Diapositive 1

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UMR7618 Biogeochemistry and ecology of
continental ecosystems
Belowground-aboveground interactions S.
Barot http//millsonia.free.fr/
sebastien.barot_at_ird.fr
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A bibliographic analysis
2007
To describe the position of soil ecology within
ecology
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How and why describing the position of soil
ecology within ecology?

• Ecology is an integrative science to study an
  ecosystem we need to study all its biotic and
  abiotic compartments

• At the same time, scientists need to specialize,
  to develop their own tools to get better
  insights on the compartments/organisms/
  processes they study

• Beginning to work in the field of soil ecology I
  have been surprised by the relative isolation
  of the field and some peculiarities

? Difficult to publish soil ecology in generalist
ecology journals?
? Relative lack of modeling/theory in soil
ecology?
? Relative lack of evolutionary thinking in
ecology?
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How and why describing the position of soil
ecology within ecology?

• All our work and partially our thinking is
  organized by the publication system

• Comparing journals and counting/classifying
  publications allows a quantitative analysis of
  the way Ecology is structured and the way we
  think/work

• Comparison of generalist journals,
  theory-oriented journal, evolution-oriented
  journals, soil ecology journals

? Comparison of 18 journals
? About 23000 articles scanned from 1997 to 2004
? Classification of papers related to soil
ecology / using modeling / based on
evolutionary thinking
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Results

• Journals dealing with soil ecology are less
  theory-oriented

• Journals dealing with soil ecology are less
  evolution-oriented

• Journals dealing with soil ecology have lower
  Impact Factors

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Results

• Soil Biology Biochemistry quote few generalist
  / theoretical / evolutionary journals

• SBB is hardly cited by generalist / theoretical
  / evolutionary journals

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Results

• Few soil papers in generalists/theoretical/evolut
  ion journals

• Even within generalist journals, soil ecology
  papers are less model- and evolution-oriented
  than other papers

• It seems that modeling and evolutionary thinking
  help to produce general results and to publish
  in generalist journals

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Interpretation not that easy
Relative independence of the development of soil
ecology
? Necessity to develop many tools
? Historical links with agronomy, more
applied-sciences
? Close to the abiotic pole of ecology,
functional ecology
Soil ecology could benefit from the development
of its own theories / using theories developed in
general ecology
The publication system is polarized by the
gradient particular general and impact
factors maybe this is not that relevant
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A new field

• It is very clear that soils are not
  independent from aboveground

• In my opinion the most important recent
  breakthroughs in soil ecology are coming
  from the study of below- /above-ground
  interactions

• This is an other way to say that the key is
  to better link soil ecology and general ecology

Wardle et al. 2004 Science
The study of belowground-aboveground
interactions,a field whose time has come!
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Goals of this lecture

• Give some examples of the impact of aboveground
  processes on belowground processes

• Give some examples of the impact of belowground
  processes on aboveground processes

• To show that complex feedbacks build up between
  belowground and aboveground processes

• To show that these feedbacks may cascade on the
  whole ecosystems

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Some issues to be discussed at the end of the
lecture
Has soil ecology really developed in isolation
from general ecology?
What are the more striking knowledge gaps in
belowground-above ground interactions?
How can we exploit belowground-above ground
interactions in agriculture, for sustainable food
production?
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Impact of aboveground processes on belowground
processes
Examples?
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Example of Amazonian pastures

• Primary forests are cut down and replaced by
  pastures with a very low biodiversity

• Increasing the plant biodiversity of pastures
  could increase their sustainability?

• Could and increase in plant diversity impact
  positively soil functioning and biodiversity?

An experimental approach focusing on soil
macro-fauna
Laossi et al. 2008
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Treatments
A Arachis pintoi
Sans bétail
BLAS
BLA
B
LA
TB
A
LAS
BA
L Leucaena leucocephala
BAS
S
BL
LS
L
AS
BS
BLS
B Brachiaria brizantha
X 3 pastures 3 blocs
S Solanum rugosum
10 X 10m plots
Laossi et al. 2008
16 treatments combining the 1, 2, 3 or 4 species
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Example of Amazonian pastures results

• No positive effect of plant diversity on the
  diversity or density of soil macrofauna

• Positive effect of total plant biomass and the
  presence of A. pintoï on the diversity of soil
  macrofauna and the density of some groups of
  macrofauna (earthworms)

• Lower soil compaction in presence of A. pintoï

What are the mechanisms?
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Example of Amazonian pastures results

• Plants as an habitat and effect on microclimate

• Plants as a source of food

• Soil macrofauna seems to be more sensitive to
  the quantity and quality (A. pintoï) of food
  than to its diversity Generalists?

