The world’s largest rainforest and a crucial store of carbon dioxide gets most of its phosphorous, an important nutrient, from an unexpected source: fires in Africa.
Strange as it may seem, we thought that the Amazon got much of its phosphorus from dust whipped up from the Sahara Desert and transported across the Atlantic on the wind.
Cassandra Gaston at the University of Miami, US, and her colleagues had set out to quantify the effect of the phosphorous in Saharan dust on the Amazon’s growth. To do this, they collected and analysed particles caught in filters from a hilltop in French Guiana, at the northern edge of the Amazon Basin. But at the same time, they used satellites to track smoke from fires in Africa — both people burning wood and natural forest fires — drifting Westwards across the ocean. It turned out that the arrival of patches of smoke coincided with high levels of phosphorous being detected in the filters.
Gaston and her team then estimated how much of the phosphorus deposited on the Amazon Basin comes from African biomass burning. They found that, in Spring, smoke from the fires was responsible for most of the nutrient entering the Amazon Basin.
“Smoke from African biomass burning was a very important source of not just phosphorus, but soluble forms of phosphorus that are readily available for biological processes,” says Gaston.
This changed in autumn, when the dust travelling across the ocean dropped.
The findings suggest that people burning wood and other materials in Africa might have an impact on how much the Amazon grows and therefore how much carbon it stores in future. But it is not easy to predict what the effect might be.
At the moment, households in Sub-Saharan Africa overwhelmingly depend on wood for cooking and Gaston says that the growing population in Africa will probably result in an increase in burning. But at the same time there may be a drop in the number of wildfires as land use changes.
Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1906091116
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