Soluble iron nutrients in Saharan dust over the central Amazon rainforest

Abstract

The intercontinental transport of aerosols from the Sahara desert plays a significant role in nutrient cycles in the Amazon rainforest, since it carries many types of minerals to these otherwise low-fertility lands. Iron is one of the micronutrients essential for plant growth, and its long-range transport might be an important source for the iron-limited Amazon rainforest. This study assesses the bioavailability of iron Fe(II) and Fe(III) in the particulate matter over the Amazon forest, which was transported from the Sahara desert (for the sake of our discussion, this term also includes the Sahel region). The sampling campaign was carried out above and below the forest canopy at the ATTO site (Amazon Tall Tower Observatory), a near-pristine area in the central Amazon Basin, from March to April 2015. Measurements reached peak concentrations for soluble Fe(III) (48 ng m−3), Fe(II) (16 ng m−3), Na (470 ng m−3), Ca (194 ng m−3), K (65 ng m−3), and Mg (89 ng m−3) during a time period of dust transport from the Sahara, as confirmed by ground-based and satellite remote sensing data and air mass backward trajectories. Dust sampled above the Amazon canopy included primary biological aerosols and other coarse particles up to 12 µm in diameter. Atmospheric transport of weathered Saharan dust, followed by surface deposition, resulted in substantial iron bioavailability across the rainforest canopy. The seasonal deposition of dust, rich in soluble iron, and other minerals is likely to assist both bacteria and fungi within the topsoil and on canopy surfaces, and especially benefit highly bioabsorbent species. In this scenario, Saharan dust can provide essential macronutrients and micronutrients to plant roots, and also directly to plant leaves. The influence of this input on the ecology of the forest canopy and topsoil is discussed, and we argue that this influence would likely be different from that of nutrients from the weathered Amazon bedrock, which otherwise provides the main source of soluble mineral nutrients

Long-term observations of cloud condensation nuclei over the Amazon rain forest – Part 2: Variability and characteristics of biomass burning, long-range transport, and pristine rain forest aerosols

“…The ACSM-derived organic-to-sulfate ratio confirms that the biomass burning pulse comprised a predominantly organic aerosol. In general, the measured κ(S, D a ) levels respond inversely to the organic-to-sulfate ratio, confirming previous studies which stated that organic matter and sulfate constituents mostly define the aerosol's hygroscopicity in the Amazon (Roberts et al, 2002;Gunthe et al, 2009).…”

Long-term study on coarse mode aerosols in the Amazon rain forest with the frequent intrusion of Saharan dust plumes

“…A comparison of results from the ATTO and ZF2 sites is justified by the fact that on average similar atmospheric conditions and air mass advection patterns prevail at both locations, as shown in Pöhlker et al (2018). Also the aerosol (optical) properties, analyzed in Saturno et al (2017a), are comparable. However, some degree of uncertainty is associated with this comparison and has to be taken into account carefully.…”

“…2c by means of a decreasing difference of the three σ sp time series during LRT episodes. Quantitatively, this effect is shown by Saturno et al (2017a) in a multi-year å sca time series from the ATTO site with clear å sca decreases during the presence of African dust aerosols. Mostly, the LRT episodes occurred in a rather defined time window from December to April, which was described previously as the "Amazonian dust season" (Andreae, 1983;Swap et al, 1992;Formenti et al, 2001;Rizzolo et al, 2017).…”

“…In addition to the OPS data analysis, which is the focus of the present study, we used supplementary data from the following online instruments: a condensation particle counter (CPC, model 5412, Grimm Aerosol Technik, Ainring, Germany), a scanning mobility particle sizer (SMPS, model 3082, TSI Inc., Shoreview, MN, USA), an ultra-high sensitivity aerosol spectrometer (UHSAS, DMT, Longmont, CO, USA), a wide-band integrated bioaerosol sensor (WIBS, model 4A, DMT), a three-wavelength integrating nephelometer (Ecotech Aurora 3000, λ = 450, 525, and 635 nm), a multi-angle aerosol absorption photometer (MAAP, model 5012, Thermo Electron Group, λ = 670 nm), carbon dioxide (CO 2 ), methane (CH 4 ), and carbon monoxide (CO) monitors based on cavity ring-down spectroscopy (G1301, G1302 analyzers, Picarro Inc, USA) and a ceilometer (CHM15kx, Jenoptik, Germany). The equivalent black carbon (BC e ) mass concentrations, M BCe , were obtained from the MAAP aerosol absorptivity measurements based on ATTO-specific mass absorption coefficients (MAC) as discussed in Saturno et al (2017a). The ceilometer has been operated at the ATTO site since December 2014.…”

“…These dry season LRT events, which typically do not transport substantial dust loads and, thus, are less relevant for the coarse mode, have not been addressed in the context of this study. Details regarding dry season LRT aerosols can be found in Saturno et al (2017a, b). Results referring to the "wet season without LRT" represent average data from the wet season time frame with the corresponding LRT episodes in Table 1 being excluded.…”