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Example of the invasion of Californian grasslands

• Invasive plants are a worldwide problem

• Invasion of Californian grasslands by exotic
  grasses

• What are the consequences for soil microbial
  communities?

• What are the consequences for soil functioning?

An experiment in mesocosms
Hawkes et al. 2006
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Example of Californian grasslands results

• Higher abundance of nitrifying bacteria with
  exotic grasses

• Higher diversity of nitrifying bacteria

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Example of Californian grasslands results

• Exotic grasses increase gross nitrification

• No significant effect of exotic grasses on
  mineralization Significant increase with all
  plants

• Slightly more N immobilized by bacteria in
  presence of exotic grasses

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Example of Californian grasslands interpretation
What are the underlying mechanisms?

• Exotic grasses partially control microbial
  community

Exotic grasses

• Through root exudates?

• Through the quality of the litter?

Invasion
N fluxes
What are the consequences for the grassland?For
the exotic grasses?

• Change in some nutrient fluxes

• Increase the capacity of grasses to invade ?

Feedback!!!
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Impact of belowground processes on aboveground
processes
Examples?
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Effect of mycorrhizae on seedlings

• Mycorrhizae help plant to absorb mineral
  nutrient and especially P

• Plant seedlings have a very small root system
  and could benefit from preexisting hyphal
  network !!!

A mesocosm experiment
Addition of seeds to communities of adult plants
These communities were grown with no
mycorrhizae 1 mycorrhizae taxa (among 4) the
4 taxa
van der Heijden 2004
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Effect of mycorrhizae on adult and seedling
Effect on adult and seedling growth

• In most cases mycorrhizae have a positive
  effect on adults and seedlings

• Mixtures of mycorrhizae have particular
  effects

• Different effects on seedlings and adults

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Effect of mycorrhizae on adult and seedling
Effect on shoot content in P

• Indeed mycorrhizae improve P nutrition

• These positive effect depend on the mycorrhizae
  taxa

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Effect of mycorrhizae on adult and seedling
Interpretation

• Mycorrhizae do help seedling growth through
  mineral nutrition

Consequences?

• Influencing seedling growth is likely to impact
  seedling survival

• Seedlings are very important for plant demography

• If mycorrhizae impact the recruitment of new
  seedlings they are likely to impact the
  structure of plant community presence
  /absence of species et relative abundance of
  species

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Effect of microbial diversity

• Biodiversity is thought to have a positive
  influence on ecosystem functioning this has
  been demonstrated aboveground especially for
  plants

• But what about the influence of soil
  biodiversity?

A mesocosm experiment with 6 grasses

• 4 levels of microbial biodievrsity

How did they obtain these levels?

• Mesocosms with monocultures or community of the
  four plants

Bonkowski 2005
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Effect of microbial diversity
Effects on soil functioning

• Soil respiration increases with microbial
  biodiversity

• Microbial biomass increases with microbial
  biodiversity

• Specific respiration decreases with microbial
  biodiversity

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Effect of microbial diversity
Effects on soil functioning

• Ammonium concentration decreases

• Nitrate concentration increases in leached
  water

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Effect of microbial diversity
Effects on plant communities

• Microbial biodiversity changes plant
  biomasses

• This effect changes between monocultures
  and polycultures

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Effect of microbial diversity
Effects on plant communities

• Microbial biodiversity changes plant
  biomasses

• This effect changes between monocultures
  and polycultures

• Microbial diversity changes the
  competitive hierarchy between plants

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Effect of microbial diversity
Interpretation
Why does microbial diversity influence soil
functioning?

• Complementarity?

• Facilitation?

Why does microbial diversity influence plant
competition?

• Through the effects on nutrient fluxes?
  Plants having different optimums for N, nitrate,
  ammonium availabilities

• Through more specific rhyzospheric interactions
  between microbes and plant species?

