Nutrient Acquisition And Cycling Flashcards

0
Q

Give five factors which influence soil formatim

A

1) Parent material 2) Climate 3) Topography 4) Organisms 5) Time

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1
Q

Give the main soil types present in the uk

A

Podzol Brown Earth Gleysol

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2
Q

Describe Podzol

A
  • Often found in cold and wet areas (boreal or northern coniferous forests) but can also be found in parts of the tropics - Generally of low agricultural value - Acidic conditions (layers not merged and tend to be well defined) - Grey/light coloured surface ‘E’ horizon due to severe leaching - Al and Fe oxides accumulate in subsurface ‘B’ horizon
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3
Q

Describe Brown Earth

A
  • Typical of mainly deciduous woodland and grassland - High agricultural value (naturally fertile) -Widespread in UK (not highlands) - Some leaching occurs (not heavy) - Al and Fe oxides more dispersed (hence brown colour) - Earthworm activity may merge horizon boundary
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5
Q

Describe Gleysol soil

A
  • Form on unconsolidated materials (exclusive of recent alluvial deposits). Show mottling and reduction (anaerobic) within 50 cm of soil surface. - Poorly drained – in tropics used for rice planting. - If drained then often can be used in agriculture.
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6
Q

Describe the mineral component of soil

A
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7
Q

Describe the Organic Component of Soil

A

Soil would not have structure if no organic component missing i.e. sand v soil.

Organic component due to life in soil.

Organic structure acted upon by micro-organisms & some compounds not fully decayed i.e. lignin - parts broken off to form ‘humus’ – not a compound per se but still a nutrient reservoir.

Soil very heterogeneous environment.

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8
Q

Give three functions of a root system

A
  1. Anchorage
  2. Uptake of Water
  3. Uptake of Nutrients
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9
Q

Describe the distribution of roots and state why they are distributed this way

A

Roots are unevenly distributed and tend to aggregate

To avoid root overlap and therefore competition between roots

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10
Q

Describe a study which shows the effects of root length density on nutrient uptake

A

Re-analysis of exsiting data by Fitter (1976)

Negative relationship found between root length density and ion uptake (P and K) (greater density = slower uptake) due to competition between the roots for ions.

K ions more mobile than P. Depletion zones for K more likely to overlap as RLD increases

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11
Q

Define two variables which dtermine the amount of nutrients that roots require

A
  1. Diffusion (D) of the ion (and time)
  2. Root Length Density (RLD) or Lv (Length per unit volume
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12
Q

Describe how plants show plasticity at different levels

A

Cellular: Root hair production, Carrier (ion) uptake

Tissue: Root diameter

Organ: Root architecture

Between organ: Root/shoot partitioning

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13
Q

Describe Reynolds & D’Antonio’s 1996 study

A

Lierature Study examining changes in rescource allocation between roots and shoots, and determining if there is any pattern among the species

Method: Using measured root eight ratio (RWR) (the ratio of root weight to total plant weight). An increase in RWR = greater allocation to roots

Findings: RWR decreases as N availability increases (75% cases). No species pattern (not related to form, life history, competitive ability, fertility etc.

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14
Q

Why does RWR decrease with higher nutrient availability?

A

The plant doesn’t need to waste resources on roots when N availability is high

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15
Q

Give pros and cons of using RWR as a measurement in studies and describe alternative measurements

A

Pro: easy to measure

Con: Includes lignified support roots (good for anchorage, not involved in nutrient uptake). Takes no account of root system architecture (differences in surface area)

Alternatives:

  • Length and density of root hairs
  • Length and density of roots
  • Root diameter
  • Root Lifespan
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16
Q

Describe pros and cons of measuring nutrient uptake potential using root hairs

A

Few studies on root hairs

Generally species from infertile habitats increase root hair length and density under low nutrient availability

Pro:

  • Role in P capture (P depletion zone 0.2mm, root hair can grow outside zone)

Con:

  • Typically less than 1mm in length
  • No role in N capture (N depletion zone around root is 20mm - root hair can’t grow out of that)
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17
Q

