Lecture 3: OM and soil organisms Flashcards
OM compositio
active (decomposing matter)
humus (stabilised OM)
fresh residue (OM from debris)
living (soil organisms)
decomposition
mineralisation to convert organic to simple inorganic form
immobilisation incorporate C into microbes
carbon cycle
plants photosynthesise
animals eat, excrete and die
mineralisation by microorganisms so C back to atmosphere
immobilisation SOM enter microbe pool
microorganisms respire
C immobilised as humus, peat, fossils
manures and residues depend on
climate
vegetation
land use
timber
rhizosphere activity
plant litter
cabrohydrates and proteins which get immobilised by microbes
OA, waxes, structural polymers, chlorophyll, chitin
high C low N
immobilisation
low C high N
mineralisation
soils C:N
low (more mineralisation)
microbes in soil food web
basis of this
rhizobia feed off root exudates
organisms consume OM
organisms consume microorganisms (earthworms)
mycorrhizal role and evolution
access nutrients for plants
ectomycorrhiza
fungal sheath (mantle) envelops the root
hartig net puts hyphae into root BETWEEN cortical cells
endomycorrhiza
AM form arbuscules and hyphae penetrate INSIDE cells
how do endomycorrhiza find a host
spore generates and sends MYC-factors to initiate branching in roots
host plant produces strigalactones which induce hyphal branching
appressorium formed when root and hyphae find eachother
endomycorrhiza post-appressorium formation
fungal MAMPs recognised so immune response
mycorrhiza invaginates membrane by suppressing local plant defences
branching starts
nutrients transported through interfacial apoplast
non-nutritional effects of mycorrhizae
manipulate and manage host ecosystem to protect against disease by priming immune system
PGPR soil microbes
indirect: pathogen outcompetition
direct: hormone, antagonist, quorum sensing to improve nutrient uptake and resist pathogens
PGPR and AMF together
AMF benefits PGPR colonisation to prime plant immune system
PGPR help immune priming
callose deposition
simulate fungal MAMPs so prime responses
stages of immune priming in AMF and PGPR
plants signal AMFs via strigalactones
triggers immune response so signals for PGPR
fungal effectors switch off defences and prime to recruit PGPR
trigger immunity
issues with agriculture and soil organisms
fertiliser changes microbes
ploughing breaks fungal hyphae
pesticides kill microorganisms
crops evolve to stop using mycorrhiza
humification
OM decomposed and stored for long-term
humus properties
particles bound to clay-silicate surfaces
store and release N and C
buffer
cation and anion exchange with affinity for ions
soil priming
microbes can prime soils to produce C and humus
planting trees in humus-rich soils
bad as can mine humus and release CO2 (GHGs)
