G. J. Clark

Organic amendments initiate the formation and stabilisation of macroaggregates in a high clay sodic soil

Subsoil constraints present a substantial problem for crop production in many agricultural regions. In particular, soils in temperate grain production areas of Australia are often poorly structured due to high content of sodic clay. An alternative to the standard practice of addition of gypsum is to incorporate organic amendments deep into the subsoil. An incubation experiment was performed for 174 days using several organic amendments. These consisted of wheat shoots, lucerne pellets, canola and chickpea stubbles, chicken manure, peat, and sawdust. Gypsum, an inorganic amendment commonly applied to sodic soil, was included for comparison. The change over 174 days in soil structural properties was measured using wet-sieving. Formation of slaking-resistant macroaggregates >2 mm was most rapid with ‘green plant material’, wheat and lucerne, while the ‘stubbles’ were markedly slower in obtaining the equivalent level of aggregation. However, the largest growth in aggregates after day 56 was shown by the ‘stubble’ and chicken manure amended soils. The gypsum amendment was not capable of forming large, slaking-resistant aggregates >2 mm; this may be attributed to the inability of gypsum to stimulate soil biological processes. Peat and sawdust failed to initiate slaking-resistant macroaggregates. The study demonstrated that a variety of organic amendments have the ability to improve the physical fertility of sodic subsoil, and in the case of the green plant materials within 1 week of incubation.

Cd hyperaccumulative characteristics of Australia ecotype Solanum nigrum L. and its implication in screening hyperaccumulator.

A pot culture experiment was used to determine the differences in uptake characteristics of a cadmium hyperaccumulator Solanum nigrum L. discovered in China, an ecotype from Melbourne, Australia and a non-hyperaccumulator Solanum melogena. Australian ecotype was not significantly different to the China ecotype. In particular, Cd concentration in leaves and shoots of S. nigrum collected from Australia were 166.0 and 146.3 mg kg−1 respectively when 20 mg kg−1 Cd spiked, and were not significantly different to the ecotype imported from China which had 109.8 and 85.3 mg kg−1 respectively, in the stems and leaves. In contrast, the tolerance of the eggplant to Cd was significantly less than the two S. nigrum ecotypes. Although some morphological properties of S. nigrum collected from Australia were different from that of the plants collected from China, Cd hyperaccumulator characteristics of two ecotypes were similar. The results suggested that the tolerance and uptake of Cd may be a constitutive trait of this species.

Contrasting effects of organic amendments on phytoextraction of heavy metals in a contaminated sediment

Background and aimsSoil amendments are often added to polluted soils to increase phytoremediation efficiency. Here we investigated the potential of a range of organic amendments for phytoextraction of heavy metals in a contaminated sediment.MethodsTwo experiments compared adsorption and phytoextraction of heavy metals by a Cd-hyperaccumulator Carpobrotus rossii grown in the contaminated sediment amended with six organic amendments.ResultsThe adsorption capacity as measured by Langmuir adsorption maximum followed the order of Cr > Zn > Cu > Cd, and the effect of organic amendments followed the order of chicken manure > cow manure > brown coal > golden wattle biochar > blue gum biochar > radiata pine biochar. The addition of amendments increased the adsorption of heavy metals, with brown coal resulting in the lowest concentrations of water-extractable Cd, Cu and Zn. Two manures resulted in the highest concentrations of these water-extractable heavy metals in the rhizosphere soil of C. rossii. Furthermore, brown coal resulted in higher shoot accumulation of these heavy metals than three wood-derived biochars, whilst the manures generally had the lowest accumulation of Cd and Cu although they increased shoot biomass.ConclusionsThe addition of brown coal decreased whereas manure addition increased the mobility (water-extractable fraction) of heavy metals in rhizosphere soil. Phytoextraction of Cd and Cu was greater with brown coal than with biochars or manures. Brown coal is suitable for enhancing phytoextraction of these heavy metals because it could increase their accumulation in shoots of C. rossii and decrease the risk of leaching of these heavy metals into groundwater.

Experimental warming and antecedent fire alter leaf element composition and increase soil C:N ratio in sub-alpine open heathland

Plant communities in alpine ecosystems worldwide are being altered by climate warming. In the alpine open heathland of the Bogong High Plains, Australia, warming and fire have affected the growth and phenology of plants, and have recently been found to alter soil nutrient availability. We examined the effects of nine years of passive warming by open-top chambers and nine years post-fire on (i) the soluble and extractable nutrients and toxic elements available for plant uptake in the soil and (ii) on the element composition of leaves of seven dominant sub-alpine open heathland plants. Warming increased soil C, soil C:N, and decreased soil δ13C, indicating an accumulation of soil organic matter and C sequestration. Warming increased soil δ15N, indicating increased N mineralization, which concurred with the increased availability of NH4+ (measured by ion-exchange membranes). Leaf element composition varied among the plant species in response to changes in soil element availabilities, suggesting the importance of species-specific knowledge. Warming decreased leaf N concentration and increased leaf C:N, generally in the plant community, and specifically in Asterolasia trymalioides, Carex breviculmis, Poa hiemata, and Rytidosperma nudiflorum. Warming increased soil P availability, but did not significantly affect leaf P in any species. Antecedent fire increased soil C:N, and decreased concentrations of Ca and Mg in Celmisia pugioniformis more than in the other species. The results suggest that warming and fire changed the nutrient composition of plants and increased soil C:N, which might lead to progressive N limitation in the alpine ecosystem.

Inundation of a floodplain lake woodlands system: nutritional profiling and benefit to mature Eucalyptus largiflorens (Black Box) trees

River management continues to challenge riparian systems worldwide, with climate change impacts and anthropogenic extractions escalating. The Murray–Darling basin (MDB) in Australia is critical to agricultural production and habitat provision to maintain biodiversity. Concern for the condition of native trees and biota in the MDB has led to substantial research investment to increase ecosystem function understanding and improve floodplain and wetland management. This field study offers new insights into tree nutrition and physiology as interpreted against the plant-soil-environment dynamics of recent flooding. Black Box (Eucalyptus largiflorens (Myrtaceae) is the only key native riverine MDB tree restricted to that region; and appears stressed at the far reaches of certain significant floodplain ecosystems. Here, nutritional and ecophysiological comparisons were made between Black Box trees that had just been inundated, and those nearby that had not. Leaf stomatal conductance, transpiration, total soil aluminium (Al) concentration, soil pH, and soil conductivity were different between inundated and dry sites. Soil moisture increased due to inundation, thus reducing tree water stress across the three study locations. Changes in leaf chemistry were not detected at the very early stages of flooding examined in this study. An increase in soil acidity due to inundation may also enhance bioavailability of nutrients to trees. New insight into immediate plant benefits gained from this study suggests further investigation is warranted to elucidate the influence of flood and drought on nutrient balance and how future wetland management can benefit from a more holistic understanding of plant-soil-environment dynamics.