Describe root length and specific root length

A
  • Specific root length (SRL, cm mg-1) = length of root per unit root weight
  • SRL an estimate of root thickness (i.e. diameter) & tissue density
  • Nutrient uptake is more closely related to root length (than root hair amount?)
  • Higher SRL often found in nutrient rich zones - indicating a greater length of thinner roots
  • SRL varies among species - also a plastic root response
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18
Q

Describe Robinson and Rorisons 1983 study into the effects of N availability on SRL

A

Method: Supplied N to root system of 3 grass species. After 14 days some plants were removed and grown in a split root system with N only supplied to half of the root system. Controls had all roots exposed to N

Results: Compared to controls, SRL increases in the split root system supplied with N in all three grass species. SRL in N- split root system differed between species: after 35 days it was higher than control for 2 species (lolium and holcus) (Why? - SRL decreases as experiment goes on - decrease in SRL as roots age. In split system, burst of growth occurs on N side to maximise capture, resulting in longer length of thinner roots. For N- side, SRL starts low but increases with age as new, thinner, anatomically different (not good for N uptake) roots produced.)

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19
Q

Describe two methods of viewing roots

A

Rhizotrons -trenches dug and perspex panel put up - plants grown between panels (potential to cause stunted plants)

Minirhizotron and boroscope - Glass tube with regular interval markings, inserted at an angle. Insert camera and record roots (use markings to know depth). Used for counting roots which appear and die in the same place.

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20
Q

How is root life-span measured?

A

Cohort analysis - measure the roots until they are all dead. Half life is length of time it takes for half of the roots to die

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21
Q

Describe the life span of a root (Fitter)

A
  • Surprisingly short lived
  • Small fraction of roots survive indefinitely (probably structural)
  • Most roots die rapidly following exponential decay curve - fine roots involved in nutrient absorption first
  • Survivorship under elevated CO2 is shorter (higher turnover rate)
  • Plants extract N from roots before root destroyed
  • 70% studies - root half life less than 100 days
  • 46% studies - Root half life less than 60 days
  • Grass root half life can be less than 10 days
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22
Q

How are root systems heterogeneous?

A

Root systems are modular and the number of modules is not fixed therefore allowing plants to respond to changes in the soil.

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23
Q

Describe physiological responses of plants to incraesed nutrient availability

A

Increased ion uptake:

  • Jackson et al (1990) showed that nutrient uptake is higher in plants from soil with enriched phosphate patch (increased uptake prob. more imp for ions of high mobility. Immobile ions limited by diffusion to root.)
  • Drew (1975) put plants in pots with three levels - HHH or LHL nutrient conc pattern. P = increased root growth in higher conc. area (reduced in low conc.), N = increased root growth in higher conc. area (Reduced in low conc.), K = No effect (Don’t know why - maybe because plants don’t respond to K and when they do they are good at distributing it)
  • Campbell et al (1991) used a dripping system to create patches of depletion on developing root system. Showed that there is no evidence to suggest that plants from infertile habitats allocate more biomass to roots, and that generally 10/20% new dry matter allocated to root system in patchy environment. Dominance in the mixture (plant community) positively correlated with increment to root biomass in nutrient rich quadrants (more biomass, more competitve the species)
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24
Q

Describe plants’ approach to foraging and give evidence

A

Larger plants respond in scale, smaller plant in precision (they have fewer roots to place)

Campbell (1991) - shows greatest precision in foraging in subordinate species, however relationship weak

Fitter (1994) (using campbells data) - plotted relationship between precision of allocation and plant size, showed that as plant size increases, precision decreases

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25
Q

The percentage that the root system contributes to net primary production (npp) is…

A

… generally greater than 60%

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26
Q

Define the Rhizosphere

A

The rhizosphere is the narrow region of soil that is directly influenced by root secretions and associated soil microorganisms

Rhizosphere further divided into:

Endorhizosphere - Within the root

Ectorhizosphere - Outside the root

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27
Q

Define Mycorrhizosphere

A

Volume of soil influenced by the mycorrhizal root (normal rhizosphere + mycorrhizal mycelium)

