Sediment and pollutant transport from grazing lands of Australia

Abstract

Transport of sediment, nutrients and other pollutants from grazing lands of Australia into surface water bodies is becoming a major water quality issue with frequently occurring drought and reduced flow in the country's river systems. Murry-Darling is one such river system which is under pressure both for the quality and the quantity of its water. A large scale field experiment was conducted over a 6-year period on a sheep grazing property in western Queensland to study the impacts of different grazing management systems on various soil, water and environmental issues that the graziers of the region have to deal with. The study site included four small to medium size catchments and four large plots all fully instrumented for flow measurement and sampling of runoff and eroded sediments for subsequent physical, chemical and biological analysis. The two commonly used grazing systems rotational grazing and continuous grazing were adopted, each applied to two both small catchments and large plots. Results show large quantities of soil and soluble nutrients being lost to the receiving water bodies under the traditional system of continuous grazing. The concentrations of sediment as well as soluble phosphorus and nitrogen in the runoff leaving both plots and catchments were reduced significantly under the rotational grazing system. As this new method of grazing, through providing long rest periods for the land, also contributed to a substantial increase in plant growth, surface mulch accumulation, and more uniform animal distribution over the confined cells. All these changes may have caused less sediment transport and higher rate of plant uptake of soluble nutrients leaving less of these contaminants to be washed away by runoff. The presence of a higher quantities of litter and above ground organic materials on lands under rotational grazing reduced hoof pressure and soil compaction under this system of grazing. Less compaction in turn resulted in higher infiltration of rain water and lower surface runoff. Continuous grazing allows grazing animals to selectively graze patches of the pasture with more palatable grass varieties and congregate in camp sites. Both of these two actions contributed to uneven distribution of trampling, animal dropping and plant and residue covers, which resulted in the creation of bare patches which are highly susceptible to erosion and camp sites highly susceptible to nutrient loss into surface runoff. Changing the grazing system from continuous to rotational grazing helped reducing the formation of denuded patches of land, animal aggregation sites, and washoff of nutrients and surface residue as well as higher and more sustainable grass production