• Less abundant species disappear with decreasing
  biodiversity, but could strongly influence
  plants through the production of hormones

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Complex feedbacks and cascades between
aboveground and belowground processes
Examples?
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General framework
Effects on growth, competitive hierarchy,demograp
hy
Plants
Mineral nutrient availability
Carbon availability
Soil organisms / microbes
More direct interactions
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A bit of modeling
Nitrogen cycling

• All fluxes interact

• All fluxes depend both on plants and soil
  microbes

• Consequences of all these interaction may
  be predicted using differential equations

Bacteria
Microbes
Boudsocq 2009
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Nitrification inhibition by plants

• Indeed nitrification inhibition decreases
  nitrogen losses

• This increases primary production

• Success of African grasses in South America?

Boudsocq 2009
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Nitrate or not nitrate
b is the proportion of nitrogen absorbed as
ammonium

• An intermediate preference minimizes losses
  and maximizes primary production

• This supports the idea that plants do not
  only absorb nitrate

Boudsocq 2012
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Nitrate or not nitrate influence on competition

Mutual invasion by strategies b1 et b2
Stable coexistence
2 eliminates 1
1 eliminates 2

• Coexistence is possible for contrasted
  strategies

• The plant that prefers ammonium tends to
  outcompete the other

• Inhibiting or increasing nitrification modifies
  the pattern

Boudsocq 2012
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Impact of herbivores

• Herbivores modify the growth and physiology of
  plants

• In turn this changes the quality of the litter,
  the quantity and quality of root exudates

• In turn this should impact soil microbial
  communities

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Impact of herbivores a field experiment

• Center of France

• Two intensities of grazing

• Two positions in the watershed

• Description of nitrogen fluxes

• Description of microbial communities

Patra et al. 2012
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Impact of herbivores a field experiment

• Different communities (PLFA)

• Effect on the quantity of bacteria

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Impact of herbivores a field experiment

• In turn the modification of microbial
  communities impact soil functioning

• High grazing intensity tends to reduce
  mineralization

• High grazing intensity tends to reduce
  nitrification

• High grazing intensity tends to reduce
  denitrification

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Consequences
For the ecosystem?

• Less nitrate is produced, the ecosystem should
  lose less nitrogen

• The ecosystem should have a higher fertility

For the grasses?

• The interaction between grass, herbivores and
  microbes leads to a higher fertility

• In turn this should favor the regrowth of the
  grasses and even a better primary production

Have these interactions been selected along the
evolution of plants?
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Impact on aboveground food web
Belowground processes may impact the whole
aboveground food web
The larvae of a butterfly eat the leaves
The fly larvae are attacked by a parasite wasp
The larvae of a fly eat the roots
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Impact on aboveground food web
Choice of the parasitewasp
RH Root herbivore
LH Leaf herbivore
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Impact on aboveground food web
Analyze of the volatile moleculesproduced by the
plant
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Impact on aboveground food web
Interpretation
Why is the wasp avoiding plants with root
herbivores?

• Because plants with root herbivores have leaves
  with a lower chemical quality (less N?) ?

• Because plants with root herbivores have leaves
  with a lower chemical quality (less N?) and
  thus butterfly larvae with a lower quality?

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General framework
Aboveground food web
Plants
Mineral nutrient availability
Carbon availability
Soil organisms / microbes
Belowground food web
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Conclusion
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Take home messages

• Plants link together all belowground and
  aboveground processes

• These links between aboveground and belowground
  processes may create complicated feedbacks
  that cascades on large scale properties

Plant invasive capacity
The structure of plant communities
Primary production

• It is important to study the effect of
  belowground organisms on plant demography!!!

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Microcosm study of four species communities
Four annuals
Cerastium glomeratum
Veronica persica
Trifolium dubium
Poa annua
Lumbricus terrestris
Change in the biomass
Change in the number of individuals after one
generation
The grass is favored in terms biomass, the legume
in terms of the number of individuals
Laossi 2009
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Microcosm study of four species communities
Four annuals
Cerastium glomeratum
Veronica persica
Trifolium dubium
Poa annua
Lumbricus terrestris
Why is it so important to study the impact of
belowground processes on plant demography?
(survival, fecundity )
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Some issues to be discussed at the end of the
lecture
Has soil ecology really developed in isolation
from general ecology?
What are the more striking knowledge gaps in
belowground-above ground interactions?
How can we exploit belowground-above ground
interactions in agriculture, for sustainable food
production?