Transport of nutrients in runoff from lands under different management regimes on the Atherton Tableland, North Queensland
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Scientific investigations have linked the declining health of the Great Barrier Reef with concentrations of sediments and nutrients flowing into the lagoon from adjacent catchments. Land use practices within these catchments have changed significantly since European settlement, when clearing for agricultural purposes began. Today grazing practices comprise an estimated 77% of the entire catchment and have intensified over the last decade. Increased grazing pressures have coincided with a growing awareness of the impact of such land use practices on sediment and nutrient loads in terrestrial runoff, and their subsequent impact on catchment water quality and health. To date water quality measurements have been made in a number of the inland and coastal water ways, however there has been no land based study which has endeavoured to clarify or quantify the impact of land use and farm management techniques as the source of contamination. In an effort to address this issue a project was designed to examine the on-site effects of different land-uses and management techniques on the off-site movement of nutrients detrimental to water quality. The investigation was conducted on grazed land in the headwaters of the Johnstone and Barron rivers, two of the major and most heavily impacted river systems within the Great Barrier Reef catchment. Rainfall simulation techniques were used to measure and compare nitrogen levels in runoff from beef and dairy farms with different management regimes. Results showed that the mean concentration of total-N, dissolved nitrate -N and dissolved ammonium-N from all management techniques exceeded the ANZECC (2000) guidelines set for the maintenance of aquatic ecosystem health. A comparison of dissolved nitrate-N concentrations in runoff from beef sites and dairy sites showed that dairy sites had significantly higher concentrations than beef sites. Dairy industry in the study area is therefore a bigger contributor to surface water pollution by nitrate-N then the other forms of land use. The levels of dissolved nitrate-N and ammonium-N in runoff were significantly influenced by the management techniques applied to the farms. Beef farms produced the lowest concentrations of these two forms of N in runoff followed by organic farms, effluent applied farms and urea applied farms in an ascending order. The positions within the landscape also significantly influenced N-concentration in runoff with the highest concentration being in the lower toe-slopes. Runoff coefficients were calculated for each rainfall simulation site and statistically analysed. Significant differences in runoff coefficient were found between different land-use, management techniques and positions within the landscape. The highest rate of runoff was associated with the beef farms where animal trampling effect was the highest amongst the different land uses studied. The significance and implications of the results to best management practices in the beef and dairy industries are discussed in this paper.
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