Phillip Hirst

The acid flux dynamics of two artificial drains in acid sulfate soil backswamps on the Clarence River floodplain, Australia

The export of acidity, iron, aluminium, and sulfate to an estuary from 2 drains in acid sulfate soil backswamps was monitored over 18 months. The backswamps had similar geomorphology, stratigraphy, and drainage density, and comparable soil and groundwater acidity. However, the flux rates, temporal dynamics, and export pathways of acid and other sulfide oxidation products varied greatly and were controlled to first order by (i) the saturated hydraulic conductivity (K) of sulfuric horizons and (ii) the tidally influenced groundwater gradients. The site with very high K and large tidally influenced groundwater gradients had high acid flux rates (5300 mol H+/ha.year), chronic acid discharge, high drain water acid and metal concentrations, and the primary flux pathway was direct groundwater seepage (interflow/bypass flow) to the drain. The site with lower K and smaller groundwater gradients displayed low acid flux rates (50 mol H+/ha.year), infrequent, highly episodic discharge, and the primary flux pathway was dilute surface runoff following dissolution of sulfide oxidation products accumulated on the soil surface. Importantly, the majority of acid export at both sites occurred while the backswamp groundwater level was within a very narrow elevation range.

Artificial drainage of floodwaters from sulfidic backswamps: effects on deoxygenation in an Australian estuary

The Clarence River estuary experienced extensive oxygen depletion and fish kills following overbank flooding in 2001. This paper examines the chemical composition and volume of surface water draining from two floodplain sulfidic backswamps into the Clarence River estuary after the flooding. Water draining from the backswamps was severely deoxygenated ( 6 days post-peak) of flood recession. In the absence of artificial drainage, most of the floodwaters with high deoxygenation potential would have been retained in the landscape and not exported to the estuary as observed during this flood.

Alteration of groundwater and sediment geochemistry in a sulfidic backswamp due to Melaleuca quinquenervia encroachment

Extensive encroachment of the native tree species Melaleuca quinquenervia (Cav.) Blake has occurred on a coastal floodplain sulfidic backswamp in eastern Australia. Almost 50% of the open swamp area c. 1870 is now monospecific M. quinquenervia forest. Encroachment has been associated with shortened hydroperiods and land management changes following drainage for agriculture. Large differences to shallow groundwater and sediment geochemistry were observed beneath both individual M. quinquenervia trees and encroaching forests compared to open swamp. Groundwater beneath M. quinquenervia had enhanced titratable acidity and acidic metal cations, increased concentrations of other ionic species (Cl–, SO42–), altered ionic ratios, and increased dissolved organic carbon. Soil beneath M. quinquenervia displayed enhanced accumulation of acidity and soluble ions, with concentration profiles suggesting vertical redistribution towards the surface. Deepening of the sulfide oxidation front in the soil beneath encroached M. quinquenervia suggests that enhanced sulfide oxidation may be occurring. Changes in soil pH, redox potential, and Fe mineral precipitation/dissolution were also evident. These changes appear to be the result of interactions between M. quinquenervia physiology and the unique groundwater and sediment geochemistry of the surrounding sulfidic/sulfuric horizons. Mechanisms to explain the observed changes are discussed along with potential management implications.

Changes in surface water quality after inundation of acid sulfate soils of different vegetation cover

Surface soils from an acid sulfate soil (ASS) backswamp were inundated in a temperature controlled environment and surface-water chemistry changes monitored. The soils had contrasting in situ vegetative cover [i.e. 2 grass species, Cynodon dactylon and Pennisetum clandestinum (Poaceae), and litter from Melaleuca quinquenervia (Myrtaceae)]. The different vegetation types had similar biomass and carbon content; however, there were large differences in the quality and lability of that carbon, which strongly influenced decay/redox processes and the chemical composition of surface waters. The grass species had more labile carbon. Their surface waters displayed rapid sustained O2 depletion and sustained low Eh (~0 mV), high dissolved organic carbon (DOC), and moderate pH (5–6). Their soil acidity was partially neutralised, sulfides were re-formed, and reductive dissolution of Fe(III) led to the generation of stored acidity in the water column as Fe2+(aq). In contrast, M. quinquenervia litter was high in decay-resistant compounds. Its surface waters had lower DOC and low pH (<4) and only underwent a short period of low O2/Eh. Soluble Al caused M. quinquenervia surface waters to have higher titratable acidity and soil pH remained consistently low (~3.8–4.0). Concentrations of Cl– and Al in surface waters were strongly correlated to initial soil contents, whereas the behaviour of Fe and SO42– varied according to pH and redox status. This study demonstrates that changes in vegetation communities in ASS backswamps that substantially alter either (a) the pool of labile vegetative organic carbon or (b) the concentration of acidic solutes in surface soil can have profound implications for the chemical characteristics of backswamp surface waters.

New estimates of dry matter intake of lactating beef cows grazing three pasture systems.

Estimates of dry matter intake reported by Barlow el al. in this journal in 1988 appeared low for grazing beef cows during early lactation. Further estimates for 144 crossbred and Hereford cows at Grafton, NSW, support the contention that the previous estimates for early lactation were biased downward. This appears to have been simply a scale effect as rankings of pastures and cow genotypes remained unchanged, although the magnitude of differences did alter.