The rock cycle

WEATHERING

Physical and Chemical

•Physical weathering- Changes in the degree of consolidation with little or no chemical and mineralogical changes in rocks and minerals. Frost wedging, salt weathering in arid climates, thermal expansion, plant and animal disruption.

•Chemical weathering - chemical changes rocks and mineralogical composition via dissolution, hydration, hydrolysis, acidolysis, chelation, and oxidation/reduction.

•Dissolution

•minerals most affected - salts, sulfates, carbonates.

•H2O + CO2 H+ + HCO3- H2CO3

•CaCO3 is a congruent reaction - entire mineral is weathered and results in completely soluble products

•CO2 from microbes contribute to breakdown of silicates in soils

Hydration and hydrolysis:

Hydration - incorporation of water molecules into minerals which results in structural and chemical change.

CaSO4 + 2 H2O CaSO4 . 2H20

anhydrite gypsum

Gypsum is relatively soluble and can undergo dissolution whereas anhydrite is less soluble.

Hydrolysis - incorporation of H+ or OH- in to mineral.

KALSi3O8 + H+ HALSi3O8 + K+ (incongruent)

Feldspar

FeOOH + 3H+ Fe3+ + 2H2O (congruent)

goethite

Pathways of water near the land surface.

Acidolysis:

Similar to reaction of hydrolysis where H+ is used to weather minerals, however, H+ supply not form water but organic and inorganic acids.

Humic and fulvic acids, carbonic, nitric, sulfuric and low molecular weight organic acids such as oxalic acid.

Chelation:

Organic acids can also cause weathering by chelation. A chelator is a ligand capable of forming multiple bonds with metal ions such as fe, Al, Ca resulting in ring-type structures with the metal incorporated. Large complex acids in soils can strip metals from minerals. EDTA (ethylene diamine tetraacetic acid) - common artificial chelator used in labs.

Oxidation and reduction:

Oxidation and reduction reactions weather minerals by the transfer of electrons. Minerals containing elements that can have multiple valence states such as Fe, Mn, S are susceptible.

Fe3+ + 2H2O FeOOH + 3H+

Goethite

Mg-olivine (Fosterite)

•Mg2SiO4 + 4CO2 + 4H2O 2Mg++ + 4HCO3 + H4SiO4

• 2Mg++ + 4HCO3- 2MgCO3 + 2CO3

- + H2O

HCO3- is a good indication of weathering.

Mg precipitates as magnesite, thus 4 moles of CO2 are taken from air, 2 return when Mg precipitates. So, for every mole of fosterite--2moles of CO2 get fixed into carbonate.

Minerals in soils are divided into primary and secondary minerals.

Primary minerals, which occur in igneous rocks, metamorphic , and sedimentary rocks, are inherited by soil from the parent material.

Secondary minerals form in soils and include layer-silicate clays, amorphous (non-crystalline) minerals, carbonates, phosphates, sulfide, sulfates, oxides, hydroxides, and oxyhydroxides.

Primary minerals are typically larger than secondary - high surface area of secondary minerals make them extremely reactive in soils.

Basic tetrahedral unit –Si ion shares charge equally with four oxygen ions.

Second most common building block of phyllosilicates is the Al octahedral polyhedron.

Kaolinite

Gibbsite

Feldspar

•Na-Feldspar (Albite) alkaline solution + kaolinite

2NaAlSi3O8 + 2CO2 + 11H2O Al2Si2O5(OH)4 + 2Na+ + 2HCO3- +

4H4SiO4

•Ca-Feldspar (Anorthite) kaolinite, 1 mole Ca, 2 moles HCO3-, but no

H4SiO4

•Ca2+ +2HCO3- CaCO3 +CO2 +H2O, 1 mole of CO2 returned to

atmosphere

•In addition to carbonic acid, other organic acids -citric acid from plant roots,

•phenolics (tannins) - decomposition,

•fungi - oxalic acid

•fulvics and humics weathering

•Also chelation (e.g., Fe & Al) combine with fulvic acids -- can percolate to lower profile.

•Fe and Al are relatively insoluble, found as crystalline and hydrous oxides.

•Crystal oxide

Fe-- goethiteAl-- gibbsite

•Hydrous oxide

Fe--hematite

Al--boehmite•Oxides are common in tropical soils where high

temperatures and decomposition leave little humics to chelate.

•Can use ratio of Si to Al as indicative of weathering

•kaolinite - 1:1 ratio - more weathered montmorillonite - 2:1 ratio

•some 2:1 clay minerals hold H2O and NH4 in crystal lattice. Can represent 10% of total N.

•Bauxite - Residual weathering mineral composed of more than 50% of Al, Fe, and Ti oxides and hydroxides.

•The primary Al ore occurs in 2 varieties

1. Lateritic-occurs with Al-Si rocks: granite, basalt.(most widespread)

2. Karst-occurs with carbonate rocks: limestone, dolomites

Structure of a 1:1 (kaolinite)

and a 2:1 (montmorillonite) layer-silicate clay mineral.

