Bennett MacDonald

Spatial distribution and management of total actual acidity in an acid sulfate soil environment, McLeods Creek, northeastern NSW, Australia

Abstract The spatial distribution of total actual acidity (TAA), defined as the total amount of acidity which exists in a soil at the time of sampling, was examined across an acid sulfate soil floodplain in northeastern New South Wales (NSW), Australia. Despite generally uniform soil conditions, there is considerable variation in the amount of acidity, and the amounts of soluble and exchangeable ionic species in the soil profile are positively correlated with this acidity. The surface hydrology of the site has been extensively modified for sugarcane production. It is hypothesised from air photo interpretation and the spatial soil data that variation in TAA is a result of the natural geomorphic environment, and that the current distribution pattern is a remnant of past natural land formation and hydrological processes controlling pyrite oxidation and acidity export. The degree to which land drainage caused the acidity is unclear, but the drainage systems provide the conduit for its increased transfer to estuaries. By investigating the distribution of acidity in the landscape, ‘hotspots’ can be identified and land managers can target these areas. Currently, the acidity is managed by a containment program in which it is kept within the soil profile and discharge into the estuary is minimised. Work is under way to apply emerging technology from the mining industry so that any acidity that enters drains is neutralised prior to discharge from the site.

The distribution of soluble cations within chenopod-patterned ground, arid western New South Wales, Australia

Abstract The soluble cation (Na, Mg, K, Ca) concentrations of the soils underlying different vegetated/soil surface units were measured within a chenopod-patterned ground complex. It was found that distribution of soil cations within the chenopod-patterned ground is not uniform across the landscape. The data shows that there are at least three chemically distinct zones: the bare, intermediate, and vegetated areas, within chenopod-patterned ground. The bare areas, which are considered salt dumps, are dominated by sodium and its concentration decreases towards the centre of the vegetated arcs. The magnesium and calcium ions have a similar pattern of distribution across vegetated–bare ground transects, but in the vegetated arcs they are relatively more concentrated than the sodium ions. The potassium ions are concentrated in the vegetated arcs and decrease in the bare ground. A model is proposed to explain how the spatial distribution of the soluble soil cations is maintained.

Preliminary Investigation into the Suitability of Permeable Reactive Barriers for the Treatment of Acid Sulfate Soils Discharge

Publisher Summary The generation and release of acidic water from acid sulfate soils are an environmental problem of international importance. This chapter focuses on the suitability of permeable reactive barriers packed with neutralizing agents such as calcite in the treatment technology for assisting in the management of drainage from acid sulfate soils. However, various factors need to be resolved prior to installation of such a barrier in an acid sulfate soils region. Preliminary data required for design include the water chemistry and local hydrology of the region. It is also particularly important to develop an understanding of the variability in flow and acid discharge through storm events, as these potentially constitute the times of greatest impact with respect to acid transport. Although significant concern exists that armoring by iron and aluminum oxide precipitates may limit the reactivity and longevity of the permeable reactive barrier, maintenance of saturated conditions and installation of effective subdrainage and/or flushing procedures can be effective in overcoming problems and yield an effective, low maintenance solution to an increasingly troublesome problem.

Gaseous Nitrogen Losses from Coastal Acid Sulfate Soils: A Short-Term Study

Abstract NO x and N 2 O emissions from coastal acid sulfate soils (CASS) cultivated for sugarcane production were investigated on the coastal lowlands of northern New South Wales, Australia. Two series of short-term measurements were made using chambers and micrometeorological techniques. Series 1 occurred during the wet season, the water-filled pore space (WFPS) was between 60%-80% and the site flooded during the measurements. Measurements were made directly after the harvest of soybean crop, which fixed an estimated 100 kg N ha −1 , and the emission amounted to 3.2 kg NO x -N ha −1 (12 d) and 1.8 kg N 2 O-N ha −1 (5d). Series 2 was made towards the end of the dry season when the WFPS was less than 60%. In Series 2, after an application of 50 kg N ha −1 , emissions were markedly less, amounting to 0.9 kg N ha −1 over 10 d. During both series when the soil was moist, emissions of NO x were larger than those of N 2 O. The emission of NO x appeared to be haphazard, with little time dependence, but there was a clear diurnal cycle for N 2 O, emphasising the need for continuous measurement procedures for both gases. These results suggest that agricultural production on CASS could be important sources of greenhouse gases and nitrogen practices will need to be optimised to reduce the offsite effects of atmospheric warming, acidification or nitrification. Many questions still remain unanswered such as the emissions during the soybean bean filling stage and crop residue decomposition, the longer-term losses following the fertiliser application and emissions from CASS under different land uses.

