UMR7618 « Biogeochemistry and ecology of
continental ecosystems »
Ecological interactions in soils
S. Barot
http://millsonia.free.fr/
sebastien.barot@ird.fr
What is ecology?
 It is the science that studies interactions between organisms and
between organisms and their environment
Species 1
Abiotic environment
Higher
scales
Species 2
 This means we must study biotic interactions but also interactions
between the biotic and abiotic world
 We must also study purely abiotic interactions
 All these interactions must be studied to predict properties at
higher scales (population, ecosystem, biosphere…)
Species 2
How can we classify ecological interactions?
Biotic interactions
Species 1
Negative
Neutral
Negative
Neutral
Positive
Competition
Amensalism
Predation
Parasitism
Neutralism
Commensalism
Positive
Classical classification in ecology!!!
Symbiosis
Mutualism
How can we classify ecological interactions?
Biotic-abiotic interactions
 Historically ecology has emphasized biotic interactions
… competition is at the heart of the most famous
ecological theories
 “general ecology” has often forgot about interactions with
the abiotic environment
Species
Abiotic environment
Organisms also modify their physico-chemical
environment
 Notion of ecosystem engineer
Goal of this lecture
 To show that all types of ecological interactions occur in
soils
 To show that all these interactions are involved in
well-known soil processes
 To show that biotic-abiotic interactions are particularly
important in soils
Some issues to be discussed at the end of the lecture
Can we identify some important knowledge gaps?
Are some types of ecological interactions unique
in soils?
Can soil ecology throw new lights on general
ecology?
Competition
Examples?
Competition for a limiting resource
Interspecific competition
Intraspecific competition
Species 1
Species 2
Limiting resource
 It is a key biotic interaction in ecology … tightly linked to
many principles of ecology
 Should be very influential for evolution and the structure
of communities
Example 1 : competition between earthworms
Butt 1994
Intraspecific competition
What do they compete for?
What is the limiting factor?
Baker 2002
Interspecific competition
Example 2 : bacteria
Rich and poor mediums to cultivate a community
of bacteria
Nutrient-rich medium
Nutrient-poor medium
Travisiano 2000
 Competition is stronger when resource is poorly available
 This leads to the exclusion of many species
Example 2: bacteria
 Many experimental studies in controlled conditions
But, what is the real impact of competition on the structure
of bacterial communities in soils?
 Cooperation between bacteria is also studied (quorum
sensing, production of exoenzymes …)
 Soil is extremely heterogeneous :
Many micro-habitats
Many types of molecules
Consequences?
 More complementarity, less competition, more bacteria
coexist?
Example 3: a well known case in soils
Competition between plants and soil microorganisms for
mineral nutrients
 Plant mostly absorb mineral nutrients under their mineral
forms
 Bacteria and fungi can exploit these nutrients under both
their mineral and organic forms
 This should strongly influence primary production
Who is the better competitor?
 Large surface-area:volume ratios and rapid growth rates
compared with plant roots
Micro-organisms are better competitors?
Predation
Examples?
Intricate food webs
Are these food webs different from aboveground food webs?
The brown food web
 Aboveground food web is based on the consumption of
the biomass of primary producers (green food web)
 Belowground food web is based on the consumption of
dead organic matter (brown food web)
What are the consequences of predation in soils in terms of
nutrient and carbon cycling?
 Release of CO2
 Release of mineral nutrient
 Stimulates the growth of microbes and mineralization
 Top-down control of microbes by predators : mineral
nutrients are more available to plants
Example of the microbial loop
 Root exudates stimulate the growth of bacteria
 Subsequently this
attracts protozoa that eat
bacteria
 These protozoa release
ammonium
Without predation, plants
would not benefit from
their exudates
Bonkowski
Symbiosis / mutualism
Examples?
Difference between the two?
Famous example 1 : mycorrhizae
 What is exchanged between the
mycorrhizae and the primary
producer?
 The PP gives organic matter
 Mycorrhizae give mineral nutrients
Why are mycorrhizae so efficient at gathering mineral
nutrients?
Famous example 2 : nitrogen fixation
 What is exchanged between the
bacteria and the legumes?
 The legume gives organic matter
 Bacteria give nitrogen
Consequences at higher scales
What is the effect of nitrogen fixation for the ecosystem?
 Nitrogen enrichment
 Nitrogen fixing plants are very important at the beginning
of successions (young soil)
 Young soils tend to be N limited,
while old soils tend to
be P limited
Vitousek
 Cycles between fixer and non-fixers
Consequences at higher scales
 Cycles between fixer and non-fixers
Jenerette 2004 Oikos
War and peace in symbioses
What can we expect about the evolutionary stability of
symbioses?
