umr7618 « biogeochemistry and ecology of continental ecosystems » belowground-aboveground...
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UMR7618 « Biogeochemistry and ecology of continental ecosystems »
Belowground-aboveground interactions
S. Barot
http://millsonia.free.fr/ [email protected]
A bibliographic analysis
2007
To describe the position of soil ecology within ecology
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?
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
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
Results
Soil Biology & Biochemistry quote few generalist / theoretical / evolutionary journals
SBB is hardly cited by generalist / theoretical / evolutionary journals
Results
Few soil papers in generalists/theoretical/evolution 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
Interpretation … not that easy
Relative independence of the development of soil ecology
Soil ecology could benefit from the development of its own theories / using theories developed in general
ecology
► Necessity to develop many tools
► Historical links with agronomy, more applied-sciences
► Close to the abiotic pole of ecology, functional ecology
The publication system is polarized by the gradient particular – general and impact factors… maybe this is not that
relevant
A “new field”
It is very clear that soils are not independent from aboveground
Wardle et al. 2004 Science
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
The study of belowground-aboveground interactions,a field whose time has come!
Goals of this lecture
Give some examples of the impact of aboveground processes on belowground processes
To show that complex feedbacks build up between belowground and aboveground processes
Give some examples of the impact of belowground processes on aboveground processes
To show that these feedbacks may cascade on the whole ecosystems
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?
Impact of aboveground processes on belowground processes
Examples?
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
Treatments
S: Solanum rugosum
A: Arachis pintoi
L : Leucaena leucocephala
B: Brachiaria brizantha
BLAS
TB A LAS BA
BLA B LA
BASSBLLS
L AS BS BLS
X 3 pastures = 3 blocs
16 treatments combining the 1, 2, 3 or 4 species
10 X 10m plots
Sans bétail
Laossi et al. 2008
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?
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?
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
Example of Californian grasslands : results
Higher abundance of nitrifying bacteria with exotic grasses
Higher diversity of nitrifying bacteria
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
Example of Californian grasslands : interpretation
What are the underlying mechanisms?
Exotic grasses partially control microbial community
Through root exudates?
Through the quality of the litter?
What are the consequences for the grassland?For the exotic grasses?
Change in some nutrient fluxes
Increase the capacity of grasses to invade ?
Exotic grasses
N fluxesInvasion
Feedback!!!
Impact of belowground processes on aboveground processes
Examples?
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 experimentAddition 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
Effect of mycorrhizae on adult and seedling
Effect on adult and seedling growth
Mixtures of mycorrhizae have particular effects
In most cases mycorrhizae have a positive effect on adults and seedlings
Different effects on seedlings and adults
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
Effect of mycorrhizae on adult and seedling
Interpretation
Mycorrhizae do help seedling growth through mineral nutrition
Consequences?
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
Influencing seedling growth is likely to impact seedling survival
Effect of microbial diversity
Biodiversity is thought to have a positive influence on ecosystem functioning… this has been demonstrated aboveground especially for plants
A mesocosm experiment with 6 grasses
4 levels of microbial biodievrsity
But what about the influence of soil biodiversity?
How did they obtain these levels?
Mesocosms with monocultures or community of the four plants
Bonkowski 2005
Effect of microbial diversity
Soil respiration increases with microbial biodiversity
Effects on soil functioning
Microbial biomass increases with microbial biodiversity
Specific respiration decreases with microbial biodiversity
Effect of microbial diversity
Ammonium concentration decreases
Nitrate concentration increases in leached water
Effects on soil functioning
Effect of microbial diversity
Microbial biodiversity changes plant biomasses
This effect changes between monocultures and polycultures
Effects on plant communities
Effect of microbial diversity
Microbial biodiversity changes plant biomasses
This effect changes between monocultures and polycultures
Effects on plant communities
Microbial diversity changes the competitive hierarchy between plants
Interpretation
Why does microbial diversity influence soil functioning?
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?
Complementarity?
Effect of microbial diversity
Facilitation?
Why does microbial diversity influence plant competition?
Less abundant species disappear with decreasing biodiversity, but could strongly influence plants through the production of hormones
Complex feedbacks and cascades between aboveground and belowground processes
Examples?
General framework
Plants
Soil organisms / microbes
Mineral nutrient availability
Carbon availability
More direct interactions
Effects on growth,competitive hierarchy,
demography
Nitrogen cycling
A bit of modeling
All fluxes interact
All fluxes depend both on plants and soil microbes
Bacteria Microbes
Boudsocq 2009
Consequences of all these interaction may be predicted using differential equations
Nitrification inhibition by plants
Success of African grasses in South America?
This increases primary production
Indeed nitrification inhibition decreases nitrogen losses
Boudsocq 2009
Nitrate or not nitrate
is the proportion of nitrogen absorbed as ammoniumis 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
Nitrate or not nitrate : influence on competition
Mutual invasion by strategies 1 et 2
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
Herbivores modify the growth and physiology of plants
Impact of herbivores
In turn this changes the quality of the litter, the quantity and quality of root exudates
In turn this should impact soil microbial communities
Two intensities of grazing
Impact of herbivores : a field experiment
Two positions in the watershed
Description of nitrogen fluxes
Description of microbial communities
Patra et al. 2012
Center of France
Impact of herbivores : a field experiment
Different communities (PLFA)
Effect on the quantity of bacteria
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
Consequences
Less nitrate is produced, the ecosystem should lose less nitrogen
The interaction between grass, herbivores and microbes leads to a higher fertility
For the ecosystem?
For the grasses?
The ecosystem should have 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?
Belowground processes may impact the whole aboveground food web
Impact on aboveground food web
The larvae of a fly eat the roots
The larvae of a butterfly eat the leaves
The fly larvae are attacked by a parasite wasp
Impact on aboveground food web
RH = Root herbivoreLH = Leaf herbivore
Choice of the parasitewasp
Impact on aboveground food web
Analyze of the volatile moleculesproduced by the plant
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?
General framework
Plants
Soil organisms / microbes
Mineral nutrient availability
Carbon availability
Aboveground food web
Belowground food web
Conclusion
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!!!
Microcosm study of four species communities
-6
-4
-2
0
2
4
6
8
10
TD PA CG VP
b
-20
-15
-10
-5
0
5
10
15
20
TD PA CG VP
a
% %
Change in the biomass Change in the number of individuals after one generation
Cerastium glomeratum
Veronica persica
Trifolium dubium Poa annua
Lumbricus terrestris
Four annuals
The grass is favored in terms biomass, the legume in terms of the number of individuals Laossi 2009
Microcosm study of four species communities
Cerastium glomeratum
Veronica persica
Trifolium dubium Poa annua
Lumbricus terrestris
Four annuals
Why is it so important to study the impact of belowground processes on plant demography? (survival, fecundity …)
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?