Nutrient Acquisition And Cycling Flashcards
Give five factors which influence soil formatim
1) Parent material 2) Climate 3) Topography 4) Organisms 5) Time
Give the main soil types present in the uk
Podzol Brown Earth Gleysol
Describe Podzol
- 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
Describe Brown Earth
- 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
Describe Gleysol soil
- 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.
Describe the mineral component of soil
Describe the Organic Component of Soil
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.
Give three functions of a root system
- Anchorage
- Uptake of Water
- Uptake of Nutrients
Describe the distribution of roots and state why they are distributed this way
Roots are unevenly distributed and tend to aggregate
To avoid root overlap and therefore competition between roots
Describe a study which shows the effects of root length density on nutrient uptake
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
Define two variables which dtermine the amount of nutrients that roots require
- Diffusion (D) of the ion (and time)
- Root Length Density (RLD) or Lv (Length per unit volume
Describe how plants show plasticity at different levels
Cellular: Root hair production, Carrier (ion) uptake
Tissue: Root diameter
Organ: Root architecture
Between organ: Root/shoot partitioning
Describe Reynolds & D’Antonio’s 1996 study
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.
Why does RWR decrease with higher nutrient availability?
The plant doesn’t need to waste resources on roots when N availability is high
Give pros and cons of using RWR as a measurement in studies and describe alternative measurements
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
Describe pros and cons of measuring nutrient uptake potential using root hairs
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)
Describe root length and specific root length
- 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
Describe Robinson and Rorisons 1983 study into the effects of N availability on SRL
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.)
Describe two methods of viewing roots
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.
How is root life-span measured?
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
Describe the life span of a root (Fitter)
- 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
How are root systems heterogeneous?
Root systems are modular and the number of modules is not fixed therefore allowing plants to respond to changes in the soil.
Describe physiological responses of plants to incraesed nutrient availability
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)
Describe plants’ approach to foraging and give evidence
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
The percentage that the root system contributes to net primary production (npp) is…
… generally greater than 60%
Define the Rhizosphere
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
Define Mycorrhizosphere
Volume of soil influenced by the mycorrhizal root (normal rhizosphere + mycorrhizal mycelium)
Give three effects of the roots on soil
- Water - H2O lower in the rhizosphere
- Nutrients - selective uptake of ions
- Oxygen - roots consume or release CO2
Describe how the uptake of nutrients affects the rhizosphere soil
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
Describe how change in pH can affect the availability of nutrients in the soil
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
Describe how root systems get oxygen
- 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
How are aerenchyma formed
- 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