Saikat Chowdhury

Nitrification potential in the rhizosphere of Australian native vegetation

The rhizosphere influences nutrient dynamics in soil mainly by altering microbial activity. The objective of this study was to evaluate the rhizosphere effect on nitrogen transformation in Australian native vegetation in relation to nitrification potential (NP). Microbial activity, NP, and nitrifiers (ammonia-oxidising bacteria, AOB) were compared between rhizosphere and non-rhizosphere soils of several Australian native vegetation under field conditions. These parameters were also measured with increasing distance from the rhizosphere of selected plant species using plant growth experiments. To examine the persistence of nitrification inhibitory activity of rhizosphere soil on non-rhizosphere soil, the soils were mixed at various ratios and examined for NP and AOB populations. The rhizosphere soil from all native vegetation (29 species) had higher microbial activity than non-rhizosphere soil, whereas 13 species showed very low NP in the rhizosphere when compared with non-rhizosphere soil. Nitrification potential and AOB populations obtained in the soil mixture were lower than the predicted values, indicating the persistence of a nitrification inhibitory effect of the rhizosphere soils on non-rhizosphere soils. In plant growth experiments the microbial activity decreased with increasing distance from rhizosphere, whereas the opposite was observed for NP and AOB populations, indicating the selective inhibition of nitrification process in the rhizosphere of the Australian native plants Scaevola albida, Chrysocephalum semipapposum, and Enteropogon acicularis. Some Australian native plants inhibited nitrification in their rhizosphere. We propose future studies on these selected plant species by identifying and characterising the nitrification inhibiting compounds and also the potential of nitrification inhibition in reducing nitrogen losses through nitrate leaching and nitrous oxide emission.

Recycled Water Irrigation in Australia

Access to water has been identified as one of the most limiting factors to economic growth in Australia’s horticultural sector. Water reclaimed from wastewater (sewage) is being increasingly recognised as an important resource, and the agricultural sector is currently the largest consumer of this resource. An overview of the Australian experience of using reclaimed wastewater to grow horticultural crops is presented in this chapter. The wastewater treatment process and governing regulations are discussed in relation to risk minimisation practices which ensure that this resource is used in a sustainable manner without impacting adversely on human health or the environment. A case study covering the socio-economic and environmental implications of recycled water irrigation is also presented.

Soil sampling strategies for site assessments in petroleum-contaminated areas

Environmental site assessments are frequently executed for monitoring and remediation performance evaluation purposes, especially in total petroleum hydrocarbon (TPH)-contaminated areas, such as gas stations. As a key issue, reproducibility of the assessment results must be ensured, especially if attempts are made to compare results between different institutions. Although it is widely known that uncertainties associated with soil sampling are much higher than those with chemical analyses, field guides or protocols to deal with these uncertainties are not stipulated in detail in the relevant regulations, causing serious errors and distortion of the reliability of environmental site assessments. In this research, uncertainties associated with soil sampling and sample reduction for chemical analysis were quantified using laboratory-scale experiments and the theory of sampling. The research results showed that the TPH mass assessed by sampling tends to be overestimated and sampling errors are high, especially for the low range of TPH concentrations. Homogenization of soil was found to be an efficient method to suppress uncertainty, but high-resolution sampling could be an essential way to minimize this.

Zeolite for Nutrient Stripping From Farm Effluents

Abstract Many countries including New Zealand, Australia, and South Korea discharge of farm effluents containing large reserves of plant nutrients into surface waters. Such discharge is currently considered a discretionary activity and requires legal consent that demands the effluent nutrient concentration to be minimized before entering surface waters. This can be achieved by land disposal or nutrient stripping of the effluent by tertiary treatment. Although the pond system (ie, biological treatment) is effective in removing suspended solids and carbon, there has been some debate about its efficiency in removing nutrients. Porous materials such as zeolite, a naturally occurring and electrically charged aluminosilicate material, can be used to adsorb nutrients from effluents. Then the nutrient-enriched material can be recycled as a soil conditioner or nutrient source. This chapter examines the potential of zeolite in nutrient stripping from wastewater streams and its value as a nutrient source.