FINAL EXAM REVIEW QUESTIONS
11. How do fungi affect soil fertility?
Mycorrhizal fungi form symbiotic association with roots of most plants - use 10% of carbohydrates plant passes
from leaves to roots - the fungi can't manufacture carbohydrates – grow out and search for nutrients/moisture
Plant well colonized with the fungi will have the equivalent of 10x more roots than without fungi
The fungi can prevent all root pathogens and damaging nematodes from attacking the plant root
Mycorrhizal association stimulated when there is ample light, adequate pH of the soil, good aeration, humus,
and moderate soil fertility - inhibited by the presence of many chemical fertilizers.
Important role in plants grown in infertile soils where P, Zn, and Cu especially scarce - assist tree growth
Mycorrhizal associations contribute to mineralization increase fertility
12. Describe two major types of mycorrhizae. How do these differ? What benefits do plants derive from mycorrhizal
Mycorrhizas commonly divided into ectomycorrhizas and endomycorrhizas
Ectomycorrhizal - hyphae penetrate root and develop in free space around cells of cortex but do not penetrate
cortex of root cell walls
Associated primarily with temperate or semi-arid region trees and shrubs (e.g. pine, birch, hemlock, beech, oak,
spruce, fir) – stimulated by root exudates – cover surface of feeder roots with a fungal mantle – primarily visible
white rootlets with characteristic Y shape
Feeder Roots - A dense network of slender branching roots that spread close to the surface of the soil and
absorb most of the nutrients for a tree or shrub.
Endomycorrhizal - hyphae penetrate cortical root cell wall
Most important endomycorrhizae – arbuscular mycorrhizae – arbuscules (highly branched structures - transfer
mineral nutrients from fungi host plant and sugars from plant fungus)
Mycorrhizae – ―fungus root‖
Important in stabilizing soil structure
Soil tillage destroys hyphal networks, so does use of non-host species, soil bareness, heavy phosphorous
Greatly enhance ability of plants to take up phosphorous and other relatively immobile nutrients present in low
concentration in soil solution
Plant roots alone may be incapable of taking up phosphate ions demineralized in soils with basic pH -
mycelium (mass of branching, thread-like hyphae) of the mycorrhizal fungus can access and make them
available to plants colonized
Water uptake improved – better resistance to drought and salinity
Protect from soil-borne diseases and parasitic nematodes by producing antibiotics, altering root epidermis,
competing with fungal pathogens for infection sites
Mycorrhizal mycelia much smaller in diameter than the smallest root can explore greater volume of soil,
providing larger surface area absorption
Also, the cell membrane chemistry of fungi is different from that of plants (including organic acid excretion which
aids in ion displacement
Mycorrhizae are especially beneficial for the plant partner in nutrient-poor soils
The absence of mycorrhizal fungi can also slow plant growth in early succession or on degraded landscapes.
Fungi protect plants rooted in soils with high metal concentrations, e.g. acidic/
13. Are soil organisms always beneficial to soils and ecosystems?
EFFECTS OF SOIL ORGANISMS ON PLANT COMMUNITIES:
Organic Material Decomposition
Breakdown of Toxic Compounds
Not so good…
Plant Pests, Diseases and Parasites Termites – emission of greenhouse gases
Termite mounds – move around organic matter so its distribution different
Root pathogens – armoleria???
Fungi at certain concentrations – toxic to plants
Fungal wars and root rots?
Organisms beneficial in small quantities bad in quantities
Suppression of beneficial organisms
Too many earthworms - negative in ecosystems that used to not have them (e.g. North America) -
worm casts have breakdown of original organic matter + amount of available nutrients up Boreal -
ecosystem - slow turnover - speeds up - O horizon chewed up faster
14. What roles do soils play in global carbon cycling?
Soils contain a huge and dynamic pool of carbon (C
Critical regulator of the global carbon budget as repository for > 3/4 of the earth’s terrestrial C, soils store 4.5 x amount of
C contained in vegetation
Small changes in soil C cycling have potential to release large amounts of CO2
Soil gas release is temperature sensitive global warming will probably create a positive feedback ability of soil to
sequester and retain C goes down
Diagram of carbon cycle in billions of tons of carbon per year. Yellow numbers are natural fluxes, red are human
contributions in billions of tons of carbon per year. White numbers indicate stored carbon.
Plants take CO2 from atmosphere through photosynthesis, energy of sunlight trapped in C-C bonds organic molecules.
