Trace element biogeochemistry

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Ph.D Winter School, 16 February 2016, Piacenza

Trace element biogeochemistry

in the rhizosphere

Why bother with plant-mediated

physical-chemical processes?

Matthieu Bravin

Recycling and Risk group

La Réunion, France

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2

Introduction

Ph.D School Piacenza

Réunion: location and main interests

…

Doelsch et al. 2010 Geoderma, 155, 390-400

www.reunion.fr

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3

Introduction

Réunion: but also an open-sky laboratory

reunionnais.n et

www.terdav.com Cirad

lenergeek.com

 Pedological and climatic diversity

 Agronomic diversity

 High pedogeochemical background Ni, Cr, Cu, and Zn

 Space limitation to recycle organic wastes



Soil contamination

 Low Cu and Zn in forage



Cow diet deficiency

 French and EU regulations



Environmental protection

0.5 m/y

1.2 m/y

4 m/y 5 m/y

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4

Introduction

Trace element biogeochemistry

a sound issue

 Soil fertility



Plant micronutrients

 Crop biofortification



Human micronutrients

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5

Introduction

Phytoavailability concept

Root Solution Solid Phase

-TE

Organic Matters

-TE

Oxy-hydroxides

-TE

Clays

-TE

Precipitates

TE-DOM

TE-L_inorg

Uptake

TE

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6

Introduction

Objectives

Summary

 Relevance of rhizosphere chemistry

for trace element phytoavailability

 Range of plant-mediated physical-chemical processes

 Experimental and analytical tools

 Part I. Case study 1: Cu phytotoxicity

 Part II. Rhizosphere basics & techniques

 Part III. Case study 2: As phytostabilization

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7

Part I. Cu Phytotoxicity

Cu contamination in former vineyard soils

Top

Soil

Deeper

Layer

Vine

Vitis sp.

Cu Cu Cu Cu Cu Cu >1000 mg kg-1

Cu

Bordeaux Mixture Cu Cu Cu Cu Cu Cu 5-30 mg kg-1 Durum wheat

Triticum turgidum durum

80 000 ha Replaced by Annual crops

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8

Part I. Cu Phytotoxicity

Cu toxicity to wheat in calcareous soils

(Michaud et al. 2007, Plant Soil)

Cu toxicity to wheat



Fe chlorosis

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9

Part I. Cu Phytotoxicity

Toxicity occurrence

≠ Bulk-soil chemistry

Soil solution pH Cu2+ _{activity, pCu}2+ 6 7 8 9 5 6 7 8

 Bulk-soil chemistry predicts

a much higher Cu toxicity

in strongly acidic soils

than in calcareous soils

Strongly acidic soils Calcareous soils High Low

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10

Part I. Cu Phytotoxicity

Field: Cu phytoavailability not related

to bulk-soil pH

0 60 120 180 240 4 5 6 7 8 Total Cu in roots, mg kg-1 Bulk-soil pH 50 < Cu  100 100 < Cu < 200 Total Cu in soil, mg kg-1 Cu  50 Michaud et al. 2007

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Part I. Cu Phytotoxicity

Field: Rhizosphere alkalization decreases

Cu availability in strongly acidic soils

pH (Rhizosphere minus Bulk soil)

R2 _{= 0,82}*** n = 40 Bulk soil pH 4 5 6 7 8 -0.5 0 0.5 1 1.5 2 Cu-CaCl₂, mg dm-3 0 0.1 0.2 0.3 0.4 4 5 6 7 8 Bulk Soil Rhizosphere

Bulk soil or Rhizosphere pH

Michaud et al. 2007 Plant and Soil, 298, 99-111

Rhizosphere alkalisation

as a function of bulk soil pH

Drastic decrease

in Cu in solution

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Part I. Cu Phytotoxicity

Lab: Rhizosphere alkalization decreases

Cu phytoavailability

NO₃- _Rhizosphere

Alkalization

 Root

exposure to Cu

NH₄+ Rhizosphere Bulk soil 5 6 7 8 9 10 3 4 5 6 7 8 Cu2+ _{activity, pCu}2+ pH 0 50 100 150 200 250 NH₄+

