Spatial variability of African dust in soils in a montane tropical landscape in Puerto Rico

Abstract

Dust deposition provides rock-derived nutrients such as phosphorus (P) to terrestrial ecosystems. Over pedogenic timescales, as bedrock sources of P are depleted, dust sources of P may support productivity in certain ecosystems, but controls on the spatial variability of dust in montane forested systems are largely unknown. Here, we use neodymium (Nd) isotope ratios in 31 ridgetop surface soils to evaluate the spatial variability of dust contributions to soil across ~100 km2 in the Luquillo Mountains, Puerto Rico. Dust from the Sahara-Sahel region of Africa carries a distinct isotopic signature of -12 εNd. Local bedrock, in contrast, has a εNd value of +7. End-member mixing calculations based on εNd reveal a wide range in dust influence on surface soils, with between 0% and 92% of the top 20 cm of ridgetop soil Nd derived from African dust. Using εNd paired with both dust and soil Nd content, the current soil dust content was calculated, ranging from 0 to 8.0%. There were no correlations between current dust content of soil and 10Be-based denudation rate, elevation, rainfall, longitude, or forest type. Current soil dust content is significantly higher in soils developed on volcaniclastic sandstone, breccia and mudstone than in soils developed on quartz diorite bedrock, which we attribute to greater retention capacity in the volcaniclastic soils. Current soil dust content also increases with increasing ridge-width, implying that small-scale topographic effects and other factors such as wind speed and turbulence influence local dust deposition rates. Higher current dust content of soil is also positively correlated with biologically cycled fractions of soil P on quartz diorite bedrock (r2=0.24 and p=0.002 for sum of extractable NaHCO3-P + NaOH-P), suggesting that atmospheric dust inputs contribute to the fertility of Luquillo Mountain ecosystems. February 28, 2015To:Editor in Chief Chemical GeologyRe: Manuscript submission Dear Editors, Please find attached our manuscript: "Spatial variability of African dust in a montane tropical landscape in Puerto Rico", which we are submitting for publication in Chemical Geology.In this paper we explore the spatial variability of dust in the wet forested Luquillo Mountains of northeastern Puerto Rico. Dust inputs represent a significant influence on soil chemistry, ecosystem nutrient supply, and dissolved watershed solute fluxes. Little is known about the controls on spatial variability of dust deposition, especially when dry deposition is a major fraction of total deposition, as is the case in Puerto Rico.We use neodymium isotopes in surface soils to quantify African dust in Luquillo Mountain soils. We believe we have an especially robust situation for this tracer that allows for simple two end-member mixing calculations. Our choice to use surface soil provides an integrated look at dust inputs over hundred to thousand year timescales, which are relevant to the nutrient capital of the ecosystem. Our results highlight that dust input fluxes are highly spatially varia...

The effects of lithology on trace element and REE behavior during tropical weathering

“…Likewise, the LREE enrichment in the surface layers of the regolith profiles (Fig. 4,7), although also somewhat affected by pedogenic processes, is consistent with input of Saharan dust, in agreement with previous studies that found dust to be an important source of trace metals and limiting nutrients to LCZO soils (Chapela Lara et al, 2017;McClintock et al, 2015;Pett-Ridge, 2009;Pett-Ridge et al, 2009b). In fact, atmospheric inputs allow LCZO soils to maintain relatively high cation stocks despite the neartotal depletion of the bedrock source .…”

“…attributed to the greater total abundance of sorbing phases, such as clays and metal (hydr)oxides, in the volcaniclastic as compared to the granitic regolith McClintock et al, 2015).…”

Does soil erosion rejuvenate the soil phosphorus inventory?

“…Selected soil P fractions (total P, apatite P, labile P and occluded P at 0-20 cm depth) on ridgetops in Puerto Rico were not significantly controlled by erosion rates or soil residence time, however, erosion rates and residence times varied little between sites (McClintock et al, 2015). Data from slope transects in Brazil showed that young upper slope soils (Entisols) have higher apatite P and lower labile P concentrations than Inceptisols in mid and lower slope positions (Agbenin and Tiessen, 1994).…”

“…Uplift is typically associated with tectonic plate margins and a major control of erosion rates that are inversely related to soil residence times. Soil residence time in these studies is defined as the length of time that is required for soil material to be removed by erosion and replaced by soil production, and during which soil particles experience physical and biogeochemical conditions at the top of the weathering profile (Almond et al, 2007;Dere et al, 2013;McClintock et al, 2015). Compared to chronosequences developed in flat landforms, Amundson et al (2015) suggested that residence times for most hillslope soils in temperate climates give rise to neither N nor P limitation.…”

“…There are few data that directly link individual P fractions to absolute soil residence times (McClintock et al, 2015). Additionally, previous studies of soil P on eroding hillslopes are largely limited to tropical landscapes (Abekoe and Tiessen, 1998;Agbenin and Tiessen, 1994;Araújo et al, 2004;Mage and Porder, 2013;McClintock et al, 2015;Porder and Hilley, 2011;Porder et al, 2007b;Vitousek et al, 2003). In these actively eroding tropical systems, deep chemical alteration of bedrock causes soils to be depleted in apatite P, which provides the first indication that the optimal window hypothesis may not be applicable globally.…”