Unit 11 - Soil Carbon

Soil organic matter is composed of:

Recognizable organic structures

Humus

Examples of ______ include sugars, polysaccharides, proteins, amino acids, fats, waxes, lignin, cellulose, and hemicellulose

Recognizable organic structures

_____: Polymaxized organics formed during microbial and chemical decomposition

Humus examples

Humus is not 

crystalline

Humus is _____ charged

negatively

Humus contains dissociated _____

OH, COOH, and phenol groups

Humus is composed of complex chain and ring structures of ____

C, H, N, O

Humus exhibits a pH-dependent

charge

Under neutral to high pH, ___ has a very high negative charge

SOM

_______ exceeds the combined content of the organic matter in the forest floor and above ground biomass under most vegetation

Mass of humus

The traditional way of studying humus to understand its decay and mobility was to separate it into different pools based on its reaction with _____

acid and alkali

Soils were treated with a strong base (alkali) and whatever was insoluble was called _____

humin

The dissolved part was treated with an acid and whatever was insoluble in an acid was called _____

humic acid

Anything still dissolved after treatment with an acid was called ______

fulvic acid

These different pools were characterized by compounds of _____

different molecular weight and colors

After new techniques became available to examine SOM in situ, it was discovered that most of the compounds comprising these fractions were not actually found in soils but rather created by _____

the extraction process

A better way to understand humus and its decay and movement is by its ____

location

SOM within aggregates can be ____ from microbial attack

physically protected

SOM associated with clay form ____; the bonds between the SOM and clay require more energy to break and are resistant to microbial attack

organo-clay complexes

The pool of carbon in these complexes is called _____

mineral associated organic carbon (MAOC)

SOM compounds dissolve in water are ____ - dissolved organic carbon (DOC)

very mobile

The magnitude of the negative charge from the organic colloids depends on _____

the pH of the soil solution 

Hydrogen dissociates (i.e., deprotonation) from colloid functional groups _______

Hydroxoyl groups: --OH > –O^ + H+

Carboxylic acid groups: --COOH > --COO- + H+

In most temperate regions, negative charges from humus far exceed any positive charges from _____

variable-charge soil minerals

The CEC of the soil will increase with pH due ____

mostly to SOM

At low pH values, CEC is generally low

As pH is raised, negative charges _____

increase thereby increasing CEC

Because SOM is connected to so many processes even while making up a relatively minor fraction of the soil by mass, the addition of SOM can have _____

profound positive effects on soil properties

Effects of SOM addition: When added as _____ > protection of the soil surface from raindrop impact, cooler temperature, prevent or reduce evaporation

mulch

Effects of SOM Addition: Creation of soil aggregates > ____ that facilitate soil water infiltration

create macropores

Effects of SOM Addition: Serve as a ____ to increase microbial diversity and activity

food source

The carbon cycle is a sequence of movements, reactions, and transformations of _____ in air, soil, and organisms

carbon-containing substances

The C cycle couples with other nutrient cycles especially ____

nitrogen

Carbon inventory: Ocean pool

40,000 PgC

1 Pg = 1×10 (^15)g

Carbon inventory: Upper 1 m of soil (SOC)

1,500 PgC

SOC - Soil organic carbon

Carbon inventory: Upper 1 m of soil (SIC)

800-900 PgC

SIC = soil inorganic carbon

Carbon inventory: Atmosphere

750 PgC

_____ is rapid in litter and roots

Decomposition

Root respiration and the decomposition of root detritus (dead OM) will evolve, and is _____ the CO2 compound with the decomposition of litter

~2.5 x

Dissolved Organic Carbon can move ___ in the profile

lower

dissolved organic carbon can complex with ____ and Ca > very resistant to microbial attack

clay minerals

dissolved organic carbon controls the downward movement of Fe and Al in soils through a process called _____

“chelation”

Plants fix carbon from CO2 in the atmosphere into organic matter through ____

photosynthesis

Plant roots respire some of that C as ______

CO2

In wet soils, the C is returned to the atmosphere as ____

CO or CH4

The humus fraction is recalcitrant (hidden or tend to break down) and slowly accumulates depending on:

Soil temperature

Soil moisture

Soil texture

inorganic Carbon pools: CO2 + H2O = H2CO3 _____

(carbonic acid)

inorganic carbon pools: H2CO3 = HCO3- _____ + H+

(bicarbonate)

inorganic carbon pools: HCO3- = CO3 (^2-) ______ + H+

(carbonate)

Inorganic carbon pools: Ca2+ + CO3(2-) = CaCO3 _____ (SIC)

(calcite)

Carbon is sequestered in the SIC fraction only if the proton has a sink which is often a _______

Ca-bearing feldspar (e.g., anorthite)

Because this carbon reaction is very slow, it is hard to sequester carbon _____

inorganically

Most carbonates in soils are a _____ of existing carbonate rather than sequestration

product of reprecipitation