Plant uptake Cu flux, pg Cu m-2 _s-1

*

NO₃

-Alkalization

 Cu

phytoavailability

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Part I. Cu Phytotoxicity

Rhizosphere effect in acidic soils

Solution Solide Phase

Large

Availability

Bulk Soil

Initial state

Cu

2+

Cu

pH 4-5

Strongly acidic soils pH<5

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Part I. Cu Phytotoxicity

Rhizosphere effect in acidic soils

Root Solution Solide Phase

Final state

pH 7-7.5

Cu

2+

Cu



pH



Uptake

Low

Phytoavailability

Rhizosphere

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Part I. Cu Phytotoxicity

Field: Cu-induced Fe deficiency

1 cm

Root Fe concentration, mg kg-1

Root Cu concentration, mg kg-1

Non calcareous soils Calcareous soils Calcareous soils with Fe chlorosis

Michaud et al. 2007 Plant and Soil, 298, 99-111

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Part I. Cu Phytotoxicity

Phytosiderophores release in calcareous

soils

0 0.25 0.5 0.75 1 - Fe + Fe

*

Phy to siderophore exud atio n , µmol Cu g -1 dry root Michaud 2007 PhD thesis

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Part I. Cu Phytotoxicity

Rhizosphere effect in calcareous soils

Solution Solid Phase

Low

Availability

Bulk Soil

Initial state

pH ≈ 8

Cu2+

Cu

Calcareous soils pH

8

Cu phytotoxicity

Fe3+

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Part I. Cu Phytotoxicity

Rhizosphere effect in calcareous soils

Final state

pH ≈ 8

Cu

2+

Cu

Rhizosphere



Phytosiderophores

Fe3+

X

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Part I. Cu Phytotoxicity

Take-home message

 Rhizosphere effect can be so strong

that rhizosphere chemistry can contradict bulk-soil chemistry

 A given plant species can manipulate its rhizosphere

in very different ways depending on soil conditions

 Evidence rhizosphere effects under realistic conditions

and study them in depth under controlled conditions

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Part II. Rhizosphere

Basics & Techniques

Rhizosphere effects

-TE

Organic Matters

-TE

Oxy-hydroxides

-TE

Clays

-TE

Precipitates

TE-DOM

TE-L_inorg

Uptake

TE

According to Hinsinger et al. 2005 New Phytologist, 168, 293-303

[TE]

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Part II. Rhizosphere

Basics & Techniques

Studying the soil-root interface

TE TE TE TE TE TE TE TE TE TE TE TE TE

Soil-Root Interface

-TE

-TE Transfert

From natural to controlled conditions Polyamide mesh 30 µm Nutritive solution Root mat Soil Vertical positionning Horizontal positionning Nutritive solution Soil layer (~ mm thick) Polyamide mesh, 30 µm Root mat

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Part II. Rhizosphere

Basics & Techniques

Cu depletion in wheat rhizosphere

Bravin et al. 2009 Environmental Science & Technology, 43, 5686-5691

0 3 6 9 12

Distance from roots, mm pCu, soil solution

Roots

5.75

6

6.25

6.5

Bulk soil pCu = 5.8

Modelling Cu depletion

by considering only

root uptake

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Part II. Rhizosphere

Basics & Techniques

pH gradient drives Cu depletion

0 3 6 9 12

Distance from roots, mm pCu, soil solution

Roots

5.75

6

6.25

6.5

Bulk soil pCu = 5.8

Bravin et al. 2009 Environmental Science & Technology, 43, 5686-5691

Rhizosphere alkalisation

by +2.8 pH units

pH 4 5 6 7 8 0 3 6 9 12

Distance from roots, mm

Bulk soil pH = 4.7

Cu depletion

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Part II. Rhizosphere

Basics & Techniques

pH: Principle of electrical neutrality

In root cells = Solution

Uptake

C

+

_C

₊

C

+

C

+

C

+

C

+

A

-A

-C

+

A

-Root uptake >

_A

-C

+ Root uptake <

_A

-C

+

C

+

A

-A

-

Uptake

pH



H

+

pH



OH

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Part II. Rhizosphere

Basics & Techniques

pH: Nitrogen forms vs. Metabolic drivers

Bulk soil pH

Tobacco – 100 % NO

₃

Loosemore et al. 2004 Plant and Soil

260, 19-32 Bulk soil pH

Rape

– 100 % NO

₃ 3 4 5 6 7 8 Chaignon et al. 2009 Environmental Pollution 157, 3363-3369

Only

alkali-zation

Both

alkalization

and

acidification

Only

acidification

3 4 5 6 7 8 3 4 5 6 7 8 R h iz o sp h er e p H Bulk soil pH

Wheat

– 100 % NO

₃ 3 4 5 6 7 8 3 4 5 6 7 8 Bravin et al. 2009 Plant and Soil 318, 257-268

Acidification

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Part II. Rhizosphere

Basics & Techniques

pH: Soil effects

Fescue

– 100 % NO

₃

Tomato

– 100 % NO

₃

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Part II. Rhizosphere

Basics & Techniques

DOM: Underlying mechanisms

TE-DOM

Uptake

TE

Exudation

Phytosiderophores Organic anions Phenolics Amino acids

Neumann and Römheld 2007 In: Pinton R, Varanini Z, Nannipieri P (eds)