•Phosphorus-

Limited in supply to plants, apatite contains P.

Ca5(PO4)3 OH + 4H2CO3-- 5Ca++ + 3HPO4

-- + 4CO3- +H2O

•Initially non-occluded then taken up and bound by Al & Fe oxides-- “occluded”

•Animal P

1. Hydroxyapatite (Bones)

2. Flouroapatite (Teeth)

Cation Exchange Capacity

•Silicate clay minerals in temperate soils have a net negative charge.

1. Mg++ substitute for Al+++ in montmorillonite- unsatisfied negative charge in crystalline lattice

2. OH- radicals along edge of clay particles.

Depending on pH, H+ can be more or less reactive with the radical

Organic matter phenolic (-OH) and organic acid (-COOH) radicals contribute.

Diagram illustration the development of positively and negatively charged sites on

surfaces of soil constituents, at low and high pH.

•CEC = Total negative charge (meq/100g soil)

•Assuming equal molar concentrations, cations are held in sequence and displace one another:

Al+++>H+>Ca++>Mg++>K+>NH+>Na+ Ca2+ forms bases of Ca(OH)2

Si:Al ratio - 2:1 indicates greater exchange capacitythan 1:1 ratio

•Tropical soil has no exchange capacity except via organic matter.

•Tropical soils have strong CEC - thus, can resist acid rain

•However, acid rain in Northeast U.S. can dissolve gibbsite Al2O3-leading to Al+++ which is toxic to organisms.

Anion Adsorption Capacity

Soils dominated by oxides and hydrous oxides of Fe and Al have variable charge depending upon pH.

a. low pH adsorbs H+ from solution, results in positive charge - mineral AlOH+

b. high pH adsorption H+ dissociation AlOH H+

c. pH>9 - additional H+ dissociates - surface becomes negative - AlO- H+

PO43- >SO4

-->Cl->NO3-

There is a low availability of P in soils

Soils with poorly developed crystalline forms of Fe and Al oxides have greater anion adsorption capacity (AAC).

Tropical soils all controlled by organic matter.

SOILS

SOIL LAYERS

A.Forest Floor

L-Layer or Oi--undecomposed litter

F-Layer or Oe--fungi and bacteria (Fermentative)

H-Layer or Oa--humus amorphous organic matter, increase in % mineral

In tropics decomposition occurs rapidly so there is little time for development of soil structure (L-Layer, F-Layer, H-Layer are all the same)

O-Horizon -- all organic; A Horizon--alluvial processes (removal). Organic matter and minerals - chelation with organic acids - Fe and Al percolate down from forest floor - referred to as podzolization, not common in tropics because the decomposition is too complete.

E-Horizon-some minerals remain, podzolization is less intense; B-Horizon--illuvial (deposition); C-Horizon -- lowest soil layer least weathered and most similar to parent rock material

Grassland soils (i.e., tallgrass prairie at base of Rocky Mountains).

•Classified as Mollisols - high organic content and base saturation

•In forests precipitation exceeds evapotransportation, however, not typically in grasslands. Thus, less H2O, slower decomposition, high pH and Ca - same as in East African grasslands.

•Clays complexed with organic acids

•In great plains- leaching is so limited CaCO3 precipitates and accumulates in calcic horizons.

Spodosols - intense podzolization (common in temperate northeast U.S soils)

Alfisols - low podzolization

Utisols - southeast yellowish, deep-reddish color in B horizon

Soils in Tropics are many meters thick and have endured millions of years without disturbance

Deserts

•Entisols - recent with little profile development

•Chemical weathering proceeds slowly

•Soils in southwest U.S. deposited by alluvial transport from adjacent Mountain Ranges

CaCO3 - horizons called caliche

NaCl

can contain illuvial clays - eolian dust

Typical soil horizon

sequence for a Spodosol

developed under a coniferous

forest (left) and a Mollisol

developed under grasses and herbaceous

plants.

Soil Organic Matter and Humic Substances

The term soil organic matter (SOM) is generally used to represent the organic constituents in the soil, including undecomposed plant and animal tissues, their partial decomposition products, and the soil biomass.

Thus, this term includes:

1.identifiable, high-molecular-weight organic materials such as polysaccharides and proteins, 2.simpler substances such as sugars, amino acids, and other small molecules, 3.humic substances.

Soil is a complex, multicomponent system of interacting materials, and the properties of soil result from the net effect of all these interactions.

One of the major problems in the field of humic substances is the lack of precise definitions for unambiguously specifying the various fractions.

The term humus is used by some soil scientists synonymously with soil organic matter, that is to denote all organic material in the soil, including humic substances.

SOM consists of humic and nonhumic substances.

Nonhumic substances are all those materials that can be placed in one of the categories of discretecompounds such as sugars, amino acids, fats, etc.

Humic substances are the other, unidentifiablecomponents. Even this apparently simple distinction, however, is not as clear cut as it might appear.

Humic acids - the fraction of humic substances that is not soluble in water under acidic conditions(pH < 2) but is soluble at higher pH values. Humic acids are the major extractable component of soil humic substances. They are dark brown to black in color.

