UMR7618 « Biogeochemistry and ecology of continental ecosystems » Belowground-aboveground...

transcript

UMR7618 « Biogeochemistry and ecology of continental ecosystems »

Belowground-aboveground interactions

S. Barot

http://millsonia.free.fr/ sebastien.barot@ird.fr

A bibliographic analysis

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

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 on shoot content in P

Indeed mycorrhizae improve P nutrition

These positive effect depend on the mycorrhizae taxa

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

Soil respiration increases with microbial biodiversity

Effects on soil functioning

Microbial biomass increases with microbial biodiversity

Specific respiration decreases with microbial biodiversity

Ammonium concentration decreases

Nitrate concentration increases in leached water

Effects on soil functioning

Microbial biodiversity changes plant biomasses

This effect changes between monocultures and polycultures

Effects on plant communities

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?

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

Different communities (PLFA)

Effect on the quantity of bacteria

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

RH = Root herbivoreLH = Leaf herbivore

Choice of the parasitewasp

Analyze of the volatile moleculesproduced by the plant

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

TD PA CG VP

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?

UMR7618 « Biogeochemistry and ecology of continental ecosystems » Belowground-aboveground...

Documents