UW researchers study how Saharan dust changes and supplies phosphorus to various ecosystems | New


September 24, 2021

This illustration shows the changes in the chemical composition of phosphorus due to acidification and particle sorting during the transatlantic transport of dust from Africa to Cape Verde and Puerto Rico. Mengqiang Zhu, associate professor in the department of ecosystem science and management at the University of Wyoming, and Than Dam, his doctorate. student, investigated how the chemical form of phosphorus in wind dust helps determine how efficiently the dust fertilizes the ecosystem with phosphorus. Their research was published in the September 10 issue of the journal Environmental Science and Technology. (Illustration by Mengqiang Zhu and Than Dam)

Saharan dust is a major source of phosphorus in remote and nutrient-depleted regions of the world’s oceans and tropical rainforests in the American lowlands. However, the bioavailability of this phosphorus largely depends on its chemical form.

Mengqiang Zhu, associate professor in the Department of Ecosystem Science and Management at the University of Wyoming, and his doctorate. The student investigated how the chemical form of phosphorus in wind dust – or the windblown silt and clay that settles in an ecosystem – helps determine how efficiently dust fertilizes the ecosystem with phosphorus.

“Our goal is to determine the chemical form of phosphorus in the dust source, Saharan soils, and how the phosphorus in dust changes chemical form during long-range transatlantic dust transport, and through what physical and chemicals, “Zhu says.” We have found that the composition of phosphorus has undergone substantial changes that probably increase the availability of phosphorus in dust in marine ecosystems, but decrease its availability in tropical rainforests in America. “

Zhu is the corresponding author of an article titled “X-ray spectroscopic quantification of phosphorus transformation in Saharan dust during transatlantic dust transport” which was published September 10 in Environmental Science & Technology. The bi-weekly peer-reviewed journal covers research in environmental policy, environmental science and environmental technology.

The main author of the article is Than Dam, Ph.D. student from Hai Phong, Vietnam, studying soil science at UW’s Department of Ecosystem Science and Management. The research is part of his thesis.

Other contributors to the article were from the Hebrew University of Jerusalem in Jerusalem, Israel; the University of Haifa in Haifa, Israel; the University of Leeds in Leeds, United Kingdom; the University of Saskatchewan in Saskatoon, Saskatchewan, Canada; Argonne National Laboratory in Lemont, Ill .; the University of Puerto Rico at Rio Piedras, Puerto Rico; and the University of Aveiro in Aveiro, Portugal.

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Mengqiang Zhu

Phosphorus is an essential nutrient required by all forms of life. Mineral dust is known to be an important source of phosphorus in many nutrient-depleted regions of the world, including open oceans and tropical rainforests.

Every year, 182 million tonnes of Saharan mineral dust are washed away from Africa. About 28 million tonnes of this amount are deposited in the Amazon basin in winter and spring, fertilizing tropical soils poor in phosphorus. The remaining amount of dust is deposited along its transport path, providing an important source of nutrients for oceanic oligotrophic gyres – large circular ocean current systems formed by global winds and forces created by Earth’s rotation – and Caribbean Sea .

Cape Verde and Puerto Rico were the two study sites chosen because of their location along the transport route from Africa to the tropical rainforests of America, which also lies within the transport route of dust. Cape Verde, an island in the Atlantic Ocean, is located approximately 570 kilometers from the west coast of Africa. Puerto Rico is a site of American rainforest and is located approximately 4,000 kilometers from the west coast of Africa.

“Our results show that atmospheric acidification and particle sorting cause changes in the chemical composition of phosphorus in Saharan dust,” Zhu explains. “This chemical composition changes from phosphate dominated by apatite to phosphate which is mainly bound to iron and aluminum oxides during transatlantic dust transport.”

Apatite is a compound of calcium phosphate and the main phosphorus mineral in the dust source. It is also the mineral that makes up our bones. Acidification is the reaction between dust particles and acidic air pollutants, including nitrogen oxides and sulfur dioxide, Zhu explains.

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That the dam

Acidification dissolves apatite and produces reactive iron and aluminum oxides that absorb the phosphate released by dissolving apatite, Zhu explains. Particle sorting refers to the earlier deposition of larger dust particles. So, as the dust plume moves, the dust particles that have remained in the plume become smaller and smaller. As the smaller dust particles are poor in apatite but rich in iron and phosphorus bound to aluminum, sorting the particles also results in an increase in the proportions of aluminum and phosphorus bound to iron.

“Saharan dust inputs have been an important supply of phosphorus nutrients to support the American rainforests, which are a major carbon sink and sequester a large amount of carbon,” Zhu explains. “The soils of the tropical rainforest are acidic and poor in phosphorus, which severely limits the primary productivity of the forest.

“When dust arrives in Puerto Rico, it becomes rich in phosphorus bound to iron and aluminum, a form of phosphorus that is not readily available to plants in acidic soils,” he continues. “Thus, only part of the phosphorus contained in the dust deposited in tropical forests is effective in fertilizing the tropical forest. This discovery allows ecologists to estimate more precisely the importance of the contributions of dust on the fertilization of the American tropical forests.

On the other hand, phosphorus bound to iron and aluminum is more soluble and therefore available in marine environments because seawater is alkaline and promotes the dissolution of phosphorus, Zhu explains. After long-distance transport, the phosphorus in dust becomes more effective in fertilizing phosphorus-limited oceans, such as the Caribbean. This information would be useful to marine chemists studying phosphorus limitation in nutrient-depleted oceans.

“Our results will inform atmospheric chemists that not only atmospheric acidification – but also particle sorting – can alter the chemical forms of phosphorus, and possibly other elements as well, in wind dust,” Zhu said. . “The way of sorting particles has been neglected in the literature.”

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