Kuzyakov 2002 J. Plant Nutr. Soil Sci. 165, 382-396

Exudation

Priming effect CO₂

-TE

Organic Matters

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Part II. Rhizosphere

Basics & Techniques

DOM: Concentration change

Rhizosphere * * * * * * * 0 15 30 45 60 Bulk soil DOC, mg C dm-3 4.8 5.3 5.6 5.9 6.4 6.9 6.9 7.5 Bulk soil pH

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Part II. Rhizosphere

Basics & Techniques

DOM: Impact on metal availability

Bravin et al. 2012 Geochim. Cosmochim. Acta 84, 256-268

4.7 5.4 5.7 5.8 6.4 6.8 7.0 7.4 Bulk Soil pH 0 20 40 60 80 Bulk Soil Rhizosphere DGT Cu-Flux, ng m-2 _s-1 7.4 6.9 7.0 7.1 6.9 7.1 7.2 7.2 Rhizosphere pH

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Part II. Rhizosphere

Basics & Techniques

DOM: Change in metal binding properties

Bulk soil pH = 7.4 Total Cu = 0.5 µM 0 10 20 30 40 50 -1 -0.8 -0.6 -0.4 -0.2 0 0.2

% [Cu] Total Solution

Deposition Potential (Edep), V

Cu2+ + Cu-Linorg Cu-DOM Rhizosphere pH = 7.2 Total Cu = 0.6 µM

DOM Reactivity

Bravin et al. 2012

Geochim. Cosmochim. Acta 84, 256-268

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Part II. Rhizosphere

Basics & Techniques

DOM: Change in metal binding properties

5 6 7 8 9 10 11 5 6 7 8 9 10 11 pCu2+ _modelled pCu2+ _!easured Fulvic Ac. Reactivity 42 %

Bulk Soil Solution

Rhizosphere Solution

Fulvic Ac. Reactivity

27 %

Bravin et al. 2012

Geochim. Cosmochim. Acta 84, 256-268

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Part II. Rhizosphere

Basics & Techniques

DOM: Generalize quantitative & qualitative

changes

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Part II. Rhizosphere

Basics & Techniques

Redox: Reduction in dicot rhizosphere

-TE

Fe oxy-hydroxides dissolution

Uptake

TE

Reductase activity

Oxic conditions

Respiration 



pO

₂

-TE

Redox sensitive TE e.g. Cu

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Part II. Rhizosphere

Basics & Techniques

Redox: Reduction in dicot rhizosphere

Soil redox potential, mV

Soil-root contact time, d

Cornu et al. 2007 Plant Soil 292, 63-77

Bulk-soil Rhizosphere

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Part II. Rhizosphere

Basics & Techniques

Redox: Oxidation in reduced soils

Bulk soil Fe(II) Fe(II) Fe(II) Fe(II) Fe(II) Rice root Begg et al. 1994 New Phytol 128, 469-477

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Part II. Rhizosphere

Basics & Techniques

Redox: Oxidation in reduced soils

Bulk soil Rhizosphere Atmospheric O₂ O₂ O2 O₂ O₂ O₂ O₂ O₂ O₂ Fe(II) Fe (III) O₂ O₂ Rice root Begg et al. 1994 New Phytol 128, 469-477 Fe(II) Fe(II)

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Part II. Rhizosphere

Basics & Techniques

Redox: Oxidation in reduced soils

Bulk soil Rhizosphere Atmospheric O₂ O₂ O2 O₂ O₂ O₂ O₂ O₂ O₂ Fe(II) Fe (III) O₂ As (III) _mobile As (V) strongly adsorbed Rice root Begg et al. 1994 New Phytol 128, 469-477 Liu et al. 2006 ES&T 40, 5730-5736

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Part II. Rhizosphere

Basics & Techniques

Redox: Oxidation in reduced soils

Soil redox potential, mV

Soil-root contact time, d

Control

roots coatingsRoots Controlroots coatingsRoots

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Part II. Rhizosphere

Basics & Techniques

Root system:

Limitation of root mat approaches

Polyamide mesh 30 µm Nutritive solution Root mat Soil

 Average rhizosphere effect

 Geometry issue

 Coupled with invasive and/or

destructive analytical tools

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Part II. Rhizosphere

Basics & Techniques

Root system: pH effect

Larsen et al. 2015 Plant Soil DOI 10.1007/s11104-015-2382-z

Blossfeld et al. 2010 Plant Soil 330, 173-184

mm

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Part II. Rhizosphere

Basics & Techniques

Root system: Redox effect

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Part II. Rhizosphere

Basics & Techniques

Root system: DOM effect

Days after sowing

Dessureault-Rompré et al. 2006 Plant Soil 286, 99-107 Dessureault-Rompré et al. 2008 ES&T 42, 7146-7151