 Each member of the symbiosis could evolve the
capacity to cheat = to receive more than it gives
 Various hypotheses
The organic matter and minerals that are exchanged
correspond to a luxury consumption
Complex pathways of signalization have evolved to
control the interaction and allow to punish cheaters
Denison, Kiers …
War and peace in symbioses
Why plants have not all evolved symbiotic nitrogen
fixation?
 Very important issue because answering this question
allows understanding why nitrogen so often limits
primary production
 Two complementary hypotheses
Maintaining the symbiosis and feeding the bacteria is too
costly
Nitrogen fixing plants are too vulnerable to cheaters =
non-fixing plants (that do not support the cost of fixation
but indirectly benefit from fixation)
Biotic-abiotic interactions
Examples?
Uniqueness of soil as an habitat/ecosystem
Living in the soil
 For macro-organisms living and moving in the soil requires
modifying the soil
Earthworm galleries
Roots dig their way through the soil
/ bring together soil particles / limit
erosion
Uniqueness of soil as an habitat/ecosystem
Living in the soil
 For micro-organisms living in the soil might also influence soil
structure
Plant fragment
Fungal hypha
Ciment that brings together the agregate
Partially composed of bacteria material
Uniqueness of soil as an habitat/ecosystem
Eating in the soil
 Detritivores are often obliged to eat both organic matter and
mineral material : eat the soil altogether
 Eating organic matter transforms the organic matter
(it is only partially digested)
 Eating organic matter and moving in the soil mix the soil and its
organic matter (bioturbation)
 Feces = new aggregates
are produced
Earthworm cast
Consequences for other organisms
Why could ecosystem engineering be more influential than any
biotic interaction in the soil?
 Ecosystem engineers modifies the physico-chemical environment
that is shared by all organisms
 They indirectly influence all organisms while biotic interactions
tend to be more specific
Species 1
/ group of species
Ecosystem engineer
Species 2
/ group of species
Physico-chemical
characteristics
of the soil
Potentially
all species
are influenced
Consequences for soil ecology
All these arguments suggest that ecosystem engineers are
very influential for soil functioning !!!
Example : earthworm invasion!!!
 European earthworms are invading the north of USA and Canada
Modification of soil properties
Bohlen 2004
What is the
mechanism?
Example : earthworm invasion!!!
Increase in microbial biomass
What is the
mechanism?
Groffman 2004
Example : earthworm invasion!!!
Negative
impact on
plants
What is the
mechanism?
Hale 2006 Ecology
Example : earthworm invasion!!!
Where are the earthworms?
Are all these effects good or bad?
Feedbacks
+
Ecosystem
engineer
Environment
+ Engineers may be impacted by their own engineering activities
What can be the consequences?
 Negative feedbacks : negative density dependence
 Positive feedbacks : positive density dependence
Niche construction !!!
Feedbacks
+
Ecosystem
engineer
Environment
-/+
 This can lead to complicated dynamics :
Equilibrium
Cycles
Chaotic dynamics
Barot 2007 SBB
Conclusion
All types of interactions
 Of course all types of ecological interaction can be encountered
in soils
 Importance of feedbacks between soil organisms and soil
physico-chemical properties
 These interactions even when they involve very small organisms
may have consequences for the whole ecosystem functioning
Soil structure
Soil fertility
Primary production
All types of interactions occurring at the same
time
Interactions between plants and bacteria
 Perfect symbiosis?
PP need decomposers and
decomposers need PP
 But plants and bacteria also
compete for mineral nutrients!!!
 But bacteria also produce many hormone-like molecules
that manipulate plant growth and architecture … this may
have positive and negative effect on plants
How all these interactions have evolved?
All types of interactions occurring at the same
time
Interaction between earthworms and bacteria
 Earthworms stimulate bacteria activities in their guts
These bacteria produce enzymes that help earthworms to digest
more organic matter
Again, a perfect symbiosis?
 But earthworms also digest a part of the bacteria they ingest :
predation
 But earthworms and bacteria somehow also compete for the same
organic matter … the more OM is digested by bacteria the less
food have earthworms …. competition
Taken together, what is the status of the interaction? Evolution?
Some issues to be discussed at the end of the lecture
Can we identify some important knowledge gaps?
Are some types of ecological interactions unique
in soils?
Can soil ecology throw new lights on general
ecology?