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28
Q

Give three effects of the roots on soil

A
  1. Water - H2O lower in the rhizosphere
  2. Nutrients - selective uptake of ions
  3. Oxygen - roots consume or release CO2
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29
Q

Describe how the uptake of nutrients affects the rhizosphere soil

A

Influences pH

Uptake of nitrate (NO3-) releases bicarbonate and/or hydroxyl ions (increases pH) and uptake of ammonium (NH4+) releases protons. Roots do this to balance net internal charge

Rhizosphere pH will depend on plant species, N source, and buffering capacity of the soil

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30
Q

Describe how change in pH can affect the availability of nutrients in the soil

A

At high soil pH, P availability is reduced as it begins to form sparingly soluble calcium phosphates (decreasing pH will increase P availability)

Decreasing pH too much will also decrease P availability and also increase Al and MN toxicity

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31
Q

Describe how root systems get oxygen

A
  • Diffusion (although O2 diffusion is 105 lower in water than air - problematic for water-logged soils)
  • O2 transport from the shoot to roots and the rhizosphere via aerenchyma in the root cortex
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32
Q

How are aerenchyma formed

A
  • Through cell death and removal (lysigenous) or by cell seperation without collapse (schizogenous)
  • Can form in existing roots or the plant produces new adapted roots for large aerenchyma (dependent on plant species)
  • Mediated by ethylene (presence of ethylene promotes aerencyme formation)
  • Hypoxic conditions cause aerenchyma to form
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33
Q

Define radical oxygen loss (ROL) and state the problems/benefits with it

A

Some O2 released from the root (caused by oxygen flowing down the aerenchyma)

Problem: Oxygen loss can be too great (if ROL exceed the capacity of aerenchyma to diffuse O2 to growing tissues) therefore making aerenchyma detrimental to plant

Benefit:

  • ROL important for rhizosphere micro-organisms = oxidation zone in rhizosphere (extends c.1-3 mm from root)
  • ROL may also benefit plant as prevents toxic forms of ions (e.g. Fe2+ Mn2+ H2S) in immediate vicinity of root (through oxidation)
34
Q

Define sustainability

A

General: the capacity to maintain a certain process or state indefinitely

Ecological: The ability of an ecosystem to maintain ecological processes, function, biodiversity and productivity into the future

35
Q

State how to be sustainable

A

Earth’s resources used at a rate at which they can be replenished

Biological sustainability: A sustainable amount of production harvested; no net additional inputs (e.g. fertiliser); what comes out must be replaced

36
Q

Give a brief history of agriculture

A

Ancient: Civilisations built around rivers where annual flooding replenished soil with silt deposits. Nile delta/mesopotamia. Were only sustainable forms of agriculture

Agricultural revolution (1750-1880): Changed from subsistence farming to larger enclosures (fields). Mechanisation (seed drill), selective breeding, and crop rotation occurred. Resulted in significant yield increase (e.g. 35% increase in legumes) which helped drive industrialisation and population growth

1945 onwards: Green revolution (introduction of pesticides, herbicides, feritilisers, development of high yield crops etc.). Mexico in 1943 imported half of it’s wheat, by 1956 it was self-sufficient, by 1964 it was a net exporter of wheat

37
Q

Describe differences between traditional crops and green revolution crops

A

Traditional: Little response to fertiliser (increased vegetative growth, resulsts in lodging), needs long growing season, great variability in fields

New high yield cultivars: Semi-dwarf rice and wheat, uniform, good reproductive response to fertiliser (grain yields increased), earlier maturing

38
Q

Describe problems caused by the green revolution

A
  • Original rapid increases in yield now dimishing but population still increasing
  • Modern practices have:
    • caused environmental problems, e.g. excessive pesticide use by untrained farmers
    • Increased cost of production
  • Soil and water (resources) have become depleted in certain places (through mismanagement)
39
Q

Describe an event caused by mismanagement of soil

A

1930’s dustbowl events in american midwest: One of the worst environmental disasters of the 20th century caused by humans removing drought resistant wild prairie grasses and replacing it with fragile wheat, allowing livestock to overgraze pastures, and leaving fields unused with no plant cover, compounded with the natural drought period. Crops failed due to drought leading to more exposed soil and dust storms. 770 million metric tones of topsoil were lost, 3.5 million people displaced, and caused a lack of jobs and social tension. These events contributed to the Great Depression.

