Nitrogen is a major component of the fertilisers that are added to crops such as sugarcane and bananas, which are the primary crop types grown in the Great Barrier Reef catchment. It can also be delivered from soil erosion, which is dominated by grazing lands. Understanding all of the sources of nitrogen is important for developing mitigation strategies to reduce nitrogen loads from large river basins draining to the Great Barrier Reef.
A Tropical Water Quality Hub project led by Dr Rebecca Bartley from CSIRO is evaluating the effectiveness of gully remediation strategies on reducing the sediment and nutrient losses from gully erosion. A recent publication from this project found that a significant proportion of the nitrogen mobilised when gully soil erodes is comprised of very small colloidal soil particles. These particles are so small that they pass right through the filters normally used to identify particulate nitrogen, and so they would normally be defined as ‘dissolved’ nitrogen.
This component of the research was led by Dr Jonathan Judy (formerly of CSIRO but now with the University of Florida, USA) who said nearly half of the particles were previously thought to be dissolved nitrogen.
“This study demonstrates that up to 45% of the soil nitrogen passed through a 0.45-micron filter,” he said.
Dr Bartley said the findings had significant implications for nitrogen management in the Great Barrier Reef.
“For many years we have been perplexed by the relatively high levels of dissolved inorganic nitrogen (DIN) in water running off non-fertilised grazing lands,” she said.
“Now we know that much of this nitrogen is not dissolved but is actually attached to very small soil particles.
“This finding has implications for establishing and evaluating water quality targets, and also for how we prioritise investment in catchment remediation.
“It appears that managing soil erosion can reduce the amount of what is typically treated as dissolved nitrogen.
“This project brought research techniques developed for studying the transport of contaminants in soils to a new environmental challenge, with broad implications for water quality management,” she said.
The research article describing these findings is available in the Nature Publishing Journal Scientific Reports and more information on Rebecca Bartley’s Tropical Water Quality Hub project can be found online.