Soil Organic Matter Flashcards
what is soil organic matter important for? (4 things!)
- helps develop structural strength through bonding
- improves porosity and pore size distribution (infiltration rates, permeability, water availability)
- releases nutrients through decomposition
- possesses cation exchange capacity to supply nutrients to plants and to act as a buffer
what are labile pools in SOM?
product of decomposition + substrate for further decomposition –> CO2 and methane emissions
what is the global distribution of organic carbon in soil?
lots in tropics and northern/boreal forest, very little in peri-glacial/glacial/desert environments
what are the two environmental parameters controlling SOC?
water content and temperature
what are the input and outputs that SOM relies on?
- rate of input of plant tissue (plant productivity)
- rate of decomposition by soil organisms, erosion and leaching by water
what is the chemistry equation for decomposition?
carbon and hydrogen containing compounds + oxygen - (enzymatic oxidation) -> CO2 + water + energy
what are 5 controls on rate of organic matter decomposition in soils?
- composition of plant tissue and organic matter
- temperature
- moisture
- nutrient content of soil
- macro and micro organisms in soil
which compounds are fastest to decompose?
sugars, starches and simple proteins which are rich in nutrients
how is humus produced?
it is produced with the resistant components (complex molecules) of plant residue that slowly decay and accumulate. It is then slowly mineralized.
what part of the complex molecules from humus provide cation exchange capacity of SOM?
carboxylic acid groups
what is the relationship between nitrogen and rates of decomposition?
the higher the nitrogen content in residue, the faster it will decompose because microorganisms will quickly get at it
what is the new understanding of soil organic carbon cycling?
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
what is the optimum temperature for decomposition?
25-30 degrees celsius
why does decomposition reduce after optimum temperature?
because enzymes begin to denature and microbes might die/lose their efficiency
what is the optimum water and air content for decomposition called?
field capacity
what happens when anaerobic organisms take over?
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)
why do anaerobic soils accumulate lots of partially decomposed organic matter?
- decomposition takes place much slowly
- certain products of anaerobic metabolism are toxic to microbes, acting as a preservative for organic matter
explain the cycle of microbial activity for soil decomposition?
r-strategists rapidly increase until all added compoinds have been consumed completely, and then k-strategists can be more competitive until cycle repeats itself
what happens to SOC with forest fires?
- slow build up of organic matter in post-fire recovery
- ash stabilizes in soil, in longer term, forest fires help stabilize SOC
explain loss of organic matter in agriculture
- 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
why are prairies and south of spain gaining SOC?
because of the increase in irrigation
how does the loss of SOC affect the rest of the landscape?
becomes dissolved organic carbon that leaches through landscape and eventually reaches the ground water, rivers and lakes (brown ish color)
what is biochar?
black carbon that is produced as a vehicle of carbon sequestration from biomass
what does biochar do?
- generally raises soil pH
- reduces nutrient losses through adsorption/immobilization
- leads to heavy metal, phosphorus and antibiotic sorption
- increases water holding capacity