Influence of calcium and silica on hydraulic properties of sodium montmorillonite assemblages under alkaline conditions

A sodium-washed montmorillonite was exposed to calcium and silica under alkaline conditions in order to gain insight into possible interactions of engineered clay barriers and cementitious leachates found in many waste storage facilities. The changes in physico-chemical properties of the material were investigated using a combination of dead-end filtration, electrophoresis and scanning electron microscopy. The results show minimal differentiation between unaltered Na-montmorillonite samples at the two pH values tested (9 and 12), with the structure of the resulting assemblages arising from repulsive tactoid interactions. The addition of calcium (50 mM) greatly decreases the size of the structural network, and in doing so, increases the hydraulic conductivity approximately 65-fold, with the effect being greatest at pH 12. Whilst the addition of silica alone (10 mM) produced little change in the hydraulic properties of montmorillonite, its combined effect with calcium produced alterations to the structural assemblages that could not be accounted for by the presence of calcium alone. The likely binding of calcium with multiple silanol groups appears to enhance the retention of water within the Na-montmorillonite assemblage, whilst still allowing the fluent passage of water. The results confirm that polyvalent cations such as Ca(2+) may have a dramatic effect on the structural and hydraulic properties of montmorillonite assemblages while the effects of solutions containing both silicate and calcium are complex and influenced by silica-cation interactions.

Field-based measurements of sulfur gas emissions from an agricultural coastal acid sulfate soil, eastern Australia

The emissions of biogenic hydrogen sulfide (H2S) and sulfur dioxide (SO2) play important roles in the global atmospheric sulfur (S) cycle. Field-based investigations using ultraviolet fluorescence spectroscopy show that drained acid sulfate soils (ASS) are a potentially unaccounted source of biogenic H2S and SO2. Significant diurnal variations were evident in SO2 fluxes, with average daytime measurements 9.3–16.5-fold greater than night-time emissions. Similar diurnal patterns in H2S fluxes were observed but proved statistically insignificant. The results from simultaneously collected micrometeorological measurements suggest that emissions of SO2 and H2S are most likely occurring via different processes. The SO2 fluxes are closely linked to surface soil temperature and moisture content, whereas H2S is constantly emitted from the land surface at the two study sites. Drained ASS are most likely mapped as agricultural lands rather than drained backswamps. Therefore, these areas are likely to be assigned H2S and SO2 flux values of zero in greenhouse gas species inventories. These findings suggest a need to expand these measurements to other drained ASS areas to refine regional (and possibly global) atmospheric S budgets. Further research is necessary to elucidate the sources of measured S compounds, and specifically whether they are limited to individual agricultural drainage patterns in ASS.

Dissolved organic nitrogen contributes significantly to leaching from furrow-irrigated cotton–wheat–maize rotations

Leaching of nitrogen (N) in intensive irrigated agriculture can be a significant loss pathway. Though many studies have focussed on losses of mineral N, and in particular nitrate, dissolved organic N (DON) has received less coverage. In the present study, over a 5-year period (2008–2013), 740kgNha–1 fertiliser was applied to an irrigated cotton–wheat–maize rotation on a cracking clay (grey Vertosol). Deep drainage from the undisturbed soil profile at the site was measured at 2.1m below the soil surface using a variable tension lysimeter. In total, 108mm of drainage occurred during the 5 years and the majority of the drainage and the irrigations occurred during the cotton seasons. The majority of the N loss occurred during the first 3–4 irrigations and neither the N loss nor its composition were affected by the product or timing of the fertiliser application. The N in the drainage was composed of 12.8kgNOx-Nha–1, 8.7 DON-N and 0.1 NH4+-Nkgha–1, which shows that DON is an important component (40%) of the deep drainage N from irrigated Vertosol cotton production systems. Overall the total N flux lost via deep drainage represents 3% of the applied N fertiliser.

