Elemental composition of wind-blown sediments from contrasting textured soils

Highlights

• Element composition of aeolian sediments of two different textured soils was studied.

• Ca and Mg were transported at low heights in both soils.

• Organic C and N were transported at great heights in both soils.

• S was transported at low and high heights, possibly in its organic and mineral forms.

• The transport along height was less segregated in sandy loam- than in sandy soil.

Abstract

Wind eroded sediment is sorted by height by both size and density. Less information is available about the elemental composition and enrichment ratios of these sediments. Because of that, the concentration of elements in wind-blown sediments of a sandy- and a sandy loam soil of the semiarid Argentinean Pampas were analyzed. With this purpose, a wind tunnel study was performed for obtaining the saltating sediments transported at different heights, and a sorption-based laboratory procedure was used to separate the particulate matter. Results showed that Ca and Mg were transported at low- and C and N at high heights. These tendencies were attributed to the accumulation of Ca and Mg in coarse minerals, probably apatite and Ca-feldspars, which are heavily transported by the wind, and to the accumulation of C and N in low-dense organic substances, fresh debris and polysaccharides, easily transported by the wind. Total S did not show a preference to be transported at any height, possibly due to its accumulation in organic and mineral substances. The transport of C and N was more segregated in the sandy- than in the sandy loam soil, in agreement with the better aggregation of the latter, in which some of its organic substances (carboxylic acids, amides and aromatics), form coarse and heavily transportable aggregates. From the obtained results it can be deduced that C and N would be widespread larger than Ca and Mg. Assembling of data obtained to mass fluxes, becomes relevant for modeling the biogeochemical cycles.

Introduction

Wind erosion is an important process of land degradation in arid and semiarid environments. This is the case of the semiarid Argentinean Pampas, where the soils are highly susceptible to wind action due to their coarse texture and low organic matter content (Buschiazzo and Taylor, 1993). The degradation of these soils is accelerated by tillage practices that leave the uppermost layer bare and physically disturbed (Mendez and Buschiazzo, 2010). Despite the magnitude of wind erosion of soils across a wide range of climatic and management conditions is relatively well known (Buschiazzo et al., 2007, Hoffmann et al., 2011, Zobeck et al., 2013), less information is available on the elemental composition and enrichment ratios of wind-blown sediments (Webb et al., 2012).

The transport of sediments by the wind consist of the lateral flux (saltation), occurring at low heights and short distances, and the vertical flux (suspension), at greater heights and longer distances. The first transport mode involves coarse sized particles and aggregates (70–1000 μm), while the second one, clay- and silt sized particles (Shao and Lu, 2000). Elements accumulated in low-height transported sediments might be reincorporated to the soil (Doetterl et al., 2012), but those accumulated in sediments transported at high heights can be widespread outside the eroded field, representing a potential loss of soil quality (Chappell et al., 2019, Hoffmann et al., 2008, Nerger et al., 2017, Sharratt et al., 2015). In addition, suspended sediments can influence the radiation balance, the cloud formation (e.g., Conen and Leifeld, 2014, Steinke et al., 2016), and also human health due to reduction of air quality (e.g., Dockery et al., 1993, Pope and Dockery, 2006). Once deposited, the element enriched sediments may be incorporated to other soils and water bodies (e.g., Harrison et al., 1997, Martin et al., 1991, Meskhidze, 2007). Therefore, data on the elemental composition and enrichment ratios of wind-blown sediments become relevant for modeling biogeochemical cycles.

Studies regarding the elemental composition of wind-blown sediments oftentimes refer to those released from ephemeral lakes or deserts (Gaiero et al., 2007, Lenes et al., 2012). Apart from the relevant contributions on this topic aroused from studies performed in the Sahel (Bielders et al., 2002, Sterk, 2003, Sterk et al., 1996, Visser et al., 2005), little information is available on arable dust sources worldwide, despite they represent 5.6 × 10⁶ km² of the total earth surface. In addition, arable dust has special features, like a finer granulometry and a higher soil organic carbon (SOC) content than desert dust (e.g. Funk, 2004, Mendez et al., 2011). Studies performed in Australian rangelands by Webb et al. (2013), showed that enrichment ratios for SOC vary between 2.1 and 41.9, being considerably variable across soils and land uses. Therefore, the arable lands of the semiarid Pampas, due to their composition and management, become an appropriate scenario for detailed studies on elemental contents and enrichments of aeolian sediments.

It is assumed that the concentration of elements increases in height as they tend to accumulate in fine and light materials that are selectively removed by the wind (Lal, 2003, van Pelt and Zobeck, 2007). However, studies of Funk, 1995, Aimar, 2002 showed a Ca enrichment in low-height saltating sediments of different textured soils. Additionally, Iturri et al. (2017) found that wind differently distributed in height some organic and mineral soil substances, according to their size and density. The organic compounds of low density like plant debris and polysaccharides of microbial origin, were preferentially transported at high heights, while other organic compounds of high density, like carboxylic acids, ketones and aldehydes, were transported at low heights. Similarly, clay and silt sized particles and microaggregates, all of low density, were transported at high heights, while heavy quartz grains and coarse aggregates remained at lower ones. In the sandy soils, the transport in height of these substances was more segregated than in sandy loam soils. The better aggregation of the latter, which implied the transport at low heights of clay, silt and organic matter as part of aggregates, explained their little sorted organo-mineral vertical distribution. Webb et al. (2013) demonstrated that differences in SOC enrichment across soil textures were due to the higher abrasion efficiencies of coarse- than of fine textured soils, producing higher SOC enrichment in sandy than in clay-rich aggregated soils.

The soils of the semiarid Pampas have a saltation fraction dominated by individual sand grains in the sandy soils, and by aggregates in the sandy loam (Avecilla et al., 2015). Aggregates of the latter are more abundant, coarser and more stable than those of sandy soils. Aggregates of sandy loam soils are known to be fractionated due to wind abrasion into smaller-sized ones, while those of sandy soils are most susceptible to be fully disintegrated (Avecilla et al., 2018). Moreover, the great bounding energies between clay minerals and elements like organic C, N and S (Fryrear et al., 1994, van Pelt and Zobeck, 2001), would reduce the element released efficiency from aggregates, determining their transport even at low heights in the sandy loam soils. In the sandy soils, the abundance of quartz grains would trigger aggregate disintegration, causing a side-by-side arrangement of organic and mineral substances, distributing the elements associated to coarse and heavier substances at low heights, and those to fine and light substances, at high heights. On the other hand, elements like Ca and Mg which tend to be in coarse minerals like calcite, apatites and feldspars, would follow similar trends of height distribution in both soil types.

The aim of this study is to quantify the elemental composition and enrichment of saltating and particulate materials, in two arable soils of different texture of the semiarid Argentinean Pampas. The dataset may be used for modelling the significance of wind erosion in the biogeochemical cycles of the elements. Besides, the understanding of the vertical element distribution in each soil type, would allow to elucidate the enrichment mechanisms underlying.