the living soil Flashcards
what is a lichen
symbiotic relationship between algae and fungi and sometimes cyanobacteria
what does algae provide to the lichen
it fixes CO2 (photosynthesis) which is a food source for the fungus
what does fungus provide to the lichen
it protects the algae and releases organic acids that accelerates mineral weathering
what does the cyanobacteria provide to the fungus
it fixes nitrogen
what is the early role of lichens in soil formation?
generally are the initial colonisers as they are capable of both photosynthesis and nitrogen fixation
where would you find bacteria in an aggregate
in the water filled pores or surrounded by a film of water on the walls of air filled pores and forming colonies
where would you find fungi in an aggregate
same places as bacteria but also in unsaturated pores
why don’t microbes live in all pores
- size determines what microbes can fit as the pore necks may be to small to get into
- ## size also determines the level of saturation generally thus small pores are normally saturated (anoxic) which means only anearobes can occupy them and large pores often unsaturated so only areobes can live in the and microbes like bacteria will dry out.
why do bacteria prefer pores only slightly larger than themselves
- protects them from larger predators and prevents them from drying out
how does a pore full of water affect microbe metabolism
- oxygen diffusion is much slower in water thus the middle of the pore would be anoxic and you would only find anaerobes present and the outer areas would have better oxygen diffusion
how does soil orgainic matter influence aggregation
helps to bind soil particles together and inceases water holding capacity
increased OM = increased microbial activity as they feed on it
distribution of microbes within the soil profile are driven by …
- the amount of om which decreases with depth
- the presence of the water table - oxygen requirements
where are protozoa predominately found in the soil profile
- in the rhizosphere (soil region immediately under the influence of roots )
where is algae commonly found in the soil profile
- they need access to light for photosynthesis so are only really found in the root zone
where are anearobic/ areobic bacteria found and what influences their presence
- presence of a water table will affect where these microbes are found
- but both can be found at the same depth as access to oxygen varies widely across an aggregate
why does the cation exchange capacity of straw residue increase over time
molecules become smaller and smaller and oxidised so the become water soluble and able to easily react with minerals
what microbes are primarily responsible for the decomposition of lignin
- white rot fungi which can decompose lignin to CO2 and H2O
- the lock an key method doesn’t work however as lignin is not formed by uniform structure
what are the chemical and physical protecting mechanism of OM in soils
- physical - only occurs in micropores where the pore neck is to small for the microbes to enter
- chemical = interaction with amino surface so microbes will have to use more energy to break these bonds before they can start decomposing so if there is om nearby that is in a chemical bonds microbes would prefer to expend less energy and decompose that om instead
how does the tokumaru silt loam protect Organic matter
physical = high as there will be a presence of micropores chemical = dominant in clay so will have some chemical protection (energy requirement to break bonds) but its is a pallic soil so it drains poorly and loses iron oxide which are important for the chemical protection
how well does the Waitere sandy soils protect OM
physical = low due to the high presence of macropores thus the pore necks will be large enough for microbes to enter chemical = low as there is no charge so the inorganic compounds that bond to OM to protect is won't be so present as they would be in a clay soil
how well doe the egmount silt loam + allophane soil protect OM
Physical = due to presence of micropores and the chemical properties also help to increase presence of micro aggregates chemical = high as there is the inorganic compound of allophane present which om can bond to
describe the process of nitrification
- requirements for this to take place
- where it would take place
is the conversion of ammonium (NH4+) to Nitrite (NO2-) by nitrosomonas during this nitrous oxide (bad green house gas) is released then by nitrobactor nitrite is converted to nitrate (NO3-)
- require oxygen, optimum temp, moisture and non acdic environment to take place
- thus in the outer zone of a soil aggregate as oxygen is needed
describe the process of denitrification
- requirements
- where it would take place in an aggregate
it is the conversion of Nitrate (NO3-) to nitrous oxide (N2O) then too Nitrogen gas (N2)
- this happens when the environment is anoxic as it forces microbes to use the nitrate for respiration instead of oxygen - flaculative anaerobes are responsible so if there is presence of O2 denitrification wont happen
- so would happen in the inner part of a soil aggregate
what issues arise with the denitrification process
- if the process doesn’t complete fully ie oxygen is introduced more nitrous oxide will be produced which is a harmful green house gas
leaching is one way nitrogen is lost from the soil how does this occur
if the nitrate is not take up by the plants and left in the soil and rainfall comes it will then be leached
ammonia volatilisation is a way nitrogen is lost from the soil explain how this happens
NH4+ is converted to NH3+ through an equilibrium equation (NH3+ + h2o = NH4+ + OH-) if ph increases say with the application of urea the reactions will shift to the left and ammonia will be released
what are the four ways organic N can be lost from the soil and how is it converted to NH4+ for these processes to happen
- organic n is converted to NH4+ by enzymes breaking the amine group bonds
1. ammonia volitilsation
2. nitrification ( production of nitrous oxide)
3. leaching
4. denitrification
if the C:N ratio is higher than 30 what will happen
- net immobilisation will take place as all the N will be used up for microbial biomass growth and although there will be left over C there is insufficent N for the decomposition process to continue unless mineral N (NH4+ or Nitrate) is taken up from the soil
what is the implications of a high C:N ratio for farmers?
