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!