40
Q

Describe a major way of mis-managing land in humid and sub-humid tropical areas

A

Slash and Burn agriculture.

Major cause of deforestation and land degredation

Can be ecologically stable under low population densities (rarely exists)

Juo & Manu (1996): In slash and burn agriculture, total nutrient stock declines over time. Practice only sustainable with short cultivation periods and long fallow periods (even still, P becomes depleted). Nutrients lost via run-off, erosion, or leaching as well as cropping

41
Q

What are the effects of ageing soil on nutrient availability?

A

Changes in soil P and N amounts

Highest P level in young soil 800 mg/kg, lowest P level in ancient soil 30 mg/kg

Peak total N values 8000 mg/kg, less than 5mg/kg in young soil

42
Q

Give problems with phosphorous

A
  • Low mobility
  • Enter cycle through weathering raher than fixation (derived from rock phosphate (apatite) which is the only significant global reserve of phosphate)
43
Q

Why is P balance important for sustainability?

A
  • If no fertiliser is given, natural inputs can not sustain yield
  • Soil that hasn’t receieved P in a long time has lower losses of P (less solub;e P available) , lower net P input (0.18 kg/ha)
    • When only the grain is removed and the rest returned to the soil, the maximum yield is 0.1 ton/ha/year (mean british wheat yield 6.6 kg/ha/year). This is lower than historical yields! (Farming on a nwly converted prairie show higher yields (1 ton/ha/year) which is maintained by OM, although it is not stable in the long term)
44
Q

Give case studies of sustainable/unsustainable farming practices in regard to P

A
  • Nile delta: Annual silt deposit of 1.8 mm/year which provided enough P to support lower end crops. P is borrowed from further up the river. Was ‘sustainable’ until 1960’s Aswan Dam prevented annual flooding
  • Open field systems in middle age England: included fallow period (sometimes legume crops were sown during cropping phase). Crop production maintained without obvious P inputs. However records from Cuxham show a net P defecit, therefore system not sustainable. (Yields higher than would be expected due to either higher weathering inputs or crop growth dependent on stores of availble P in soil). Sustainability problem ‘solved’ by black death
  • China farms in early 20th century: Wheat grown with higher yields and 2 crops per year. China returned human waste to field so P defecit is similar to Cuxham despite larger yield. Typhoid risk.
45
Q

Give a brief history of N2 fixation

A

Regarded as an ancient form of biotechnology

  • 12th Century B.C. Theophrastus (Greek philosopher) wrote of the ‘reinvigorating effect’ of growing legumes on exhausted soil.
  • 1813 Humphrey Davy - speculated the legume-N came from the atmosphere.
  • 1880 Hermann Hellriegel showed atmospheric N fixation depended on bacteria in nodules of legumes.
  • 1888 Martinus Beijerinck isolated Rhizobia from legume roots.
46
Q

Why is N fixation significant?

A

N is primary limiting nutrient, despite the atmosphere being 78% N2

Weathering rocks results in negligible amounts of N

Biological fixation accounts for most of the N in soil

47
Q

Describe three main types of N fixers

A

All prokaryotes

Can be:

  • Free-living
  • Associative (loosely associated with roots of some plants)
  • Symbiotic (close association)
48
Q

Describe free-living N fixers

A

Autotrophs:

  • Aerobic - Cyanobacteria
  • Anaerobic - purple/green bacteria

Heterotrophs

  • Bacillus, Klebsiella (facultative anaerobes only fix N2 when O2 limited)
  • Clostridium (an obligate anaerobe).
49
Q

Describe associative N fixers

A

Loosley associated with roots of some plants

50
Q

Describe types of symbiotic N fixers

A

Lichens: If cyanobacteria (e.g. Nostoc spp) then can supply N to fungal partner.

