Lecture 3: OM and soil organisms

Question 1

OM compositio

Answer 1

active (decomposing matter)
humus (stabilised OM)
fresh residue (OM from debris)
living (soil organisms)

Question 2

decomposition

Answer 2

mineralisation to convert organic to simple inorganic form
immobilisation incorporate C into microbes

Question 3

carbon cycle

Answer 3

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

Question 4

manures and residues depend on

Answer 4

climate
vegetation
land use
timber
rhizosphere activity

Question 5

plant litter

Answer 5

cabrohydrates and proteins which get immobilised by microbes
OA, waxes, structural polymers, chlorophyll, chitin

Question 6

high C low N

Answer 6

immobilisation

Question 7

low C high N

Answer 7

mineralisation

Question 8

soils C:N

Answer 8

low (more mineralisation)

Question 9

microbes in soil food web

Answer 9

basis of this
rhizobia feed off root exudates
organisms consume OM
organisms consume microorganisms (earthworms)

Question 10

mycorrhizal role and evolution

Answer 10

access nutrients for plants

Question 11

ectomycorrhiza

Answer 11

fungal sheath (mantle) envelops the root
hartig net puts hyphae into root BETWEEN cortical cells

Question 12

endomycorrhiza

Answer 12

AM form arbuscules and hyphae penetrate INSIDE cells

Question 13

how do endomycorrhiza find a host

Answer 13

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

Question 14

endomycorrhiza post-appressorium formation

Answer 14

fungal MAMPs recognised so immune response
mycorrhiza invaginates membrane by suppressing local plant defences
branching starts
nutrients transported through interfacial apoplast

Question 15

non-nutritional effects of mycorrhizae

Answer 15

manipulate and manage host ecosystem to protect against disease by priming immune system

Question 16

PGPR soil microbes

Answer 16

indirect: pathogen outcompetition
direct: hormone, antagonist, quorum sensing to improve nutrient uptake and resist pathogens

Question 17

PGPR and AMF together

Answer 17

AMF benefits PGPR colonisation to prime plant immune system

Question 18

PGPR help immune priming

Answer 18

callose deposition
simulate fungal MAMPs so prime responses

Question 19

stages of immune priming in AMF and PGPR

Answer 19

plants signal AMFs via strigalactones
triggers immune response so signals for PGPR
fungal effectors switch off defences and prime to recruit PGPR
trigger immunity

Question 20

issues with agriculture and soil organisms

Answer 20

fertiliser changes microbes
ploughing breaks fungal hyphae
pesticides kill microorganisms
crops evolve to stop using mycorrhiza

Question 21

humification

Answer 21

OM decomposed and stored for long-term

Question 22

humus properties

Answer 22

particles bound to clay-silicate surfaces
store and release N and C
buffer
cation and anion exchange with affinity for ions

Question 23

soil priming

Answer 23

microbes can prime soils to produce C and humus

Question 24

planting trees in humus-rich soils

Answer 24

bad as can mine humus and release CO2 (GHGs)