Some of these organic molecules used as source of energy (via respiration) by the plants themselves (especially by the
plant roots), with the C being returned to the atmosphere as CO2.
Remaining organic materials stored temporarily as constituents of standing vegetation, most of which eventually added
to the soil as plant litter (including crop residues) or root deposition
Globally, at any one time, approximately 2400 petagrams (Pg or 1015g) of carbon are stored in soil profiles as soil organic
matter (excluding surface litter), about one-third of that at depths below 1 m.
An additional 940 Pg are stored as soil carbonates that can release CO2 upon weathering.
Altogether, about twice as much carbon is stored in the soil than in the world’s vegetation and atmosphere combined.
Of course, this carbon is not equally distributed among all types of soils.
About 45% of the total organic carbon is contained in soils of just three orders, Histosols, Inceptisols, and Gelisols
15. What soil Orders contain the most carbon? In what forms is this carbon found?
• About 45% of the total organic carbon is contained in soils of just three orders, Histosols, Inceptisols, and Gelisols.
16. Describe the decomposition process in terms of the organisms involved and major carbon fluxes.
• The two groups exerting the greatest influence on soil processes are:
• Composition of Plant Residues – Plant residues are the principal material undergoing decomposition in soils and, hence, are the primary source of
soil organic matter.
– Organic compounds can be listed in terms of ease of decomposition as follows:
- DIAGRAM: r-strategist (opportunist), k-strategist (more specialized) microorganisms, and the sum of these two groups. The time required for the
process will depend on the nature of the residues and the soil. Most of the carbon released during the initial rapid breakdown of the residues is
converted to carbon dioxide, but smaller amounts of carbon are converted into microbial biomass (and synthesis products) and, eventually, into soil
humus. The peak level of microbial activity appears to accelerate the decay of the original humus, a phenomenon known as the priming effect.
However, the humus level is increased by the end of the process. Where vegetation, environment, and management remain stable for a long time, the
soil humus content will reach an equilibrium level at which the carbon added to the humus pool through the decomposition of plant residues is
balanced by carbon lost through the decomposition of existing soil humus.
17. How does the C:N ratio of organic matter affect OM decomposition and net release of N?
In A horizon, typically 20
Mulch addedTypically breaks down very fast
Ratio around 20
Breakdown slowly…. Higher values
Figure 11.5 Changes in microbial activity, in soluble nitrogen level, and in residual C/N ratio following the addition of either high
(a) or low (b) C/N ratio organic materials. Where the C/N ratio of added residues is above 25, microbes digesting the residues
must supplement the nitrogen contained in the residues with soluble nitrogen from the soil. During the resulting nitrate
depression period, competition between higher plants and microbes would be severe enough to cause nitrogen deficiency in the
plants. Note that in both cases soluble N in the soil ultimately increases from its original level once the decomposition process has
run its course. The trends shown are for soils without growing plants, which, if present, would continually remove a portion of the
soluble nitrogen as soon as it is released. 17. What role do polyphenols and lignin play in OM decomposition?
High content lignin and polyphenols, along with high C/N ratios (20 is ideal for immediate turnover), markedly slow down
decomposition, causing organic matter to accumulate while reducing nutrient availability
Cellulose and hemicellulose easy to decompose
Temporal patterns of nitrogen mineralization or
immobilization with organic residues differing in quality
based on their C/N ratios and contents of lignin and
polyphenols. Lignin contents greater than 20%, polyphenol
contents greater than 3%, and C/N ratios greater than 30
would all be considered high in the context of this diagram,
the combination of these properties characterizing litter of
poor quality—that is, litter that has a limited potential for
microbial decomposition and mineralization of plant
18. Define allelopathy.
Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the growth,
survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial (positive
allelopathy) or detrimental (negative allelopathy) effects on the target organisms.
