Geo Soils Exam #3 Review

Dr. Gilliam UWF

soil organic matter

• portion of soil composed of plant, animal, and microbial residues in stages of decomposition

• NOT the same as the O horizon, but plentiful in the O horizon

global carbon cycle

• transfer of carbon between the atmosphere, biosphere, pedosphere, hydrosphere, and geosphere

• balanced fluxes when undisturbed

carbon pools

• least to most:

  1. vegetation: 620

  2. atmosphere: 790

  3. soil: 3,000 - 4,000

  4. oceans & lakes: 38,000

  5. carbonate rocks: 75,000,000

fluxes

• photosynthesis

• respiration

• fossil fuel combustion

photosynthesis =

↓ carbon

respiration =

↑ carbon

fossil fuel consumption =

↑↑ carbon

top ACTIVE carbon pools:

• soil

• ocean

Keeling Curve

• graphed figure of atmospheric carbon since 1960

• up and down yearly balance between growing season and dormant season

• overall exponential increase of atmospheric carbon

sources of carbon

• most to least:

  1. aridisols (very high)

  2. entisols

  3. gelisols

  4. andisols (very low)

entisols

• 14.2% global carbon soil (2nd)

• mostly inorganic carbon

• lack of leaching, young, nature of parent material, low rainfall, alkaline pH

gelisols

• 12.9% global carbon soil (3rd)

• mostly organic carbon

• freezing temps, low decomposition, permafrost trapping OM, frost mixing, anoxic conditions, continuous organic input

aridisols

• 20.6% global carbon soil (1st)

• mostly inorganic carbon

• low leaching, alkaline pH, cation accumulation, subsurface mineral layer, dry climate, limited water movement

andisols

• 0.8% global carbon soil (lowest)

• low organic carbon, NO inorganic carbon

• carbon released from volcanic ash

plant residue composition

• physical: 75% water, 25% dry matter

• compounds: 45% cellulose, 20% lignin, 18% hemicellulose, 8% protein, 5% sugar and starches, 2% polyphenols, 2% fats and waxes

• elemental: 42% carbon, 42% oxygen, 8% hydrogen, 8% ash

physical plant residue composition

• 75% water

• 25% dry matter (compounds, elements)

compound plant residue composition

• 45% cellulose

• 20% lignin

• 18% hemicellulose

• 8% protein

• 5% sugars and starches

• 2% polyphenols

• 2% fats and waxes

elemental plant residue composition

• 42% carbon

• 42% oxygen

• 8% hydrogen

• 8% ash

fats & waxes

• plant matter: hydrophobic, insoluble lipids responsible for energy storage, cellular structure, and environmental protection

• 2% of plant residue

• 5th to disappear in decomp

cellulose

• plant matter: complex carbohydrate responsible for plant cell wall structures, rigidity, shape, and strength

• 45% of plant residue

• 4th to disappear in decomp

hemicellulose

• plant matter: complex polysaccharides responsible for flexible matrix binding cell walls together

• 18% plant residue

• 3rd to disappear in decomp

polyphenols

• plant matter: diverse group of organic compounds responsible for chemical defense system, stress responders, and signaling molecules

• 2% plant residue

• variable disappearance in decomp

lignin

• plant matter: complex polymer responsible for the “cement” in plant structures, waterproofing vascular tissue, biotic and physical defense

• 20% plant residue

• last to disappear in decomp (complex, hydrophobic structure)

protein

• plant matter: macromolecule of amino acids responsible for biochemical reactions, metabolic pathways, and structural adaptations

• 8% plant residue

• 2nd to disappear in decomp

sugar & starches

• plant matter: carbohydrates produced during photosynthesis responsible for storing and transporting chemical energy

