44 terms

Soil Biology Exam 1

Why is a plant residue C:N ratio of < 25:1 ideal for microbes? (three reasons)
Nitrogen is not limiting
Microbes have C:N ratios of ~8:1
Residues are generally easier to decompose
What is the best combination of residues if the goal is to increase SOM?
Oat (C:N 70) and hairy vetch (C:N 11)
What are SOM inputs?
4 of them
Different types of plant tissue (lignin, cellulose)
Different types of plant root exudates (carbohydrates, amino acid)
different animal input (manure, urine, chitin)
different microbial, plant, and animal cellular components (proteins, lipids, DNA)
Where does SOM come from?
microbial biomass
What is the ideal microbial diet C:N ratio?
How fast is decomposition of C:N ratio is > 24:1?
Much slower than if it was <24:1
What materials have a faster decomposition rate? (5 of them)
Green matter= leaves vegetation
Legume cover crop
Young cereal, brassica
Poultry manure
Manure slurries
What materials have a slower decomposition rate? (4 of them)
Brown matter - stems, roots
Mature cereals (rye, wheat, oats)
Solid manure plus C (dairy bedded with straw)
Mature compost
Why would you want low C:N ratios?
fast turnover
short term N
promote predation and further N release
Why would you want high C:N ratios? (3)
feed slow growing, more efficient microbes
more diverse microbial community
food web can help with pathogen and pest suppression
What is the benefit of cover crops when it comes to building SOM?
increases input diversity, frequency, and quality
What organisms can perform Nitrogen fixation?
Algae (freshwater)
Diazotrophs (aerobic soils)
Clostridium (anaerobic soils)
Rhizobia (Leguminous root nodules)
Frankie (non-leguminous root nodules)
Major "workhorses" in decomposition of organic matter.
Helps bind soil particles into stable aggregates
C:N ~8
Organism that is a critical component of soil food webs and "microbial loop"
Predator of bacteria, fungi, archaea
Important regulators of nutrient cycling, especially N
C:N ~15
important litter decomposers
Oribatid mites
Why is biodiversity important?
Different organisms contribute to different ecosystem processes and functions that are critical to maintaining life.
5 soil forming factors
Parent Material
Soil weight/Total volume of soil
Bulk density
Soil weight/Solid soil particle volume
Particle density
When soil begins to dry out, the matric potential becomes __________.
stronger (more negative), and the biota begins to suffer
What is the first organisms to be affected by a drying soil?
Protozoa and nematodes followed by bacteria
What organism can function at much lower water availability?
Fungal growth
What soil texture has the highest plant available water?
silt loam
Why is lignin so difficult to decompose?
Aromatic structures
Amorphous, random non-specific structures
Largely hydrophobic and rigs
Relatively large
Decomposition of SOC by microbes is controlled by:
1. Environment
2. Chemical structure of SOC
3. Location
biological catalysts that control and speed up chemical reactions
Extra-cellular enzymes
The machines of biological activity enzymes: (4)
Lower activation
Increase reaction rates
May contain cofactors
May contain coenzymes
increase chemical activity of enzymes
Maximum reaction velocity
Substrate concentration at which reaction rates are half maximal
If manure with a C:N of <15 is added, which type of microbes will dominate?
Copiotrophs at first then later oligotrophs.
Regulators of N2 fixation: (4)
Ammonia concentration
C:N ratio of detrital tissue
What is the nitrogen fixation process?
N2 -> amino acids -> proteins
Abiotic reaction in which NH4+ is converted to NH3
Occurs in alkaline conditions
Biotic gaseous losses
Nitrification and denitrification
Does denitrification require aerobic or anaerobic conditions?
Anaerobic conditions
NO3- -> NO2- -> NH4+ -> amino acids
Assimilatory nitrate reduction
NO3- -> NO2- -> NH4+
Dissimilatory nitrate reduction to ammonium
The are of the soil influenced by earthworm casings and activity. Soil organic matter
the area associated with the spaces between soil particles or aggregates. Micro- and macro- invertebrates hunt their prey
the area at the top of the soil where organic matter from the surface is being actively decomposed. Di nitrogen is fixed
the interior of an aggregate, almost all bacterial-medaled nutrient transformations can occur. Methane is produced
soil surrounding root, nutrients are taken up, microbial activity is high, nitrogen fixation