19. What management practices drive OM accumulation in soils? What drive OM decreases?
Leaving crop residue
Flooding it to kill bugs for OM turnover
In rice patties – kills weeds
Till spike in decomposition
Green manure – winter cover crops - stop erosion + OM
Rice - denitrification from flooding
60-70% pore filled - denitrification
1. What macronutrients and micronutrients are essential for plants?
Red – macro (>0.1% dry plant tissue) Blue – micro (<0.1% dry plant tissue)
C.B. HOPKiNS CaFé
Closed Monday Morning and Night
See you Zoon, the Mg 2. Describe the following processes in terms of N compounds involved and the organisms that
are essential –
Mineralization – process that releases elements from organic compounds to produce inorganic (mineral forms) – it is usually the
last step in the decomposition process
Ammonification – release of ammonium from organic nitrogen compounds
Nitrification – requires supply of ammonium ions and oxygen to make NO and NO io2s and is 3herefore favoured in well-drained
soils (optimum 60% pore space filled with water) – nitrifying bacteria much more sensitive to environmental conditions than
heterotrophic organisms releasing ammonium
Denitrification – an anaerobic process by which heterotrophic bacteria reduce nitrate to gases such as NO, N O, and N2 2
Dissimilatory nitrate reduction to ammonium (DNRA) – another anaerobic microbial process that in effect reverses nitrification – it
- - +
reduces NO to3NO ions2and then to NH . 4
carbon fixation is the reduction of inorganic carbon (carbon dioxide) to organic compounds by living organisms. The
most prominent example is photosynthesis.
Nitrogen fixation is a process by which N in the a2mosphere is converted into NH . 3
Immobilization – opposite of mineralization – conversion of inorganic - organic
3. How does soil chemistry, temperature and moisture content affect each of the major nitrogen
transformation processes listed above? NOT SURE HOW COMPLETE MY ANSWER IS…
Soils saturated – denitrification – oxygen depleted and bacteria respire nitrate as a substitute terminal electron acceptor
Fungi (NH3?? Producers) – pH and wetness not bad
Plants can take up ammonium – in closed forest ecosystem – little nitrate because all consumed at ammonium stage
soil pH affects the solubility of nearly all the nutrients but is often the dominant influence on the micronutrients Fe, Mn, Zn, and
Mo see reactions below – catalysts affected
Anaerobic microorganisms as well as fire can convert nitrogen and sulfur into gaseous forms
Temperature – increases metabolic rates
Nitrifying bacteria tolerate moderate moisture (above 60-70% pore filled denitrifying) and moderate pH and temp
Researchers in New York, Virginia and Massachusetts have concluded that kudzu, a bedeviling invasive plant species in the US
South, can increase soil nitrogen fixation rates by up to ten times over non-invaded soils, and increases nitric oxides, a precursor to
N fixation: 3 orgs: too dry bad
Free living orgs - Blue green algae
N fixers with legumes 4. Why are livestock production, rice growing and industrial pollution important for N cycling?
Too much manure at wrong time of year – lots of nitrogen in inorganic form
Air pollution – automobile exhaust – NO and N O2– react with water in atmosphere – come down as nitric acid (strong acid)
Till vs. no till - till - spike in decomposition
Green manure – winter cover crops - stop erosion + add organic matter
Rice - denitrification from flooding
60-70% pore filled – triggers denitrification
5. What determines the major classes of P availability in soils – readily, slowly and unavailable
forms of P?
- PO ,4HPO , etc4 readily available
- Slowly available - with Ca, high pH, Iron, Aluminum in low pH
- Not available - not mineralized or in a rock
6. What chemical compounds play a role in P fixation? 5
7. Is K availability affected by the same soil chemical reactions as N and P?
8. In what ways does Ca cycling differ from N cycling?
Major source of N in soils is organic matter
Different forms N but one form Ca
Ca no complex steps - made available through decomposition
+ - -
N – NH - 4O - NO 3 2
9. How is Ca availability affected by acid rain?
Acidification is a natural process in soil formation that is accentuated in humid regions where processes that produce H
ions outpace those that consume them. 10. How is micronutrient availability affected by soil physical and chemical properties?
– Soil pH
– Oxidation State and pH
– Organic Matter
– Role of Mycorrhizae
– Organic Compounds as Chelates
– Stability of Chelates
– For some nutrients (e.g., Ca, Mg, K), the chemical processes of cation exchange and mineral weathering largely control
the availability for plant uptake and leaching. For others (e.g., N, S, Fe, Mn), (bio)chemical oxidation and reduction
reactions play critical roles.
– Finally, soil pH affects the solubility of nearly all the nutrients but is often the dominant influence on the micronutrients
Fe, Mn, Zn, and Mo.
11. What is a “chelate” and how does it affect nutrient availability to plants?
To prevent absorbed nutrients from precipitation resulting from the interaction of nutrients, such as iron forming
precipitation with phosphorus, upon entering plant cells cationic nutrients will immediately form chelates with organic
acids such as citric acids, malonic acid, and some amino acids. This chelation process will then enable the nutrients to
move freely inside the plants.
CHELATION in soil increases nutrient availability to plants. Organic substances in the soil either applied or produced
by plants or microorganisms are the natural chelating agents. The most important substances having this nature are