• 5% plant residue

• 1st to disappear in decomp (simple structure, chemically accessible, high bioavailability/energy)

order of disappearance

1. sugar and starches

2. protein

3. hemicellulose

4. cellulose

5. fats and waxes

6. lignin

organic decay

• biological process of microorganisms and fauna (k vs r) breaking down dead organic matter into simple compounds

r-strategists

• opportunist microbes responsible for rapid decomposition

k-strategists

• specialist microbes responsible for slow decomposition

priming effect

• decomposition of soil humus during the most rapid microbial activity

decomposition/mineralization factors

• C/N ratio

• soil ecology

• lignin and polyphenol

C/N ratio

• mass proportion of organic carbon to total nitrogen in soils

• high C/N = immobilization

• low C/N = mineralization

soil ecology

• community of living organisms interacting in and with the soil

• ↑ microbes = ↑ decomp rate

• ↑ macrofauna and detritivores = ↑ decomp rate

↑ lignin/polyphenols + ↑ C/N =

↓ N (immobilization)

↓ lignin/polyphenols + ↑ C/N =

↓↓ N (immobilization)

↓ lignin/polyphenols + ↓ C/N =

↑↑ N (mineralization)

↑ lignin/polyphenols + ↓ C/N =

↑ N (mineralization)

N mineralization

• transition of organic N to inorganic N, increasing plant availability

• low C/N

• ex. soil microbes decomposing OM and releasing NH4

N immobilization

• transition of inorganic N to organic N, decreasing plant availability

• high C/N

• ex. soil microbes absorbing NH4 or NO3 into their own biomass

labile OM

• rapid carbon turnover

• easily degradable

• biomass, detritus, free particulate OM, dissolved biomolecules

• drives biological activity and nutrient cycling

humus

• protected/slow carbon turnover (through cold or wet environment)

• resistant to degradation

• protected particulate OM, protected biomolecules, char

• builds soil structure, water retention, and long-term storage

rapid carbon turnover

• fast movement of carbon through a soil ecosystem through labile OM

biomass

• labile OM: living organisms

detritus

• labile OM: free identifiable dead tissue

free particulate OM

• labile OM: free pieces of partially degraded tissue

dissolved biomolecules

• labile OM: free organic molecules and degradation products

protected carbon turnover

• slow movement of carbon through a soil ecosystem through humus

char

• humus: black aromatic products of fire

protected particulate OM

• humus: protected pieces of degraded cell walls and tissue

protected biomolecules

• humus: protected organic molecules, supramolecules, and degradation products

microbial transformations

• labile carbon: rapid movement of carbon through fire, shredding, catabolism, enzymes, biomolecules, cell fragments, dissolved compounds, atmospheric CO2 release

• stabilized carbon (humus): slow movement of carbon due to microaggregates, sorption to mineral surfaces, wet/cold environments

microbial transformations: carbon %

• 65 - 85% CO2

• 2 - 5% biomass

• 3 - 10% nonliving labile

• 10 - 30% relatively stable humus

organic carbon residues

• carbon-containing decomposition products in soils

• labile OM vs humus

allelochemical effects

• plants:

  • cell membrane and energy disruption

  • growth and germination inhibition

  • hormone and photosynthesis blockade

• soils:

  • small amount = stimulated microbial activity

  • large amount = alter OM and disrupt nutrient cycling

  • altered soil porosity and water capacity

  • decreased OM availability and absorption

allelopathic plants

• plants which release biochemical compounds through root exudation, leaf litter leaching, volatilization, and decomposition

primary effects of OM

• ↑ soil surface protection

• ↑ macropores

• ↑ biodiversity and bioactivity

secondary effects of OM

• ↑ microbial activity

• ↑ decomposition

• ↑ N fixation

• ↑ denitrification

• ↑ protected humus

• ↑ polysaccharides, glomalin, and organic acids

subsequent effects of OM

• ↑ buffering capacity

• ↑ water capacity

• ↑ organism competition

• ↑ ion adsorption capacity

• ↑ darkness

• ↑ burrows and biopores

• ↑ mineralization

• ↑ metal ion chelation

environmental effects of OM

• ↓ water for irrigation

• ↓ fertilizer

• ↓ flooding

• ↑ or ↓ pesticides

• ↑ groundwater recharge

• ↑ plant production

• ↑ CO2 sequestering

• ↓ water pollution

• ↓ land degradation

active OM

• labile, unprotected carbon

• fast turnover

• feeds microbes, releases plant-available nutrients, builds soil aggregates

• ↑ time after cultivation = ↓↓ OM

passive OM

• stabilized, protected carbon

• slow turnover

• long-term carbon storage, CEC capacity, soil structure, buffering

• ↑ time after cultivation = ↓ OM

SPAC carbon balance

• movement of carbon into soil OM and out of soil OM

• input: litter, waste, bioproducts, roots

• output: oxidation, removal, erosion, leaching

carbon input (SPAC)

