Download Soil and the Rhizosphere

Bipolar Division
Soil &
Rhizosphere
• Oxygen supply limited by
mineral particle size,
organic matter content and
water content.
• Plant roots may also add
oxygen to deeper soils or
anaerobic soils.
• Water saturation leads to
anaerobic conditions and
increased denitrification.
• Distribution of microbes
depends on organic matter
supply and source (humus
and root exudates)
Oxygen
Gradients
Within soil aggregates
oxygen decreases
toward the interior.
Creates microniches for
fermentative and
anaerobic respiring
bacteria.
Organic matter and
availability of alternate
electron acceptors (e.g.
nitrate, sulfate, carbon
dioxide) will partly
determine which
anaerobic respiring
bacteria thrive where.
Soil Close-Up
Geosmin = “earthy smell”
= Cyanobacteria
= Actinomycetes
Bulk Soil vs
Rhizosphere
• Bulk soil is that away
from the influence of plant
roots (i.e. water, oxygen,
exudates).
• Bulk soil community is
subject to greater
environmental variability.
• Rhizosphere community
has greater microbial
biomass and activity
compared with that in
bulk soil.
Nitrogen Fixing Actinomycetes
Frankia spp.
Nodulate woody
non-leguminous
plants, such as
Alnus spp (alders).
Association is
called actinorhizae.
Crown Gall Disease caused
by Agrobacterium spp.
T-plasmid Function
Mycorrhizae
Fungi
Hoch et al., 1998
Ectomycorrhizal
association between
Douglas Fir
(Pseudotsuga
menziesii) and a
boletus-like mushroom
(Suillus caerulescens).
Hoch et al., 1999
Linking sporocarp to
ectomycorrhizae
• Internal Transcribed
Spacer (ISR) – PCR
• Amplifies the region
between 18S and 28S
rRNA genes in fungi.
• RFLP analysis to
fingerprint and determine a
match (sporocarp to “tip”).
Kbp
Alu I
S1
T1a
Hinf I
T1b
S1
T1a
.
T1b
650
492
369
246
123