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Part II. Rhizosphere

Basics & Techniques

Field scale: Sampling in 10-y field trial

 2 species

• Tomato = dicot

• Fescue = monocot

 3 fertilizations

• Mineral

• Pig slurry compost

• Poultry litter compost

 Soil sampling

• Bulk soil

• Rhizosphere

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Part II. Rhizosphere

Basics & Techniques

Field scale: Analyses

 Soil solution extraction

• 1:10 soil:solution ratio

• 0.2 µm filtration

 Measurements

• pH

• DOM concentration

• Major cations and anions

• Total metals

• Free Cu

2+

 Modelling

• Estimation of DOM quality

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Part II. Rhizosphere

Basics & Techniques

Field scale: Usual soil solution properties

R h iz o sp h er e pH Fescue Tomato 5 6 7 8 9 5 6 7 8 9 DOM, mg l-1 0 10 20 30 40 50 0 10 20 30 40 50 Bulk soil 6 6.5 7 7.5 8 6 6.5 7 7.5 8

Total Cu, pCu

Bulk soil Bulk soil

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Part II. Rhizosphere

Basics & Techniques

Field scale: DOM qualitative aspects

Rhizosph

ere

Bulk soil

DOM reactivity, %FA

Fescue Tomato 0 100 200 300 400 0 100 200 300 400

Djae et al. 2015 13th _ICOBTE

Bulk soil Tomato Rhizosphere Fescue Rhizosphere Int ensit y mg -1 C Humic Fulvic or Proteins

3D DOM Fluorescence

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Part II. Rhizosphere

Basics & Techniques

Field scale: Determining Cu speciation

Djae et al. 2015 13th _ICOBTE

Rhizosph

ere

Bulk soil

Free Cu activity, pCu

2+

9

10

11

12

13

9

10

11

12

13

Fescue Tomato 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5  pCu 2+ (Rz – Bs) estimat ed pCu2+ _(Rz – Bs) measured

R

2

_{= 0.70}

***



pCu

2+

_{= 0.8}



_{pH + 1.3}



_log

10

[%FA]

+ 1.5



pCu + 1.5



log

₁₀

[MOD]

+ 0.2

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Part II. Rhizosphere

Basics & Techniques

Take-home message

 Chemical changes can be very different

depending on plant species (cultivars) and soil properties

 Coupled as many analytical techniques as you can

to unravel chemical processes involved

 Check the realism of soil-root approaches

by multiplying the treatments tested

by scaling-up your approach to the whole root system

under controlled and field conditions

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Part III. As phytostabilization

As-rich, calcareous soils

around the Au-mining area of Salsigne

Obeidy et al. 2016 Ecotox. Environ. Safe. 126, 23-29

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Part III. As phytostabilization

As dynamic in the rhizosphere

lessons from the literature

-As

Organic Matters

-As

Fe, Al oxy-hydroxides

-As

Clays

Uptake

As(V)

[As] Distance to root

Oxic conditions

?

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Part III. As phytostabilization

Microcosm experiment and analyses

 5 sampling dates

• pH

• DOM

• Ca, Mg, NO

₃

, SO

_4,

Fe

• As

 5 plant species + 1 control soil

 Plant growing during 90 days

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Part III. As phytostabilization

Exchangeable As  but As in solution 

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Part III. As phytostabilization

Fe-As co-mobilization from soil solid-phase

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Part III. As phytostabilization

As dynamic in the rhizosphere

lessons from the literature

-As

Organic Matters

-As

Clays

Uptake

As(V)

Specific exudation

Oxic conditions

Organic

ligand

Anion exchange

Fe

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Part III. As phytostabilization

Ca uptake



 As in solution

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Part III. As phytostabilization

As dynamic in the rhizosphere

lessons from the literature

-As

Organic Matters

-As

Clays

Uptake

As(V)

Uptake

Oxic conditions

Ca

Charge screening

Mg

SO

₄

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Part III. As phytostabilization

pH effect

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Part III. As phytostabilization

As dynamic in the rhizosphere

lessons from the literature

-As

Organic Matters

-As

Clays

Uptake

As(V)

OH

-

_excretion

Oxic conditions

pH

Anion exchange

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Part III. As phytostabilization

Similar As dynamic for the 4 species

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Part III. As phytostabilization

As mass-balance unravels As dynamic

  exchangeable As

was due to

• 5% As  in solution

• 35% As plant uptake

• 60% As sorbed

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Part III. As phytostabilization

Take-home message

 The dynamic of a targeted trace element

is often link with the dynamic of many others

 Plants often mediated several chemical processes

concomitantly

 Calculate the mass-balance of your targeted element

when you experimental system is closed

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Acknowledgements

Colleagues and students

More information



Matthieu.bravin@cirad.fr



http://agents.cirad.fr/index.php/Matthieu+BRAVIN/Publication_list



http://rhizotest.cirad.fr/en

Ph.D School Piacenza