Plant microbe interaction (Lecture 18)
PLANT MICROBE INTERACTIONS
-Plant-microbe interactions diverse – from the plant perspective:
- Negative – e.g. parasitic/pathogenic
- Neutral
- Positive – symbiotic
*Positive interaction
*Negative interaction
The most important positive interactions from the standpoint of plant abundance and distribution; related to plant nutrient and water supply-
*Decomposition
*Mycorrhizae
*N2 fixation
*Rhizosphere interactions
- Decomposition
Primary supplier of plant nutrients – particularly N & P
A. Raw material
Soil organic matter derived primarily from plants –
- Mainly leaves and fine roots
- Wood can be important component in old growth forests
B. Processes
- Fragmentation –
- Breakdown of organic matter (OM) into smaller bits = humus
- By soil ‘critters’ – including nematodes, earthworms, springtails, termites
- consume and excrete OM à incomplete digestion
C. N uptake by plants – Chemical form taken up can vary
1) Nitrate (NO3–)
- Preferred by most plants, easier to take up
- Even though requires conversion to NH4+ before be used, this requires a lots of energy
- But taking up & storing NH4+ may be problematic;
- More strongly bound to soil particles
- Acidifies the soil
- Not easily stored
2) Ammonium (NH4+ ) –
- Used directly by plants in soils with low nitrification rates (e.g. wet soils= low O2)
- NO3– easy for plants to take up, but easy to be leached
- Tropical soils high in Fe/Al oxides (+ve charged) will prevent leaching
- NH4+ more easily bound to soil mineral (e.g., clays) and organic matter
Advantages of nitrate over ammonium-containing fertilizers
-Mobile in the soil – direct uptake by the plant, highest efficiency.
-Nitrates synergistically promote the uptake of cations, such as K, Ca and Mg, while ammonium competes for the uptake with these cations.
-Nitrates can be readily absorbed by the plant and do not need to undergo any further conversion, as is the case with urea and ammonium, before plant uptake.
-No acidification of the soil if all the nitrogen is applied as nitrate-nitrogen.
-Nitrates limit the uptake of harmful elements, such as chloride, into large quantities.
-The conversion of nitrates to amino acids occurs in the leaf. This process is fueled by solar energy, which makes it an energy-efficient process.
-Ammonium has to be converted into organic N compounds in the roots. This process is fueled by carbohydrates, which are at the expense of other plant life processes, such as plant growth and fruit fill.
D. Controls on rates of decomposition
1) Temperature – 
- Warmer is better
- <45°C
2) Moisture – intermediate is best
- Too little à desiccation
- Too much à limits O2 diffusion
3) Plant factors (abiotic/biotic factors)– Litter quality
a) Litter C:N ratio (= N concentration)
- if amount of C relative to N is high, then N limits microbial growth (immobilization is favored),
- and supply of N to plants is lower (competition with microbes for N)
b) Plant structural material
- Lignin – complex polymer, cell walls
– Confers strength with flexibility
– Relatively recalcitrant (difficult to break down)
- High conc. – e.g. oak leaves lowers decomposition
c) Plant secondary compounds
– chemicals not used for growth or metabolism various other compounds e.g. tannins
- Control decomposition by:
-Bind to enzymes, blocking active sites à lower mineralization
-N compounds may bind to phenolics à greater immobilization by soil
-Phenolics may be C source for microbes à greater immobilization by microbes
