By Tom Nordlie
GAINESVILLE, Fla. — The Great Barrier Reef is the world’s largest coral reef and one of Australia’s top tourist destinations, but its coral colonies have been dying at a startling rate in recent years and scientists believe that high levels of nitrogen in reef waters have played a role in the crisis.
Soils found in river basins along the Queensland coast contribute sediment to river flows and have been cited as a source of nitrogen inputs to the Great Barrier Reef Lagoon, which surrounds the 2,300-kilometer reef system. In response, scientists have routinely tested Queensland-area soils and surface waters to evaluate them for the presence of dissolved nitrogen and solid particles containing nitrogen. However, this approach does not typically involve filtering procedures that would separate out extremely small solid particles, known to scientists as colloids.
A recent study led by a University of Florida Institute of Food and Agricultural Sciences researcher suggests that colloids may be responsible for transporting a significant amount of nitrogen from onshore soils to the Great Barrier Reef.
Soil and water chemist Jonathan Judy, an assistant professor with the UF/IFAS soil and water sciences department, led a team of colleagues in the effort. It was conducted in collaboration with the University of Adelaide and Australia’s national research agency, the Commonwealth Scientific and Industrial Research Organisation, or CSIRO.
The study focused on colloids because one of their defining characteristics is that they are small enough to remain suspended in liquids for long periods of time without settling out to form a layer of sediment, as larger suspended particles would.
“We looked at colloids that are easily mobilized from the soil when erosion occurs,” Judy said. “If these colloids are mobilized – due to a heavy rainfall, for example – any nitrogen adhering to the colloids will be mobilized along with them. This is important because previous research has demonstrated that colloids can transport nutrients and contaminants long distances, and we believe the same phenomenon may be occurring here with nitrogen.”
One of the on-shore locations that contributes sediment and, presumably, colloids to reef waters is the Burdekin River Basin, he said. One of Australia’s largest drainage basins, it encompasses 129,700 square kilometers, representing about one-third of the total Great Barrier Reef watershed. The Burdekin River discharges into the Great Barrier Reef Lagoon at a point roughly halfway from either end of the reef system.
In the current study, Judy and a team of colleagues analyzed soil samples collected from numerous locations within the Burdekin River Basin, including undisturbed areas, drainage features on sugarcane farms, and areas used for cattle grazing that included gullies formed by erosion. All of the collection sites were near the city of Townsville, on the northeast Queensland coast, where the research team was headquartered.
The scientists extracted water dispersible clay colloids from the soil samples, mixed the clay with water to create suspensions, then filtered the suspensions through a 0.45 micron filter, similar to methods employed for routine water quality assessments in the area. One micron is a unit of distance equal to one-millionth of a meter; the average human hair is about 100 microns thick.
Another sample of the clay suspension was filtered through an ultrafiltration unit that removed much smaller, nano-scale particles. One nanometer is a unit of distance equal to one-billionth of a meter; a human hair is about 100,000 nanometers thick.
By analyzing the two filtrates for the presence of nitrogen and comparing the results, the researchers made an interesting discovery regarding the samples collected from cattle grazing lands and undisturbed areas. In these samples, the majority of the nitrogen that routine methods would have classified as “dissolved” was actually nitrogen in an extremely small solid form or in a dissolved form adhered to the surfaces of colloids.
The team’s findings indicate that these small, easily mobilized clay particles might be an important source of soil nitrogen within the Burdekin River Basin, Judy said.
One of Judy’s collaborators agrees. Paul Bertsch, the CSIRO deputy director for science, said he had expected results similar to the team’s findings. Bertsch has carried out previous studies showing that clay colloids are responsible for long-distance transport of nutrients and other compounds in Queensland surface waters. After learning that filtration methods used in routine water quality assessments did not typically account for extremely small particles, he hypothesized that nitrogen-bearing colloids in the Burdekin River Basin might also be escaping notice.
“This study confirms this hypothesis, providing new information needed to inform soil management practices to minimize erosion and transport of nitrogen, other nutrients and pesticides from agroecosystems to the (Great Barrier Reef Lagoon),” said Bertsch, based at the CSIRO Land & Water unit in Brisbane, Queensland.
Results from the study were published Aug. 27 in the Nature Publishing Group’s open-access journal, Scientific Reports.
Judy planned and initiated the study during a three-year post-doctoral fellowship with CSIRO in Australia prior to his arrival at UF in early 2017. Funding was provided by CSIRO and UF/IFAS. He hopes to arrange a follow-up study to explore factors involved in nitrogen transport by clay colloids, such as the effect of increased water salinity on the colloids and particles adhering to them.
Other team members were based in Australia and represented two CSIRO business units and the University of Adelaide. Besides Bertsch, the team included Rebecca Bartley, Mike Farrell, Jason K. Kirby, Mike J. McLaughlin and Scott N. Wilkinson.
Contact: Tom Nordlie, 352-273-3567, email@example.com
Photo courtesy of Scott Wilkinson, Commonwealth Scientific and Industrial Research Organisation
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