Water quality in agricultural lands draining to the Great Barrier Reef: A review of causes, management and priorities

Abstract

The environmental consequences of agriculture are of growing concern. One example of these consequences is the effect of agricultural pollutants on the Great Barrier Reef (GBR), a world heritage-listed ecosystem lying off the tropical north-eastern coast of Australia. Pollutants from agricultural lands (fine sediments and attached nitrogen (N) mainly from grazing lands, and dissolved N and pesticides mainly from cropping) in catchments draining into the GBR lagoon threaten the health and resilience of this ecosystem. Government actions are prompting farmers to adopt new management practices to reduce pollutant exports from their farms. However, previous agricultural research has, with the exception of erosion, largely focussed on production rather than environmental impacts. Also, the relevance of research conducted in other regions, e.g. Europe and North America, with different climates, soils and agricultural systems may be limited. Thus, there may not be a strong knowledge base underpinning actions to improve water quality. In this paper, we review research on the relationship between management of agricultural lands and pollutant exports in GBR catchments, and compare this knowledge with experience in other regions. Despite the differences in climate and agricultural systems, there are similarities in the causes and management of N and pesticide losses from cropping lands. Substantial N fertiliser is applied to high value crops in GBR catchments, and the primary path to reducing N losses from cropped lands will be through reducing N applications. Other practices may become effective in these crops once current (high) rates of N application and reduced. Herbicides are widely used, and practices that reduce herbicide runoff have recently been developed and demonstrated in most of the main cropping systems. However, there are still uncertainties over breakdown and fate of pesticides, especially new products, in these tropical environments. The principles of reducing erosion in grazing lands are well understood, and centre on maintaining ground cover and biomass of pastures, especially during the dry season and droughts. Highly variable rainfall makes this principle challenging to achieve in practice. In addition, it has recently become clear that gully networks caused by livestock grazing are much more important sources of sediment that previously thought. Practices such as targeted vegetation management will be important strategies for reducing gully erosion. Despite these advances in practice effectiveness, it is clear that much more is needed to have agricultural systems that are compatible with a sustainable and resilient GBR. As the demand for food increases in coming decades, and agriculture expands and intensifies in tropical countries, the experience in GBR catchments will help guide the development of more sustainable agricultural systems in these countries.