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Acid Rain and Boreal Soils

Laura Pothier. Acid Rain and Boreal Soils. What is Acid Rain?. Rain is already naturally acidic (pH ~ 5.6) H 2 O + CO 2  H + + HCO 3  H 2 CO 3 Carbonic acid is a weak acid; it does not dissociate on a large scale H 2 CO 3 attacks silicate rocks, releases Si

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Acid Rain and Boreal Soils

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  1. Laura Pothier Acid Rain and Boreal Soils

  2. What is Acid Rain? • Rain is already naturally acidic (pH ~ 5.6) • H2O + CO2 H+ + HCO3 H2CO3 • Carbonic acid is a weak acid; it does not dissociate on a large scale • H2CO3 attacks silicate rocks, releases Si • Rain becomes more acidic with the addition of SO2 and NOx

  3. Acid Rain Formation

  4. Anthropogenic Sources of Acids

  5. The Boreal Forest: Basics • Cold climate and shaded, evergreen canopy (conifers dominant) • Slow decomposition of organic matter from decreased temperatures and waterlogged soil • What causes cold, wet insulated soils? • Lichens on the forest floor trap cold moisture underneath floor • Shade from evergreen canopy keeps floor constantly cool and prevents evaporation

  6. Mycorrhizal Fungi • Gather scare nutrients for plants at the roots • Help form shealth around fine active roots of plants, extending them further into soil • Move phosphorous from soil to roots • Produce cellulase and phosphatase that help decompose organic matter • Also release fulvic acid and oxalic acid to soil

  7. Al and Fe oxides: occlude phosphorous inside crystal formation Make P unavailable for plant use Oxalic acid: Inhibits the crystallization of Al/Fe oxides to make P more available Fulvic acid: water soluble Chelates Fe and Al: Forms complex with Al and Fe to make them water soluble Removes them from A-horizon Oxalic and Fulvic Acids

  8. Podzol Soil • O-Horizon: “mor” • “L” or “Oi” layer: fresh organic matter, undecomposed • “F” or “Oe” layer: partially decomposed, fragmented organic matter (bacteria and fungi here) • Fungi produce fulvic acid in decomposition • “H” or “Oa” layer: Humus layer- amorphous products of decomposition • Fulvic acid percolated here • A-Horizon: “eluvial” layer • Zone of leaching • Whitish, silicate quartz material • B-Horizon: “illuvial” layer • Contains leached products like Fe and Al that combine with silicates to form clay

  9. Podzolization • Fe and Al in A-Horizon form complex with fulvic acid from mycorrhizal fungi • Fulvic acid is water-soluble… this mobilizes Fe and Al! • Percolation downward in water, below A-horizon • What’s left in A-horizon: water-insoluble silicate quartz material

  10. Alaskan Podzol

  11. H2SO4 reacts with Al oxides to form Al3+ ion Very toxic to mycorrhizal fungi! Strong acids dominate over weaker H2CO3 acid Cause more breakdown of organic molecules like fulvic acid and oxalic acid Whoa… Less fulvic acid = less chelation of Fe and Al => no podzolization and too much Al and Fe near mycchorizal fungi => no fungi => no P availability for plants => forest decline and the loss of lichens shade => loss of lichens and shade => increase in soil temperature => faster decomposition of organic matter and acids Effects of Strong Acids on Podzol Soils

  12. Addition of strongly acidic molecules causes the reversal of many processes that characterize podzol soils Damage done to symbiotic organisms result in decline of coniferous plants, and eventually the decline of boreal forests to make way for deciduous forests Bottom line: don’t ignore acid rain on acid soils! Conclusions

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