Leaf symbiosis

  1. cyanobacteria (Nostoc) with moss (Pleurozium schreberi)
  2. cyanobacteria (Anabaena) with Azolla (small aquatic fern) – in cavities of leaf

Root symbiosis (nodule formation)

  1. Actinorhizal associations (between roots of non-leguminous angiosperms e.g. Alnus, Casuarina & Myrica and actinomycetes of genus Frankia).
  2. Rhizobium associations (between roots of some leguminous plants and e.g. Rhizobium, Mesorhizobium, Sinorhizobium, Bradyrhizobium).
  3. Coralloid roots (between roots of Cycads (an ancient group of Gymnosperms) and cyanobacteria.

Cycads have 3 root types: tap roots, adventitious roots and coralloid roots

Coralloid roots = compact structure (resembles coral).

When mature are invaded by cyanobacteria (including Nostoc spp. – fix N in heterocyst structures).

51
Q

Give the basic biochemistry of N2 fixation

A

Involves the nitrogenase enzyme complex

N2 + 8H + 8e- + 16ATP = 2NH3 + H2+ 16ADP +16Pi

Energetically expensive!

52
Q

Describe the nitrogenase enzyme complex

A

Rich in iron and molybdenum

Functionall conserved across all N2 fixing bacteria

Strongly inhibited by O2 (O2 reacts with iron)

53
Q

Describe some solutions to ‘Oxygen problem’ that nitrogen fixers have

A
  1. Avoid O2 (move to lower O2 pressure)
  2. O2 scavenging (high respiration rates/leghaemoglobin)
  3. Separation in space (fix N2 away from photosynthetic cells)
  4. Separation in time (Photosynthesise in day, fix N2 at night)
54
Q

Describe how to measure N fixation

A

Acetylene reduction technique:

Nitrogenase enzyme system cannot distinguish between N2 and acetylene (C2H2)

Incubate the system with acetylene, measure ethylene produced

Qualitative rather than quantitative

15N2 Atmosphere

Grow plant in 15N2 Atmosphere, digest plant material and measure fixed 15N using mass spec. Good, expensive (gas and purpose built sealed units reqd.)

15NH4 added to soil

Isotope added to soil. Uptake measured and compared against uptake of non-fixing plant at similar growth stage. Difference a measure of fixation rate. Not so good - cheaper

55
Q

Describe how to calculate the fixation of associative N fixers in agriculture. Discuss differences with symbiotic calculation

A

10% of total produce/(efficiency of root C value/100) = kg N/ha/year

1500kg total, 10 g C used per gram of N

150kg/0.1 = 15 kg N /ha/year

Symbiotic: C efficiency usually greater insymbiotic assocation (6 g C for every 1g N fixed)

56
Q

Describe nodule formation in rhizobia host plants

A
  1. Root releases flavonoids/isoflavonoids.
  2. Root falvonoids recognised by NodD proteins in Rhizobium, which act as a transcriptional activator of nodulation-related (nod) genes
  3. nod genes trigger synthesis and secretion of nod factors
  4. a) Flavonoids act as chemoattractants. Rhizobiia moves towards the root
  5. b) Secreted nod factors detected by recepters on host and initiate nodule formation & cell colonisation
  6. Rhizobia attached to root hair.
  7. Root hair curls around rhizobia, entrapping it
  8. Infection thread grows from rhizobia into the root cytoplasm
  9. multiplying bacteria and root cells form the nodule
57
Q

Describe the ecological significance of biological soil crusts

A

Biological crusts = specialised community of cyanobacteria, microfungi, lichens & mosses.

Important in Arid regions - in deserts, cyanobacteria crusts can add:

1 kg ha-1 yr-1 N (main N input)

Also exude polysaccharides that retain (+) charged nutrients.

Very sensitive to disturbance (climate change) – decades to centuries to recover.

