C kg−1 soil) in the mid-salinity treatments, while the SMB was highest in the high-salinity treatments (459–565mg kg−1 soil) and lowest in the low-salinity treatments (158–172mg kg−1 soil). This was attributed to increased substrate availability with high salt concentrations through either increased dispersion of soil aggregates or dissolution or hydrolysis of soil organic matter, which may offset some of the stresses placed on the microbial population from high salt concentrations. The apparent disparity in trends in respiration and the SMB may be due to an induced shift in the microbial population, from one dominated by more active microorganisms to one dominated by less active microorganisms.

Clumped and isolated trees influence soil nutrient levels in an Australian temperate box woodland

Isolated paddock trees are a common feature of agri-pastoral landscapes in south-eastern Australia. We assessed the impact of trees on soil nutrients by examining (1) changes in soil nutrients under clumped and isolated (living and dead) trees at four microsites corresponding with increasing distance from the trunk (trunk, mid-canopy, drip line, open), and (2) changes with depth under trees growing in clearly-defined clumps. We detected significantly greater concentrations of organic C, and total N and S under trees growing in clumps compared with either isolated living or dead trees. Levels of soluble Ca2+, K+ and Mg2+, pH, electrical conductivity (EC) and available P declined with increasing distance from the trees, but there were no significant trends for organic C, or total N and S. The concentration of most nutrients declined with depth, particularly at microsites close to the trunk, while pH increased with depth. We believe that differences in chemistry were largely driven by greater inputs of organic matter under the trees. This study reinforces the view that trees, whether scattered or in clumps, are important for soil nutrient conservation in agri-pastoral landscapes.

Digital soil mapping of a coastal acid sulfate soil landscape

Coastal floodplains are commonly underlain by sulfidic sediments and coastal acid sulfate soils (CASS). Oxidation of sulfidic sediments leads to increases in acidity and mobilisation of trace metals, resulting in an increase in the concentrations of conducting ions in sediment and pore water. The distribution of these sediments on floodplains is highly heterogeneous. Accurately identifying the distribution ofCASS isessential for developing targeted management strategies. One approach is the use of digital soil mapping (DSM) using ancillary information. Proximal sensing instruments such as an EM38 can provide data on the spatial distribution of soil salinity, which is associated with CASS, and can be complemented by digital elevation models (DEM). We used EM38 measurements of the apparent soil electrical conductivity (ECa) in the horizontal and vertical modes in combination with a high resolution DEM to delineate the spatial distribution of CASS. We used a fuzzy k-means algorithm to cluster the data. The fuzziness exponent, number of classes (k) and distance metric (i.e. Euclidean, Mahalanobis and diagonal) were varied to determine a set of parameters to identify CASS. The mean-squared prediction error variance of the class mean of various soil properties (e.g. EC1:5 and pH) was used to identify which of these metrics was suitable for further analysis (i.e. Mahalanobis) and also determine the optimal number of classes (i.e. k=4). The final map is consistent with previously defined soil-landscape units generated using traditional soil profile description, classification and mapping. The DSM approach is amenable for evaluation on a larger scale and in order to refine CASS boundaries previously mapped using the traditional approach or to identify CASS areas that remain unmapped.

Seawater-induced mobilization of trace metals from mackinawite-rich estuarine sediments

