Saturday, 15 July 2006

Thirty-five Year Changes in Soil Mn, Zn, B, Cu, and Fe in Response to Forest Growth.

Jianwei Li1, Daniel Richter2, Arlene Mendoza1, and Paul Heine3. (1) School of Environment and Earth Sciences, Duke Univ, Levine Science Research Center, Durham, NC 27708, (2) School of Environment and Earth Sciences,Duke University, Levine Science Research Center, Durham, NC 27708, (3) School of Environment and Earth Sciences,Duke Univ, Levine Science Research Center, Durham, NC 27708

Changes in soil micronutrients (Mn, Zn, B, Cu and Fe) over periods of years to centuries are not widely explored. At the Calhoun Experimental Forest in the Piedmont of South Carolina, repeated soil sampling of 16 0.2-ha permanent plots are used to estimate changes in micronutrient concentrations in a forest that grew from seedlings to mature trees over 35 years. Soils from 1962 and 1997 were extracted with both 0.05M HCl and acid-ammonium oxalate (AAO) to examine changes in two chemical fractions of the micronutrients. Air-drying and storage effect were also tested. Overall, the medians of AAO-extractable Mn, Zn, and Cu were 2.3-6.5 times the medians of HCl-extractable Mn, Zn, and Cu throughout the upper 60cm. The Pearson's product moment correlation coefficients of HCl and AAO extractable Mn, Zn and Cu were 0.86, 0.71 and 0.60 respectively, suggesting that both methods extracted similar soil fractions, although AAO was much more aggressive at complexing and solubilizing metals. In the 35-year period, Mn, Zn, and B were depleted throughout the profile (0-60cm), with HCl-extractable B declining significantly in each of the four soil depths (p-value<0.001). In contrast, Cu remained relatively unchanged throughout 0-35cm, and even increased significantly at 35-60cm (p-value<0.05). Converse to Cu, Fe remained unchanged at 35-60cm but increased significantly over 0-35cm (p-value<0.01). Results indicate the soil has little ability to resupply B, while Cu and Fe have remained readily available over several decades. Soil acidification, as much as 0.5-1.2 pH units decrease in the soil profile from 1962 to present, has likely been one factor in the observed patterns. This study suggests strikingly contrasting processes control the bioavailability of soil Cu, Zn, Mn, Fe, and B.

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