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

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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 ~100km<sup>2</sup> in the Luquillo Mountains, Puerto Rico. Dust from the Sahara-Sahel region of Africa carries a distinct isotopic signature of -12 ε<inf>Nd</inf>. Local bedrock, in contrast, has a ε<inf>Nd</inf> value of ~+7. End-member mixing calculations based on ε<inf>Nd</inf> reveal a wide range in dust influence on surface soils, with between 0% and 92% of the top 20cm of ridgetop soil Nd derived from African dust. Using ε<inf>Nd</inf> paired with both dust and soil Nd content, the current soil dust content was calculated, ranging from 0 to 8%. There were no correlations between current dust content of soil and <sup>10</sup>Be-based denudation rate, elevation, rainfall, longitude, or forest type. Current soil dust content in the Luquillo Mountains 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 (r<sup>2</sup>=0.24 and p=0.002 for sum of extractable NaHCO<inf>3</inf>-P+NaOH-P), suggesting that atmospheric dust inputs contribute to the fertility of Luquillo Mountain ecosystems on the relatively P-poor quartz diorite bedrock.

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McClintock, M. A., Brocard, G., Willenbring, J., Tamayo, C., Porder, S., & Pett-Ridge, J. C. (2015). Spatial variability of African dust in soils in a montane tropical landscape in Puerto Rico. Chemical Geology, 412, 69–81. https://doi.org/10.1016/j.chemgeo.2015.06.032

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