Spatial decoupling of soil nitrogen cycling in an arid chenopod pattern ground

Patterned ground is a characteristic landscape form in arid zones across the globe and is caused by the redistribution of runoff. The vegetation is concentrated in groves, which receive runoff water from the bare soil surface of the intergroves. Despite many soil chemical and physical investigations, biogeochemical cycles are not described for this landscape form. This study focuses on the distinct features of the nitrogen (N) and carbon (C) cycle in intergrove and grove areas of chenopod pattern-ground soils of the Australian rangelands. The N concentration of grove soils is low compared with the intergrove soil because of a close link between the N cycle in the topsoil and the plant primary production. In grove soils, denitrification including emission of N2O dominated the N cycle, whereas in intergrove soils, abiotic N2O uptake is the sole fate of atmospheric N2O. The intergrove soils appear to be generally poor in bacterial quantity and diversity, and lacking denitrification by bacteria, which has an impact on the C cycle as well. Production of CO2 and consumption of CH4 were substantially lower than grove soils. There was partitioning of key biogeochemical processes between the intergrove and grove soils, which results in a spatially decoupled soil N and C cycle in arid chenopod, pattern-ground ecosystems.

Quantification of NOx and NH3 emissions from two sugarcane fields

This paper reports emissions of NOx and NH3 from a rain-fed, fertilised, residue-blanketed sugarcane field at Mackay, Queensland. Emissions were measured using a micrometeorological flux-gradient technique for the whole of the 2006–07 season and for the first 2 months of the 2007–08 season. Nitrogen (N) fertiliser was applied as urea at a rate of 150 kg N ha–1 into slits 100–150 mm deep. Previous work at the site found that N2O emissions accounted for ~5 kg N ha–1, or 3% of the applied N in the 2006–07 season. In the present study, NOx and NH3 were emitted in both the 2006–07 and 2007–08 seasons and accounted for ~1.5 kg N ha–1, or ~1% of applied N. The main driver of NOx emissions appeared to be the availability of a soil mineral N source. However, the maximum N2O and NOx fluxes were offset by nearly 20 days, which indicated different emission pathways. After the soil mineral N was exhausted, the emissions of NOx were reduced. Emissions of NH3 continued at around the same rate for the whole of the growing season. Water-filled pore space, which was a main driver of N2O emissions, did not seem to influence the measured emissions of NOx or NH3.

Salinisation processes in a sub-catchment of Wybong Creek, Hunter Valley, Australia

Salinisation in the Hunter Valley, New South Wales, Australia, is a significant environmental issue which affects the regional wine, horse and beef industries. The sub-catchment of Wybong Creek causes increased salinity, sodicity and chlorine concentrations in the Hunter River. Sampling was undertaken in the scalded area of Manobalai, located in the mid-catchment area of Wybong Creek, to establish whether salt stores within the regolith, or saline groundwater discharge from deeper formations, are sources of Na+ and Cl− dominated water to Wybong Creek. Ten soil cores were collected from eight sites in the Manobalai area, with bores and piezometers installed to characterise groundwater chemistry and hydraulic head. The regolith within the Manobalai field site was non-saline, though Na+ and Cl− were the dominant ions in most regolith layers. The most saline regolith samples have coarse textures and a high moisture content, and occur in the valley floor. Cores from the salt scald have salt concentrations ranging from 274 to 2089 mg/kg, with a maximum Na+ and Cl− wt.% of 0.17. Saline groundwater in piezometers has total dissolved solid concentrations of up to 7277 mg/L. In this saline groundwater, Cl−/Br− ratios of up to 1767, Na+/Cl− ratios of 0.6–1.2 and 87Sr/86Sr ratios up to 0.709446 indicate halite dissolution and a marine source of solutes to groundwater. The Wittingham Coal Measures, which previous studies have linked to salinity elsewhere in the Hunter Valley, contain halite efflorescences and were intruded by marine water in their geological past. Salinity is due to discharge of this regional groundwater body in the Wybong catchment and not, as commonly assumed within Australia, due to dryland salinity.