Acid Rain and Boreal Soils Laura Pothier

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 Rain becomes more acidic with the addition of SO 2 and NO x

Acid Rain Formation

Anthropogenic Sources of Acids

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

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

Oxalic and Fulvic Acids 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

Podzol Soil O-Horizon: “mor” –“L” or “O i ” layer: fresh organic matter, undecomposed –“F” or “O e ” layer: partially decomposed, fragmented organic matter (bacteria and fungi here) Fungi produce fulvic acid in decomposition –“H” or “O a ” 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

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

Alaskan Podzol

Effects of Strong Acids on Podzol Soils H 2 SO 4 reacts with Al oxides to form Al 3+ ion –Very toxic to mycorrhizal fungi! Strong acids dominate over weaker H 2 CO 3 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

Conclusions 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!