What is the composition of soil?
Inorganic materials, organic materials, air, water, organisms (plants, bacteria, fungi, protists, animals)
What is the size of the global soil OM pool?
- 2500 Gigtions of carbon
- Soil Organic Carbon is GT (62%)
- Remaining is carbonates (CaCO3)
- 1 gigation = 1 empire state building
Examples of strategies for maintenance and increase of soil organic matter (SOM):
Minimize losses: erosion, limit cultivation, limit overgrazing
Increases storage: amend soil with OM, practice conservation tillage, maintain plant cover, plant deep-rooted perennials, high plant diversity, high soil microbial diversity
Mineralization
Organic nutrients (N or P) will turn into inorganic nutrients
Immobilization
turns inorganic nutrients to organic nutrients
Benefits of OM:
- carbon sink
- storage of nutrients (especially in and on aggerates)
What are the biological conditions associated with the breakdown of OM?
- Detrivitores: larger organisms break dead tissues into small pieces; include inscets, isopods, worms, nematodes etc
- Decomposers: bacteria, protists & fungi that consume the small bits of organic matter, some carbon released as CO2 (respired)
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What are the physical factors that contribute to the breakdown of OM?
- temperature
- moisture (O2 availability, lack of slowed decomposition)
- O2
- tillage (starts decomposition)
- lignin content
How does CN ratio within OM contribute to its decomposition rate?
high CN litter decomposes slowly, and low CN litter decomposes rapidly
How does the CN ratio of an amendment (and composition with bacteria) alter plant nutrient availability?
- adding OM to soil can alter the physical and nutrient environment (porosity (number of air spaces) and nutrient availability)
- ex: mulching, cover crops, and residue retention
How does the microbial loop or plant exudates increase OM mineralization?
- if OM is high C:N ratio, bacteria (which grows faster) will steal nitrogen from plants
- High CN ratio 5:1: wood chips 500:1 (immobilization)
- if OM has a Low C:N ratio bacteria will decompose other (more resistant) organic matter
- Low CN ratio 2:1: raw manure 2:1 (mineralization)
Fungi have a higher CN ratio (~20:1)
CN ratio for the baseline:
- C:N > 30:1 immobilization likely (N bound by microbes
- C:N < 20:1 immobilization unlikely (N available to plants)
- C:N between 20:1 and 30:1 generally balanced
Compare and contrast microbial loop, litter homefield, and feedback:
- Microbial loop: plants exclude carbon from roots, 50% of carbon fixed, rhizosphere ‘hot spot’ for microbes, simple sugar w/ decomposition, waste of microbes contains nitrogen, plants regulate
- Homefield advantage: plants ary in litter quality (CN ratio), plants litter can also contain chemicals (nicotine), complex communities
- feedback: plants interact with microbes, can help maintain plant species (mycorrhizal fungi, rhizobia), can cause plant turnover (diseases, parasitic nematodes)
Mycorrhizae - a symbiotic relationship
- various forms (ecto- endo- vesiular-arbuscular)
- can increase root surface area >700%
- plants provide sugars (carbon)
- fungi provides nutrients and water (P especially)
