Soil Organic Matter

Question 1

what is soil organic matter important for? (4 things!)

Answer 1

1. helps develop structural strength through bonding
2. improves porosity and pore size distribution (infiltration rates, permeability, water availability)
3. releases nutrients through decomposition
4. possesses cation exchange capacity to supply nutrients to plants and to act as a buffer

Question 2

what are labile pools in SOM?

Answer 2

product of decomposition + substrate for further decomposition –> CO2 and methane emissions

Question 3

what is the global distribution of organic carbon in soil?

Answer 3

lots in tropics and northern/boreal forest, very little in peri-glacial/glacial/desert environments

Question 4

what are the two environmental parameters controlling SOC?

Answer 4

water content and temperature

Question 5

what are the input and outputs that SOM relies on?

Answer 5

• rate of input of plant tissue (plant productivity)
• rate of decomposition by soil organisms, erosion and leaching by water

Question 6

what is the chemistry equation for decomposition?

Answer 6

carbon and hydrogen containing compounds + oxygen - (enzymatic oxidation) -> CO2 + water + energy

Question 7

what are 5 controls on rate of organic matter decomposition in soils?

Answer 7

1. composition of plant tissue and organic matter
2. temperature
3. moisture
4. nutrient content of soil
5. macro and micro organisms in soil

Question 8

which compounds are fastest to decompose?

Answer 8

sugars, starches and simple proteins which are rich in nutrients

Question 9

how is humus produced?

Answer 9

it is produced with the resistant components (complex molecules) of plant residue that slowly decay and accumulate. It is then slowly mineralized.

Question 10

what part of the complex molecules from humus provide cation exchange capacity of SOM?

Answer 10

carboxylic acid groups

Question 11

what is the relationship between nitrogen and rates of decomposition?

Answer 11

the higher the nitrogen content in residue, the faster it will decompose because microorganisms will quickly get at it

Question 12

what is the new understanding of soil organic carbon cycling?

Answer 12

that molecular structure is not the only component that determines the decomposition of SOM, also environmental factors (physical disconnection, sorption/desorption, freezing/thawing)
–> we can manage these factors in the way we treat our soil

Question 13

what is the optimum temperature for decomposition?

Answer 13

25-30 degrees celsius

Question 14

why does decomposition reduce after optimum temperature?

Answer 14

because enzymes begin to denature and microbes might die/lose their efficiency

Question 15

what is the optimum water and air content for decomposition called?

Answer 15

field capacity

Question 16

what happens when anaerobic organisms take over?

Answer 16

they reduce the decomposition of organic compounds and eventually everything is reduced to methane -> less energy is gained (but it still contains all that energy)

Question 17

why do anaerobic soils accumulate lots of partially decomposed organic matter?

Answer 17

1. decomposition takes place much slowly
2. certain products of anaerobic metabolism are toxic to microbes, acting as a preservative for organic matter

Question 18

explain the cycle of microbial activity for soil decomposition?

Answer 18

r-strategists rapidly increase until all added compoinds have been consumed completely, and then k-strategists can be more competitive until cycle repeats itself

Question 19

what happens to SOC with forest fires?

Answer 19

• slow build up of organic matter in post-fire recovery
• ash stabilizes in soil, in longer term, forest fires help stabilize SOC

Question 20

explain loss of organic matter in agriculture

Answer 20

• organic matter is removed in crops by tillage and fertilizer application speeds up decomposition
• when SOC is lost, cation exchange capacity gets lost -> water holding capacity is lost and soil structure is degraded

Question 21

why are prairies and south of spain gaining SOC?

Answer 21

because of the increase in irrigation

Question 22

how does the loss of SOC affect the rest of the landscape?

Answer 22

becomes dissolved organic carbon that leaches through landscape and eventually reaches the ground water, rivers and lakes (brown ish color)

Question 23

what is biochar?

Answer 23

black carbon that is produced as a vehicle of carbon sequestration from biomass

Question 24

what does biochar do?

Answer 24

• generally raises soil pH
• reduces nutrient losses through adsorption/immobilization
• leads to heavy metal, phosphorus and antibiotic sorption
• increases water holding capacity