Andrew Ash - Mentor for Postdoctoral Fellow

Summary

​Extensive cattle grazing is the dominant land use in northern Australia. To better understand the role of nutrient cycling in response to grazing and rainfall variability, field studies were undertaken in north-east Queensland. Organic matter in soils is critical for soil health, carbon storage, pasture productivity and nutrient cycling. Analysis of soils from the long-term grazing trial at Wambiana in north-east Queensland showed that soil organic matter was higher under heavy stocking, although grazing treatment had little effect on soil nitrogen. While this result is somewhat surprising given conventional wisdom suggests that more conservative grazing should result in higher organic matter, it is not completely inconsistent with other recent studies in northern Australia which show little effect of grazing intensity on soil organic matter. The results from this study suggest that caution is needed in recommending specific grazing management strategies as a means of storing carbon. An experimental study at the Spyglass Research Facility, where rainfall was manipulated to study the interaction between climate variability and nutrient dynamics showed that drought conditions caused an accumulation of available nitrate in the soil. While plants had higher nitrogen concentrations in drier conditions, the lower biomass resulted in less total nitrogen uptake which can explain accumulation of nitrate in the soil. The accumulation of nitrate in the soil in drought conditions can explain the high levels of animal growth following drought breaking rains. The results show that plant biomass, at least up to average rainfall, is mainly limited by water availability in this ecosystem. With higher levels of rainfall, water availability will not limit productivity and nitrogen availability will be the limiting factor.