23 July 2015

The Fertilizing Role of African Dust in the Amazon Rainforest


The Amazon region contains about half of the planet’s remaining rainforests and is an important ecosystem with a crucial role in regulating Earth’s climate. Relatively small changes in the forest cover and productivity could have important implications for the carbon cycle, atmospheric circulations, the hydrology cycle, and climate from regional to global scales. Ninety percent of soils in the Amazon Basin are deficient in phosphorous, a principal fertility factor influencing tree growth across the basin. Long-range transported African dust is recognized as a potentially important source of phosphorus. A team of scientists, including a U.S. Department of Energy researcher at Pacific Northwest National Laboratory, used 3-D aerosol measurements to estimate the dust deposition into the Amazon Basin in 2007–2013 during the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) field campaign. The team found significant inter-annual variation that is negatively correlated with the prior-year rainfall in Africa’s Sahel region—the less rainfall, the more dust. The CALIOP-based multiyear mean estimate of dust deposition matches better with estimates from in situ measurements and model simulations than a previous satellite-based estimate does. The closer agreement benefits from a more realistic geographic definition of the Amazon Basin and inclusion of a meridional dust transport calculation in addition to the 3-D nature of CALIOP aerosol measurements. They found that the imported dust could provide a phosphorus input comparable to the hydrological loss of phosphorus from the basin, suggesting an important role of African dust in preventing phosphorus depletion on timescales of decades to centuries.

Chun Zhao

The work was supported by NASA CALIPSO/CloudSat project (NNX14AB21G) managed by David Considine and the Science of Terra and Aqua project (NNX11AH66G) managed by Richard Eckman. Chun Zhao acknowledges the support by the U.S. DOE as part of the Regional and Global Climate Modeling program. The Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute under contract DE-AC05-76RL01830. We thank Kelly Elkins of NASA GSFC Science Visualization Studio for helping in laying out Figure 1. We are grateful to Paulo Artaxo for helpful discussion and Francois- Xavier Collard for sharing the PM10 measurements at Cayenne with us. We thank two anonymous reviewers for their helpful suggestions. The CALIPSO data were obtained from the NASA Langley Research Center Atmospheric Sciences Data Center. The SPI data were downloaded from http://jisao. washington.edu/data/sahel/ (doi:10.6059/ H5MW2F2Q). The GPCP version 2.2 rainfall data were downloaded from the Giovanni online data system, developed and maintained by the NASA GES DISC. The processed CALIPSO aerosol profiles, GEOS-5 wind profiles, and GOCART and WRF-Chem dust simulations are archived in NASA GSFC clusters and personal computers, which will be made available to readers per request to the corresponding author H.Y. at Hongbin.Yu@nasa.gov.