Benthic sediments in coastal acid sulfate soil (CASS) drains can contain high concentrations (~1-5%) of acid volatile sulfide (AVS) as nano-particulate mackinawite. These sediments can sequester substantial quantities of trace metals. Because of their low elevation and the connectivity of drains to estuarine channels, these benthic sediments are vulnerable to rapid increases in ionic strength from seawater incursion by floodgate opening, floodgate failure, storm surge and seasonal migration of the estuarine salt wedge. This study examines the effect of increasing seawater concentration on trace metal mobilization from mackinawite-rich drain sediments (210-550 μmol g⁻¹ AVS) collected along an estuarine salinity gradient. Linear combination fitting of S K-edge XANES indicated mackinawite comprised 88-96% of sediment-bound S. Anoxic sediment suspensions were conducted with seawater concentrations ranging from 0% to 100%. We found that mobilization of some metals increased markedly with increasing ionic strength (Cu, Fe, Mn, Ni) whereas Al mobilization decreased. The largest proportion of metals mobilized from the labile metal pool, operationally defined as Σexchangeable + acid-extractable + organically-bound metals, occurred in sediments from relatively fresh upstream sites (up to 39% mobilized) compared to sediments sourced from brackish downstream sites (0-11% mobilized). The extent of relative trace metal desorption generally followed the sequence Mn > Ni ≈ Cu > Zn > Fe > Al. Trace metal mobilization from these mackinawite-rich sediments was attributed primarily to desorption of weakly-bound metals via competitive exchange with marine-derived cations and enhanced complexation with Cl⁻ and dissolved organic ligands. These results have important implications for trace metal mobilization from these sediments at near-neutral pH under current predicted sea-level rise and climate change scenarios.

Soil organic carbon stocks in saline and sodic landscapes

Increasing salinity (high levels of water-soluble salts) and sodicity (high levels of exchangeable sodium) are serious land degradation issues worldwide. In Australia, salinity and sodicity affect a large proportion of the landscape and often coincide with agricultural land. Despite the areal extent of salt-affected soils, both worldwide and in Australia, few data exist on soil organic carbon (SOC) stocks in these areas. For this study, the level of SOC was determined in scalded (bare areas without vegetation), scalded-eroded, vegetated, and revegetated (i.e. sown pasture) soil profiles from 2 sites in the Southern Tablelands region of New South Wales, Australia. SOC concentration was significantly higher in the profiles that were vegetated with native pasture (1.96-2.71% in the 0-0.05 m layer) or revegetated with sown pasture (2.35% in the 0-0.05 m layer), and lower in those profiles that were scalded (1.52% in the 0-0.05 m layer) or scalded-eroded (0.16-0.30% in the 0-0.05 m layer). These lower SOC levels are reflected throughout the profiles of the scalded and scalded-eroded soils. The soil carbon stocks to 0.30 m are also much lower in the scalded and scalded-eroded soils that have been affected by salinity and sodicity. The profiles that were vegetated with native pasture had carbon stocks to 0.30 m of 35.2-53.5 t/ha, while the sown pasture had 42.1 t/ha. This compares with the scalded profiles with 19.8 t/ha and the scalded-eroded profiles with 7.7-11.4 t/ha to 0.30 m. The presence of vegetation ameliorates several soil properties and results in the differences in SOC and other soil properties between scalded and vegetated profiles at the surface and at depth.

Hybrid brown coal-urea fertiliser reduces nitrogen loss compared to urea alone

Synthetic nitrogen (N) fertilisers, such as urea, are susceptible to rapid dissipation from soil. More gradual release of mineral N from fertiliser may reduce the off-site movement of mineral N, thereby enhancing N supply to crops and minimising negative off-site impacts. We hypothesised that granulation of urea with humified brown coal (BC) delays mineral N release and maintains higher concentrations of N in soil than conventional urea granules. Four different brown coal-urea granules, with C:N ratios of 1-10, were prepared by pan granulation. Advanced spectroscopic and X-ray powder diffraction (XRD) techniques confirmed loading of urea-N into the BC structure. Nitrogen-release from BCU granules was slower than from urea, resulting in higher N retention over a longer period for increasing growth and N uptake by crop plants. This trend increased with higher loading of BC, emphasising the significant role of BC in N retention. These findings support the hypothesis that BC is suitable for developing slow release N fertilisers.

Changes in soil organic carbon fractions after remediation of a coastal floodplain soil

