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Acid Rain and Boreal SoilsAcid Rain and Boreal Soils
Laura PothierLaura Pothier
What is Acid Rain?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
Acid Rain FormationAcid Rain Formation
Anthropogenic Sources of AcidsAnthropogenic Sources of Acids
The Boreal Forest: BasicsThe 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 FungiMycorrhizal 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 AcidsOxalic 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 SoilPodzol 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
PodzolizationPodzolization
• 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 Effects of Strong Acids on Podzol SoilsSoils
• 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
ConclusionsConclusions• 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!