Rhizosphere Flashcards
Rhizosphere
narrow region of soil that is directly influenced by root secretions and associated soil microorganisms
Endorhizosphere
portions of the cortex and endodermis in which microbes and cations can occupy the “free space” between cells (apoplastic space)
apoplastic space
free space between cells
Rhizoplane
the medial zone directly adjacent to the root including the root epidermis and the mucilage
Ectorhizosphere
extends from rhizoplane out into the bulk soil
The rhizosphere is not
a defined size or shape
The rhizosphere consists of
a gradient in chemical, biological and physical properties which change both radially and longitudinally along the root
gradients at the rhizosphere
nutrient concentration pH Redox potential Exudates Microbial activity
Microbial activity
non-infecting rhizosphere microorganisms
infecting rhizosphere microorganisms: VA mycorrhiza, ectomycorrhiza
roots are a _____ for _____ and ______
sink
nutrients and water
nutrient ions and other ions may be _________ or _______ at the root surface
accumulated or depleted
roots release ____ or ____
H+ or HCO3-
Changes pH
Oxygen consumed or released…..
alters redox potential
Redox potential
A measure of electrochemical potential or electron availability within a system
electrons are essential to…..
all inorganic and organic reactions
Redox potential predicts..
stability of various compounds that regulate nutrients and metal availability in soil and sediment
Redox potential is determined from…
the concentration of oxidants and reductant
Oxidants: Oxidation
removal of electrons from a compound
Oxidants
oxygen, nitrate, nitrite, manganese, iron, sulfate, and CO2
Oxidation
removal of electrons from a compound
Reduction
addition of electrons to a compound
Reductants
various organic substrates and reduce inorganic compounds
root exudates
- Chelate and mobilize mineral nutrients
- provide energy for microbes
Adaptions in the rhizosphere
- significantly influence nutrient uptake
- crucial to plant adaptation to adverse environmental conditions
Organic chelating agents (peptides and sugars)
- extract metal ions from minerals and rocks
- some metal complexes in the environment are not found in some form of chelate ring (e.g. with a humic acid or a protein)
- metal chelates are relevant to the mobilization of metals into plants and microorganisms
- selective chelation of heavy metals is relevant to bioremediation
Organic chelating agents (peptides and sugars)
- extract metal ions from minerals and rocks
- some metal complexes in the environment are not found in some form of chelate ring (e.g. with a humic acid or a protein)
- metal chelates are relevant to the mobilization of metals into plants and microorganisms
- selective chelation of heavy metals is relevant to bioremediation (e.g. removal of 137Cs from radioactive waste)
Concentration of a specific ion in rhizosphere relative to bulk soil
lower
higher
similar
Factors affecting ion concentration
- concentration in the bulk soil solution
- rate of movement of ion to root surface
- rate of nutrient uptake by the root
Nutrient concentration examples
- Ryegrass roots: deplete K concentration in soil solution below 80 micro moles in the rhizosphere enhanced release of (non-exchangeable) K.
- Canola (rape) depletion of K and Mg associated with a pH decrease in the rhizosphere to 4.0 resulted in release of non-exchangeable K and Mg which introduced irreversible transformations of mica.
- water uptake is greater than ion uptake: ions accumulate in the rhizosphere
Salt accumulation in Rhizosphere:
Low solubility salts such as CaCO3 and CaSO4 accumulation
- may not be harmful to plants in acid soils, but can be a problem in saline soils
- increasing salts concentration and osmotic potential of the soil solution decreases water availability to plants
Rhizosphere pH and bulk soil pH
can differ up to two units
Rhizosphere pH and Redox potential:
Root induced factors
- imbalance in cation/anion uptake ratio and corresponding differences in net release of H+ and HCO3- or (OH-)
- Excretion of organic acids (or organic acids produced by increased microbial activity in the rhizosphere)
CO2 production by roots and microbes:
Aerated Soils
- CO2 rapidly diffuses from roots and has small effect on rhizosphere pH
- mainly the CO2 dissolved in the soil solution which affects the rhizosphere through the influence in HCO3- and H+ concentrations
Rhizosphere pH change
- pH buffering capacity of soil
- Initial pH of the bulk soil
pH buffering capacity of soil
influenced by organic matter and clay contents
-residual lime available for reaction
Nitrogen source and rhizosphere pH
- N source (NH4+ or NO3-)influences the release by roots of H+ or HCO3- to maintain electrical neutrality
- Neutral or alkaline soils
- Acid soils
Nitrogen source and rhizosphere pH: neutral or alkaline soils
ammonium (through acidification) enhances the solubility of Ca and other ions that are precipitated in the alkaline soil
Nitrogen source and rhizosphere pH: neutral or alkaline soils
ammonium (through acidification) enhances the solubility of Ca and other ions that are precipitated in the alkaline soil
Nitrogen source and rhizosphere pH: acid soils
- ammonium usually does improve nutrient uptake
- Nitrate can increase pH in the rhizosphere and increase P uptake from acid soils
Legumes and rhizosphere pH
- N requirements met with N2
- Nitrate not being taken up so more cations enter roots than anions
- Net excretion of H+
- Can influence uptake of other elements due to the acidification in the rhizosphere
Nutritional status of plants and rhizosphere pH: Micronutrient deficiencies
- Zn in cotton results in acidification in the rhizosphere
- Fe deficiency in non-grass plants
- Net release of H+ acidifies soil to increase solubility of these micronutrients
Acidification of rhizosphere also observed in
P deficient dicots
Micronutrient deficiencies: Net release of H+ acidifies soil to increase solubility of these micronutrients
- acidification is confined to apical root zone
- adaptation to improve growth
Redox potential
- redox potential changes associated with water logging and changes in nutrient availability
redox potential:
Plants adapted to water-logging main ______ in the rhizosphere by the transport of _____ from the shoot to the roots where it is released into the rhizosphere. It decreases________________________
high redox potentials
O2
phytotoxic effects of Mn and Fe
Aerated soils
- zones of anaerobic microsites occur even in well aerated soils
- such sites are more abundant in rhizosphere
- denitrification occurs under anaerobic conditions (conversions of nitrate to N gas)
- Compacted soils will have increase in anaerobic microsites
* Planted soils more likely to be compacted
* Potentially greater rates of denitrification