58
Q

Describe the biological significance of the Nostoc and Feather moss symbiosis for N fixation in boreal forests

A

This symbiosis can fix c. 1.5 – 2 kg N ha-1 yr-1.

Significant to N cycling in these systems.

P. schreberi = most common moss on Earth. Moss destroyed by wildfires – but there is a temporarily increase in N availability

59
Q

Define and describe Achlorophylly

A

•Family Monotropaceae (a series in the Ericales) consist 10 genera of entirely achlorophyllous plants. • All orchids are achlorophyllous in the early stages but most are green as adults. • These achlorophyllous plants acquire their C from the mycorrhizal fungus. • Fungus usually in an ECM symbiosis with other plants.

60
Q

Globally, what are the most important mycorrhizal types?

A

Arctic tundra - ericoid

Northern coniferous forest, temperate forest - ectomycorrhiza

Temperate grassland, dry scrub, desert - arbuscular mycorrhiza

61
Q

Describe the structure and function of orchid mycorrhizas

A

Structure: Growth of intracellular hyphae results in formation of pelotons which increase the interfacial area between the symbionts.

Function: Benefit to orchid = C, P & N, Benefit to fungus = ?

62
Q

Describe the structure of ericoid mycorrhizas

A
  • Intracellular hyphal complexes in epidermis.
  • Fungus produces loose wefts of hyphae on root surface (but no sheath or mantle).
63
Q

Describe the structure of arbuscular mycorrhiza and state how they colonise a root

A

Colonisation: Hyphae grow from inoculum source and attach to roots. 2-3 days later, swollen appressoria form. Hyphae then narrow to form a peg & penetrate cell wall & epidermis cells to the cortex

Structure:

Forms arbuscules which enter the plant cell wall but do not breach plasma membrane. Arbuscule turnover can be rapid (varies with species. Intracellular hyphae more long lived. Two types:

  • Arum: Grows longitudinally in intercellular spaces. Branches give rise to arbuscules
  • Paris: Little intercellular growth, but extensive development of intracellular coiled hyphae. Arbuscules grow from coils. Vesicles may also form
64
Q

Give three pieces of evidence which support tham AMs are the most ancient type

A
  1. Ubiquitous (in 2/3 of all plant species)
  2. Molecular evidence (glomeromycota split from other fungi >400 mya
  3. Fossil evidence: Glomalean spore present 460 mya, and devonian plants (400 mya) contained arbuscles and vesicles in underground stems
65
Q

Describe the structure of ECM and describe it’s key diagnostic feature

A

Fungus penetrates between the cortical cells and forms a complex fungal network = hartig net

Hartig net: Complicated fan-like branch system. Incontact with epidermal or cortical plant cells. Site of nutrient exchange for ECM

66
Q

Describe the effects of AM on the acquisition of P

A
  • In P defiecient soil, AM plants grow better than non-AM plants, however less well in P non-deficient soils.
  • AM roots take up P faster per unit root length than non-AM roots (under ideal conditions in growth rooms/agriculture)
  • Difficult to reproduce benefits of AM colonisation under field conditions (P not limiting, indigenous species, external mycelium grazed)
67
Q

Describe the effects of Collembola on AM

A

Collembola is a soil organisms which eats, or bites and severs, AM hyphae. Can also act as AM dispersal agents

In presence of Collembola, AM P inflow same as non AM. Phosphate inflows into leek plants stimulated by AM colonisation when no Collembola present

68
Q

Give evidence that the arbuscle is the site of nutrient exchange

A

P: Plant P uptake can be correlated with arbuscule development. Plant P transporters expressed on the plant membrane surrounding the arbuscules.

C: High affinity monosaccharide transporter 2 (MST2) found with a broad substrate range spectrum that functions at several symbiotic root location. MST2 expression closely related to mycorrhiza-specific phosphate transporter (PT2) - suggests exchange of C and P tightly coupled

N: When AM (but not roots) allowed into an organic patch (15N and 13C labelled), AM transports N back to host plant. Amount 15N in plant directly related to length in patch.