• surface:

  • plant litter/residues

  • animal wastes

  • imported bioproducts

• depth:

  • rhizodeposition

  • root residues

carbon output (SPAC)

• surface:

  • oxidation (CO2)

  • removal

  • erosion

• depth:

  • dissolved organic carbon leaching

rhizodeposition

• process of living plant roots release organic compounds and cellular material into the rhizosphere

• (SPAC carbon input)

fine root turnover

• process of fine root residues dying, decomposing, and being replaced, releasing them into the rhizosphere

• (SPAC carbon input)

agroecosystems

• human-managed food, fiber, or fuel producing ecosystem

• ↓ organic carbon in topsoil

natural ecosystems

• naturally occurring, unmanaged ecosystem

• ↑ organic carbon in topsoil

carbon levels vs soil orders

• aridisols: low carbon at all depths

• alfisols: high carbon at topsoil, low below

• spodosols: high carbon at topsoil and spodic horizon, low otherwise

• mollisols: relatively high carbon throughout, lower with depth

  • much lower with agricultural plowing

climate

• ↑ soil temp = ↑ organic carbon

• ~ soil moisture = ↑ organic carbon

texture

• ↓ grain size = ↑ organic carbon

drainage

• ↑ porosity = ↓ organic carbon

fire exclusion

• ↓ forest fires = ↓ organic carbon

agricultural management

• ↑ tillage = ↓ organic carbon (long term)

climate change

• damaging erosion

• accelerated decomposition

• agricultural feedback loops

• CO2, NH4, N2O

• wetland soils = HIGH storage

↑ C/N =

↓ decomposition (immobilization)

↓ C/N =

↑ decomposition (mineralization)

↑ N =

↑ CO2 released during decomposition

↑ C/N vs time

↓ C/N vs time

MAX mineralization variables

• ↓ C/N ratio

• ↓ lignin/polyphenols

• ↑ soil ecology activity

• ~ to ↑ moisture

• ↑ temp

• ↑ O2

• ~ pH

↑ soil moisture =

↑ decomposition rate

↑ soil temperature =

↑ decomposition rate

↑ drainage =

↓ carbon

↓ particle size =

↑ organic carbon

nutrient

• element required by plants for growth, survival, and reproduction

soil chemistry

• chemical composition, reactions, and properties of soil

soil fertility

• capacity of soil to supply essential nutrients for plant growth

limiting factors

• chemical in the shortest supply relative to plant requirements, restricting plant growth

• often: nitrogen

• sulfur: rarely

nitrogen

• often limiting factor

• gaseous cycle

• role: amino acids, proteins, enzymes, chlorophyll

• uptake: NH4, NO3, small amino acids

• mobility: highly mobile through translocation and resorption

• deficiency: chlorosis in older leaves

• oversupply: lodging, root toxicity

nitrogen roles

• amino acids

• proteins

• enzymes

• chlorophyll

nitrogen uptake

• NH4

• NO3

• small amino acids

nitrogen mobility

• highly mobile in both forms

• translocation and resorption

nitrogen deficiency

• chlorosis

• older leaves

nitrogen oversupply

• lodging

• root toxicity

translocation

• transport and movement of nutrients through plants and soil

resorption

• conservation of nutrients from dying plant organs to perennial tissue for future growth

chlorosis

• condition in which too little chlorophyll is produced, turning older leaves yellow

• nitrogen deficiency

nitrogen input

• nitrogen fixation

• deposition

nitrogen output

• denitrification