科研之友
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摘要
Aerosols play important roles in soil genesis and plant nutrition, at both local and intercontinental scales, by redistributing nutrients through the atmosphere. Aerosol capture by tree crowns is a possible mechanism by which trees significantly augment their nutrition, particularly in trace elements. Forests capture considerable amounts of aerosols relative to adjacent grasslands, but the extent to which aerosol capture occurs in small, scattered trees in savanna and the effect that such capture has on soil nutrients are unknown. We analyzed spatial variations in elemental concentration in topsoils near indigenous trees (%26lt;8 m tall) in short savannas marginal to treeless grassland in South Africa, by comparing samples taken immediately adjacent to tree trunks, under tree crowns and in the open. Soils adjacent to tree trunks were significantly enriched in Ca, Zn and Sr, and enriched in most study sites in As, Se and Mo compared with soils in the open. Soils adjacent to tree trunks were also significantly impoverished in Be, V, Fe and Co, and impoverished in most study sites in Ti, Cr, Mn and Ni compared with soils in the open. The elemental content of aerosols differs according to the aerosol origin. In natural, non-industrial aerosols, all elements (except Al and Si) are differential from the average particle of similar size in the sense that their concentrations in the aerosols particles exceed the Earth%26apos;s crustal averages. Most of the elements identified in this study as showing enrichment adjacent to tree trunks were strongly differential elements indicating atmospheric accretion. By contrast, most elements that showed impoverishment adjacent to tree trunks were weakly differential elements of ambivalent origin. This suggests that aerosol capture by tree crowns may be a significant source of enrichment of certain elements in soils under small trees at the edges of savanna. The relative impoverishment of certain elements (e.g. Fe) shown in our study could reflect the soil conditions prior to tree establishment, suggesting that the establishment of the seedlings of savanna trees may occur predominantly within micro-sites of relative soil nutrient poverty where intensity of grass competition is least. Our study confirms that several, possibly antagonistic, mechanisms influence nutrient content under trees, resulting in a complex pattern of enrichment and impoverishment in topsoils. Augmentation of nutrients such as Zn through aerosol capture may have adaptive value for the trees studied given that many soils in South Africa tend to be Zn-poor.
- 出版日期2012-11
