Abstract
Soils in the Amazon Basin are deficient in phosphorus, essential to soil fertility. Previous studies suggested that African mineral dust deposited to Amazonian soils served as an important source of phosphorus that enhances soil fertility. These studies lacked the quantitative measurements essential to validate estimates. Here we present daily measurements of mineral dust and PM10 carried in the trade winds at Cayenne, French Guiana, during 2002–2017. MERRA‐2 model dust concentrations showed excellent agreement with measurements over this period. Consequently, we used MERRA‐2 to estimate temporal and spatial deposition rates to Amazonia. Our annual deposition rate, 8–10 Tg dust, is substantially lower than previous estimates. Deposition rates are greatest over the northern and northeastern regions of South America. In contrast, rates are low in central Amazonia. Our results raise questions about the impact of African dust on soil fertility in Amazonia. However, African aerosol transport carries other aerosol species that could play a role in soil fertility, including biomass‐burning products known to contain substantial concentrations of phosphorus. Our study highlights the need for more measurements of aerosol and precipitation chemistry over wider expanses of South America in order to better characterize aerosol chemical and physical properties, to identify aerosol sources, and to constrain model estimates. We show that over 2002–2017 dust transport to South America was negatively correlated to rainfall over the Sahel in North Africa. Long‐term monitoring is needed to capture changes in transport from Africa that might occur as a result of climate change.
Plain Language Summary
The Amazon Basin plays a major role in climate by removing huge quantities of carbon dioxide (CO2) from the atmosphere and storing the carbon in vegetation. This removal partially offsets the impact of CO2 emitted by humans and the consequent rate of global warming. There is a concern about decreasing soil fertility in the Amazon Basin because of the loss of phosphorus, an essential plant nutrient, due to intense tropical weathering and biomass burning. Previous work suggested that large quantities of dust are transported from Africa to South America every year and deposited to the Amazon. Dust contains phosphorus and other nutrients that could mitigate soil losses and increase Amazonian soil fertility. However, estimates of the amount of dust carried to the Amazon are uncertain because of the lack of aerosol measurements. Our multiyear measurements of African dust in the trade winds in French Guiana, coupled with a chemical transport model, suggest that significant quantities of dust reach the heart of the Amazon Basin although the amounts are substantially less than had been previously estimated. This raises questions about the long‐term status of soil fertility in the Amazon and the future impact on the carbon cycle. We also find that the quantities of dust transported to South America are inversely linked to rainfall in North Africa. Consequently, climate change will affect dust transport to South America.
Key Points
We use 15 years of aerosol measurements in trade winds at Cayenne, French Guiana, to quantify the transport of African dust to South America in boreal spring
Dust transport may be supplying significant amounts of nutrients, including phosphorus, that are important for maintaining soil fertility in the Amazon Basin
The MERRA‐2 model yields deposition rates to the Amazon that are substantially smaller than rates from previous studies
