11. Organic Matter Flashcards
soils are a (3)… of carbon. globally, oceans buffer CO2 levels but fossil fuel combustion has accelerated increases in the…
store
source
sink
atmosphere
bulk of global carbon is in….They contain… the carbon of vegetation and atmosphere combined
soils
twice
Soil organic matter values may be estimated as 2.0 times soil organic C, although the multiplier traditionally used is …
Organic N may also be estimated from organic C values by dividing by ….for most soils,
1.72.
12
Organic compounds may be listed in terms of ease of decomposition as follows:
- Sugars, starches, and simple proteins
- Crude proteins
- Hemicellulose
- Cellulose
- Fats and waxes
- Lignins and phenolic compounds
Decomposition involves the …. When organic tissue is added to an aerobic soil, four general processes take place:
breakdown of large organic molecules into smaller, simpler components.
oxidation, release, synthesis, and protection.
It may take from days to years for the just described processes of decomposition and mineralization to run their course, depending mainly on two broad factors:
(1) the environmental conditions in the soil
(2) the quality of the added residues as a food source for soil organisms.
The environmental conditions conducive to rapid decomposition and mineralization
- sufficient soil moisture
- good aeration
–> 60% pores = water
- warm temperature
- near neutral pH
*periodic stresses such as episodes of severe drying actually accelerate overall mineralization due to the dramatic burst of microbial activity that occurs each time the soil re-wets
green manures—
plant residues used to enrich agricultural soils
The ratio of carbon to nitrogen (C/N) in organic residues applied to soils is important for two reasons:
(1) intense competition among microorganisms for available soil N occurs when residues having a high C/N ratio are added to soils,
(2) the residue C/N ratio helps determine the rate of decay and the rate at which N is made available to plants.
green plants are … water
dry plant matter is 90% of… (3)
60-90%
C, H, O
…(3) quick to decompose
…(4) slow to decompose
sugars, starch, protein
cellulose, lignin, hemicellulose, fats
anaerobic conditions = incomplete decomposition … (3) remains
alcohols
CH4
organics
C:N ratio in organics has a major influence on decomposition rates:
> 25 = N depletion and slow break down = poor quality SOM
–>wood, sawdusts, bark mulch, straw
<20 = rapid decomposition and N release = good quality SOM
–> higher N boosts primary production, C builds-up and stabilizes
–> hay
litter quality
Lignin ratio = good guide to litter quality
–> more lignin = more breakdown needed = slower decomposition
High quality litter: low lignin/polyphenol content, narrow C:N ratio
–> quick o.m. increase, quick decomposition
–> low C:N = always getting N
–> low lignin
Poor litter = slow decomposition, C build-up
–>always short of N until o.m. decomposes
Polymerization:
complex, resistant to further break down = humic substances
Humus
amorphous, colloidal from decomposition and synthesis by microorganisms
–> adds CEC
–> adds water-holding capacity
–> not a nutrient, but it enhances fertility
Humics vs non-humics
60-80% of SOM
complexing with clays (joining with others colloidal)
Non-humics: less resistance, identifiable
composting
creates humus-like material from raw residues
requires N, water, air, high surface area for more rapid decomposition
heats up to 70C when active –> sterilizes compost
= excellent mulch and potting medium
commercial example of composting:
composting straw for mushroom farming
–> add water, nutrients then aerate (mixing-windrow)
amount of SOM in soils varies. SOM depends on balance: …vs….
humification vs oxidation
Deserts - 20-30% by mass
Typical arable soil -1-4%
Mollisols and Andisols - high om in A horizon (deeper horizons = lower om)
Forests and grasslands = large stores because of higher productivity = greater SOM build up
SOM build up determined by environment:
-temperature and rainfall
(low temperatures and high rainfall = more vegetation growth = more om)
-lush growth (grasslands within fixed environment)
-fine textured soils (higher surface area, complexiation with om, clay particles (and silts) = more likely that om retained in soil instead of being oxidized)
-restricted drainage (water saturation = slower decomp.)
-little to no tillage
-sod crop rotations
-high soil fertility (especially N)
aim of mgmt of SOM = improve both…. of SOM
quality and quantity of SOM
increase active SOC by regular additions (green manures, cover crops, manures, mulch)
adequate N to boost productivity
return roots and shoots to soil
minimum tillage
perennial vegetation»_space; annuals
soils and greenhouse effect
soil = important sink for carbon
CO2 produced by all soils
HNO2 + CH4 produced by anaerobic soils
–> increase SOM to store more CO2
–> too much inorganic N-fertilizer stimulates NH4 oxidizing bacteria and suppress bacteria oxidizing CH4 so even more methane production (rice paddies, marshes)
organic soils contain more than….om depending on clay
20-30%
Histosols (US)
Organics (CAD)
-marshy depressions, anaerobic conditions
-low bulk density
-high C:N ratios
-high water and nutrient holding capacity
-oxidation by drainage = rapid break down
–> peat mined for fuel and horticulture
SOM is a complex and dynamic (why dynamic?) soil component that influences soil …. (3). It has 3 pools:…..
overtime changes with decay vs. buildup
–> decay and mineralization governs N + S cycling
–> build up and decay controlled by enviro. and om quality (C:N ratio + composition)
behaviour, properties and functions
active, slow and passive (stable)
C:N ratio stabilizes at ….so add N to ….to increase productivity
12:1
increase C
