Saharan Dust Fertilizes the Amazon Basin
NASA scientists recently studied in detail how dust from the seemingly barren Sahara Desert gets picked up by strong winds and carried more than 3,000 miles across the Atlantic Ocean, where it fertilizes the lush Amazon rainforest. So how does desert dust manage to fertilize a rainforest thousands of miles away? The answer is that this dust contains phosphorus, a key ingredient for plant growth.
Background
Phosphorus is in surprisingly short supply in the Amazon. But there's a lot of it in the Sahara — because the Sahara once had a vast lake. That prehistoric lake, referred to as Lake Mega-Chad, used to be filled with algae and other microorganisms. But more than 7,000 years ago, a changing climate caused the lake to dry out and recede (it is now the much smaller present-day Lake Chad), leaving a lot of nutrient-rich dust behind.
Cause of Depletion of Phosphorus in Amazon
This dust turns out to be crucial to the Amazon rainforest. Although plants need phosphorus to grow — it's a key ingredient of modern fertilizer — there's surprisingly little phosphorus available in the Amazon soil. NASA's press release explains why: Fallen, decomposing leaves and organic matter provide the majority of nutrients, which are rapidly absorbed by plants and trees after entering the soil. But some nutrients, including phosphorus, are washed away by rainfall into streams and rivers, draining from the Amazon basin like a slowly leaking bathtub.
Statistics
Scientists have known about the Sahara-Amazon connection since at least 2006. But this newest study in Geophysical Research Letters, led by Hongbin Yu of NASA's Goddard Space Flight Center, gives us a better understanding of how much dust is actually traveling through the air — and how important it is. Around 15% of the dust leaving the Sahara, equivalent in tons to the weight of 700,000 trucks, reaches the Amazon every year. Using NASA satellites, the authors find that winds carry, on average, about 27.7 million tons of dust from the Sahara to the Amazon basin each year, containing 22,000 tons of phosphorus. As it happens, that's about the same amount of phosphorus that washes away from the Amazon soil.
Conclusion
The dust clouds of the Sahara and the famous flying rivers are examples of the constant exchange between the Amazon rainforest and other ecosystems that, even thousands of kilometers away, nourish the soil and regulate the world’s lung rains. For now, dust particles will continue to nourish Amazonian forests and feed the great diversity that makes the Amazon a region that connects the world.