Fulvic acids - the fraction of humic substances that is soluble in water under all pH conditions. They remains in solution after removal of humic acid by acidification. Fulvic acids are light yellow to yellow-brown in color.

Humin - the fraction of humic substances that is not soluble in water at any pH value. Humins are black in color.

“Classical view” of the formation of

kerogens.

Turnover of litter and soil organic fractions in a grassland soil.

Humic acid

Linking of structural units according to Kleinhempel.

Pathway 1 – Formation of Humic Substances

Pathway 1 – Lignin Theory - Wakman’s Theory

Lignin utilized by microbes, and residuum material becomes part of soil humus. Modification of lignin includes loss of methoxyl (OCH3) groups with generation of o-hydroxyphenols and oxidation of aliphatic side chains to form –COOH groups.

Pathways 2 and 3 – Formation of Humic Substances

Pathways 2 and 3:

Pathway 3 - Phenolic aldehydes and acids are release from lignin during microbial attack and undergo enzymatic conversion to quinones. These quinones polymerize in the presence or absence of amino acids to form humic-like compounds.

Pathway 2- same except that polyphenols are synthesized by microorganisms from non-lignin sources (i.e., cellulose). Quinone-lignin theory now currently accepted by most.

Formation of brown-colored substances by reactions involving quinones is not rare – well documented in melanine formation in fruits.

quinone - any member of a class of cyclic organic compounds containing two carbonyl groups, > C O, either adjacent or separated by a vinylene group, -CH CH-, in a six-membered unsaturated ring. In a few quinones, the carbonyl groups are located in different rings. The term quinone also denotes the specific compound para- (p-)benzoquinone (C6H4O2). The quinone structure

Pathway 4 – Humic Substance Formation

Pathway 4 – Sugar-Amine Formation

Reducing sugars and amino acids, formed as by-products of microbial meatbolism and then undergo non-enzymatic polymerization to form brown nitrogenous polymers.

Sugar- amine condensation involves addition of amine group of the sugar to form the n-substituted glycosamine.

Major objection to this theory is that the reaction rates is very slow in typical soil temperatures.

Attractive feature is that the reactants are produced in abundance in soils.

Biopolymer degradation and abiotic condensation for humic substance formation.

Carbohydrates constitute 5 to 25% of the organic matter in most soils.

Plant remains contribute carbohydrates in the form of simple sugars, hemicellulose, and cellulose, but these are more or less decomposed by bacteria, and fungi, which in turn synthesize polysaccharides and other carbohydrates of their own.

Carbohydrates in Soils

The significance of carbohydrates in soil arises largely from the ability of complex polysaccharides to bind inorganic soil particles into stable aggregates. Carbohydrates also form complexes with metal ions, and they serve as building blocks for humus synthesis. Some sugars may stimulate seed germination and root elongation. Other soil properties affected by polysaccharides include cation exchange capacity (attributed to COOH groups of uronic acids), anion retention (occurrence of NH2 groups), and biological activity

(energy source for microorganisms).

The major groups of carbohydrates

They can be divided into 3 subclasses:

•1. Monosaccharides, which are aldehyde and ketone derivatives.

•2.Oligosaccharides, a large group of polymeric carbohydrates consisting of a relatively few monosaccharide units.

Monosaccharides

Oligosaccharide

3. Polysaccharides- contain many monomeric units (8 or more)

•The carbohydrates material in soil occurs as:

1. free sugars in the soil solution

2. complex polysaccharides3. polymeric molecules of various sizes and shapes which are strongly attached to clay and/or humic colloids.

Polysaccharide

Soil lipids

The class of organic compounds designated as lipids represents a convenient analytical group rather than a specific type of compound.

They represent a diverse group of materials ranging from relatively simple compounds such as fatty acids to more complex substances such as the sterols, terpenes, polynuclear hydrocarbons, chlorophylls, fats, waxes, and resins. The bulk of the soil lipids occurs as the so-called fats, waxes, resins.

In normal aerobic soil the lipids probably exist largely as remnants of plant and microbial tissues.From 2 to 6% of soil humus occurs as fats, waxes, resins.

Lipids are physiologically active. Some compounds have a negative effect on plant growth whereas others act as growth hormones. Waxes and similar materials may be responsible for the water repellent condition of certain sands.

Lipid

Amino acids

Amino acids exist in soil in the following different forms:

• As amino acids, peptides or proteins bound to clay minerals

• on external surfaces

• on internal surfaces

2. As amino acids, peptides or proteins bound to humic colloids

• H-bonding and van der Waals' forces

• In covalent linkage as quinone-amino acid complexes

3. As mucoproteins

4. As a muramic acid

Amino acids, being readily decomposed by microorganisms, have only an ephemeral existence in soil. Thus, the amounts present in the soil solution at any one time represent a balance between synthesis and destruction by microorganisms.

The free amino acids content of the soil is strongly influenced by weather conditions, moisture status of the soil, type of plant and stage of growth, additions of organic residues, and cultural conditions.