- lead to competition between the microbes and plants leading to a deficiency in plants = poor growth and yellowing.
if the C:N ration is below 20 what will happen
- net mineralisation as there will be N left over after microbes have used it for growth
what would the C:N ratio need to be for neither net immobilisation or mineralisation to occur?
- C:N ratio between 20 to 30 as all the the c and n will be used up in either microbial biomass growth or respirations as a source of energy
in decomposition how much carbon is used for respiration (energy) compared to being used for microbial biomass growth
2/3 or carbon used for respiration
1/3 for growth
what are the 3 Nitrogen fixation systems
- free N2 fixers ( source of energy = light and OM, carbon source is CO2 and its inhibited by presence of NH4+ and nitrate)
- association with plants (loose association no symbiotic relationship)
- symbiotic (mutual benefit)
why are free living N2 fixers inhibited by the presence of ammonium or nitrate
cos to fix n2 from the atmosphere bacteria have to expend a lot of energy so if there is mineral n present (NH4+ or nitrate) present they would prefer to use less energay and just us the mineral in instead
mycorrhizal fungi have symbiotic relationships with more then 90% of plant species those with thick roots like pine trees are the most dependnt on this relationship. What are the three main benefits for the plants in this relationship
- increased nutrient up take
- increased water uptake
- creating a extensive hyphal network which connects plant communities offering efficient horizontal transfers of nutrients
mycorrhizal fungi help increase uptake of phosphorus why is this important and how is this done?
- important as P is only slightly soluble and is immobile in soil particularly in allophonic soil as it is very attracted to fe and al oxides through specific adsorption these attraction forces are very strong and thus P is not often fund freelin in the soils and if it was to move via mass flow it would just get stuck> as a result for plants to take up P there need to be direct contact through root interception. The extensive hyphal connections through the soil mean the roots effectively have more reach and are able to access P even if it is no in the root zone.
what are the benefits of the mycorrhizal symbiotic relationship to plants
- plants supply the simple sugars to fungi
what are the benefits of mycorrhizal symbiotic relationships to the soil
- improve aggregation
- some produce glomalin which is crucial in aggregation
what are the 3 classifications of earthworms
- epigeic
- endogeic
- anecic
how dow you classify an epigeic worm
- feed on the organic matter on soil surface
- dont form form permanent burrows
how do you classify an endogeic worm
- burrow thru to the top soil
- feed on OM but also ingest a lot of soil
- form semi permanent burrows
how do you classify an anecic worm
- larger
- draw OM from top to their deep permanent burrows
outline earthworms seasonal activity and preffered environment
- peak casting activity in may, june and sept
- when top soils dry out worms go inactive in the sub soil
- the die more due to moisture stress than to heat
- like med textured soils (san to dry and clay to hard to burrow)
- don’t like acidity
what are the beneficial effects of earthworms (3)
- burrowing = increased aeration and improves infiltration and drainage
- mixes organic matter into soil and incorporates it deeper which inturn increase microbial activity deeper in the soil profile
- casting promotes mineralisation by bacteria, fungi and it stimulate aggregation by producing gums that have a binding effect.