Coastal floodplain soils and wetland sediments can store large amounts of soil organic carbon (SOC). These environments are also commonly underlain by sulfidic sediments which can oxidise to form coastal acid sulfate soils (CASS) and contain high concentrations of acidity and trace metals. CASS are found on every continent globally except Antarctica. When sulfidic sediments are oxidised, scalds can form, which are large bare patches without vegetation. However, SOC stocks and fractions have not been quantified in these coastal floodplain environments. We studied the changes in soil geochemistry and SOC stocks and fractions three years after remediation of a CASS scald. Remediation treatments included raising water levels, and addition of either lime (LO) or lime and mulch (LM) relative to a control (C) site. We found SOC concentrations in the remediated sites (LO and LM) were more than double than that found at site C, reflected in the higher SOC stocks to a depth of 1.6xa0m (426 Mg C/ha, 478 Mg C/ha and 473 Mg C/ha at sites C, LO and LM, respectively). The particulate organic C (POC) fraction was higher at sites LO and LM due to increased vegetation and biomass inputs, compared to site C. Reformation of acid volatile sulfide (AVS) occurred throughout the profile at site LM, whereas only limited AVS reformation occurred at sites LO and C. Higher AVS at site LM may be linked to the additional source of organic matter provided by the mulch. POC can also potentially contribute to decreasing acidity as a labile SOC source for Fe(3+) and SO4(2-) reduction. Therefore, coastal floodplains and wetlands are a large store of SOC and can potentially increase SOC following remediation due to i) reduced decomposition rates with higher water levels and waterlogging, and ii) high C inputs due to rapid revegetation of scalded areas and high rates of biomass production. These results highlight the importance of maintaining vegetation cover in coastal floodplains and wetlands for sequestering SOC.

Mobilisation, alteration, and redistribution of monosulfidic sediments in inland river systems

The accumulation of monosulfidic sediments in inland waterways is emerging as a major environmental issue. Mobilisation and suspension of monosulfidic sediments can result in deoxygenation, acidification of the water column and mobilisation of trace metals. The controls on monosulfidic sediment mobilisation and the critical thresholds for its scour and entrainment have not been established. This study examines the effect of a minor flood event (average return interval of 5 years) on sulfidic sediment scour in the Wakool River in southern NSW, Australia. Five profiles were sampled within a small (~300 m) reach before and after a minor flood event to determine the degree of sediment scour and transport. The results indicate substantial scour of both monosulfidic sediments and underlying bed sediments (approximately 2100 m(3)). Changes in the sediment geochemistry suggest large concentrations of monosulfidic sediments had been suspended in the water column, partially-oxidised and redeposited. This is supported by (210)Pb results from one of the profiles. These results suggest that these monosulfidic sediments can move as bed load during minor flood events.

Critical metals in the critical zone: controls, resources and future prospectivity of regolith-hosted rare earth elements

ABSTRACT The rare earth elements (REE) are a group of 17 metals that include the lanthanides, Sc and Y, which are critical for many modern technologies including consumer electronics, medicine and communication. One of the major controls on the concentrations of the REE in regolith material (including soils) is the abundance of these elements in the parent material. It is known that REE concentrations are largely inherited from the protolith rather than acquired during pedogenic processes but our understanding of how pedogenesis affects fractionation and accumulation of REE to produce potentially economic deposits of these critical metals is limited. This study provides a review of (1) the biogeochemical controls on REE distribution and mobility during pedogenesis and (2) the potential for REE extraction from regolith material. Factors that control mobilisation of REE during weathering include (1) the initial distribution of the REE in protolith minerals and the resistivity of these phases to weathering, (2) adsorption and absorption of REE to Fe- and Mn oxides, clay minerals and organic matter and (3) variations in pH and Eh conditions. We also discuss the relative importance of biogeochemical controls on REE mobility in soils in southern Australia, where REE concentrations are demonstrated to be largely a function of weathering of REE-enriched protoliths, the sorption of REE to weathering products and the accumulation of resistant minerals in soils.

Applied geomorphic mapping for land management in the River Murray corridor, SE Australia

Abstract The River Murray Corridor, in SE Australia, was the subject of a large integrated project aimed at providing answers to a number of important land and environmental management questions and guidance for land use planning. Airborne electromagnetic (AEM) surveys were carried out in conjunction with a light detection and ranging survey in 2007. These data were combined with various satellite remote-sensing data and field observations to produce a number of maps, including maps of landforms and surface materials. These latter maps were used to assess the value of the AEM data, identify potential surface and sub-surface flow pathways and map potential recharge on the Murray River floodplain surface. This allowed the assessment of potential salt storage and the impact of various land use options on salt storage and mobilisation.