69
Q

Give the model of N movement in AM symbiosis

A

Inorganic N taken up by extraradical mycelium (ERM), assimilated and converted in to arginine which is transloated in to the intraradial mycelium (IRM - inside root). Arginine is broken down into Urea and ornithine. Urea is further broken down into ammonia. Ammonia passes through to plant.

70
Q

What controls N mineralisation - immoblisation

A
  1. The quality (n content) of the substrate being decomposed. At the critical substrate C:N ratio, neither net mineralisation or immoblisation occurs
  2. The decomposer community’s requirement for N relative to its requirement for C
71
Q

How to calculate the critical substrate C:N ratio

A

Calculate the N required:

Total units of C - Units C respired = Total C required

C:N ratio = 15:1

N requirement = 50C/15 = 3.3 units N

Calculate Critical substrate C:N ratio

Total units C/N requirement = C:N ratio

100C/3.3 = 30.3:1

72
Q

Describe the effects of substrate C:N ratio on the availability of N for plants

A

Smaller substrate C:N ratio, more N available for plants.

73
Q

Role of soil animals in decomposition

A
  1. Physical - redistribute and break up OM
  2. Chemical - Ingest and breakdown OM in gut
  3. Biological - Consume OM. C used for respiration &metabolism, N excreted at ammonia, urea, or amino acids
74
Q

Definition of nitrification

A

Oxidation of ammonium via nitrite to nitrate

75
Q

Give factors controlling nitrification

A
  1. pH
  2. Aeration of soil (nitrification is an oxidation process)
  3. Soil moisture (Autotrophic nitrifiers very sensitive to water stress. Moist soil required)
76
Q

Give benefits and problems of nitrification

A

Benefits:

  • Results in readily available form of N (Important because large amounts of ammonia toxis to plant)

Probems:

  • Nitrite reacts with amines in soil, leading to nitrosamine, which if assimilated by plants and then consumed by humans can cause stomach cancer
  • Nitrate can be reduced to nitrate by gut bacteria in animals and human infants. Nitrite combines with haemoglobin to form methahaemoglobin which can not transport oxygen. Blue baby syndrome (brain damage/death in severe cases)
  • Leaching - nitrate in drinking water at high levels hazardous to human health. Eutrophication caused by leaching into lakes. Affects higher food chain levels
77
Q

State the products of denitrification and the methods by which it is controlled

A

N2 is ultimate product. Can also form hydroxyl ions (alter soil pH) or N2O under acidic conditions

Controlled by O2 availability in soil. When O2 conc. is low, nitrate replaces it as dominant electron acceptor. Waterlogging favours denitrification (also centre of soil aggregates)

78
Q

How to measure denitrification

A

Acetylene block technique

Add acetylene gas to sealed soil cores. Gas blocks the final reductase enzyme and prevents N2 from forming. N2O is final product which can be easily measured by gas chromatography

Problems:

  • acetylene blocks primary oxidase of autotrophic nitrification (so NO3 not renewed), therefore underestimated denitrification flux.
  • N2O can also be generated by other means in soil, thefore overestimation of flux
79
Q

Give the benefits and problems of denitrification

A

benefits

  • May protect soil from recieving heavy pollutant loads of N (i.e. acid rain)

Problems:

  • N lost from soil system
  • Potential accumulation of phytotoxic nitrate in soil
  • N2O oxidised instratosphere - thought to destro ozone
    *
80
Q

What is ammonia volatislisation and give conditions which increase/decrease it

A

gaseous loss of ammoia from soil

Increase:

  • Dry conditions, high temps, high air movement, alakline pH, low clay/OM content

Decrease:

  • presence of plants (plants roots compete for ammonia, foliage can directly absorb volatilised ammonia
81
Q

Calculate the depletion zone of an ion

A

x = 2(sqrt)diffusion coefficient x time

Nitrate D = 10-10 m2s-1

Radius of depletion zone after 106 seconds = 2x sqrt(10-10 x 106) = 2 x 10-2 m = 20mm for nitrate

82
Q
A