Introduction

• In soils and sediments there is a strong coupling betweenlocal biogeochemical processes and the distribution ofwater, electron acceptors, and acids

• Soil structures such as aggregates, roots, layers, andmacropores occurring in natural soils enhance thepatchiness of these distribution

• The root - soil interface is of specialinterest because urvival of theimportance of plants’ functioning

Roots performance itself is a function ofage and local conditions such as watercontent, soil pH and oxygen availabilityin soil (Hinsinger et al., 2009)

Root-mediated interactions are directlylinked to the local dynamics of water, pHand oxygen which are vital parametersfor living organisms

Root-mediated interactions happen in a small distance(less than cm) from the roots but the spatial distributionand temporal dynamics are difficult to access

Non-destructive imaging methods such as fluorescence

and neutron imaging have provided a uniqueopportunity to unravel some of these complex processes

Our non-invasive combined imaging technique can

directly quantity distribution and changes of watercontent, pH and oxygen

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References:

Carminati, A.; Moradi, A.B.; Vetterlein, D.; Vontobel, P.; Lehmann, E.; Weller, U.; Vogel. H.-J.; Oswald, S.E. (2010): Dynamics of Soil Water Content in the Rhizosphere, (332):163-176Hinsinger P, Bengough AG, Vetterlein D, Young IM (2009); Rhizosphere: biophysics, biogeochemistry and ecological relevance, Plant Soil (321): 117-152

Oswald, S.E.; Menon, M.; Carminati, A.; Vontobel, P.; Lehmann, E.; Schulin, R. (2008): Quantitative Imaging of Infiltration, Root Growth, and Root Water Uptake via Neutron Radiography, (7):1035-1047

Schröder, C.R.; Polerecky, L.; Klimant, I. (2007): Time-Resolved pH/pO2 Mapping with Luminescent Hybrid Sensors, (79):60-70Rudolph N, Esser HG, Carminati A, Moradi AB, Hilger A, Kardjilov N, Nagl S, Oswald SE (2012a): Dynamic oxygen mapping in the root zone by fluorescence dye imaging combined with neutron radiography.Soils and Sediments 12, 63-74Rudolph N, Voss S, Moradi AB, Nagl S, Oswald SE (2012b): Spatiotemporal optical mapping of local soil pH changes induced by roots of lupin and soft-rush. Plant Soil, submitted

Plant Soil

VZJ

Anal. Chem.

!Objectives

Gradients of water content, pH and oxygen in soils canbe visualised with high spatial and temporal resolution

• We will help to clarify the formation and architectureof the root - soil interfaces concerning oxygenconsumption (root respiration), pH dynamics (rootexudation), and water distribution (root water uptake)

•

H2OO2

CH3COO-

Figure 1: Root-mediated inter-actions concerningoxygen, pH, water

Source Collimator Object Detector

Figure 6: ICON,Neutron facility at PSI

F0 Fluorescence signal at highest pH

F Fluorescence signal at varying pHA1 Empirical value of initial value

A2 Empirical value of final value

pka‘ Point of inflectionp Width of the curve

Material and methods

→ Fluorescence foil imaging to map dynamics of oxygen and pH:

→ Image calibration:

• pH imaging based on luminescenceproperties of an acid/ base conjugate pairof the pH indicator and the permeationselectivity provided by polymer matrix

• Described by a Boltzman fit (Schröder et al.,2007):

2´

exp1

210

A

p

pKapH

AA

F

F+

÷÷ø

öççè

æ -+

-=

Figure 2: Preparation steps for fluorescence foil imaging Figure 3: Experimental setup

Figure 5: Container with lupine in a quarz sand captured by NR (corrected raw image,centre) and position of the sensor foils. Oxygen map (right) and pH map (left)

Fig. 8: Profile plots (red line) of pH and water content as functionof axial distance from the root centreFig. 7: Time series of pH profile along

the growing tap root as a function ofdistance from branching point

Figure 6: Container with lupine in a quarz sand captured by NR (corrected raw image,left) and corresponding segmented image showing the root structure (right)

400 500 600

0,0

0,5

1,0

rel.

absorban

ce/re

l.tra

nsm

issio

n

Wavelenght [nm]

Absorbance

LP 500 nm

0

1

2

3

4

5

Emission)

rel.

em

issio

n

1. Mixing permeablepolymer and dyeand stirring over night

2. Produce ahomogeneoussensor cocktail

3. Knife-coating sensorcocktail ontosupproting material

4. Attaching sensorfoil to the inner-sideof the container

• Oxygen imaging based on quenching ofspecial dyes (Transition Metal Complexes)

• Upon excitation TMC built a transfer statewhich is sensitive to oxygen

• Described by a modified Stern-Volmer-Eq.:

0 10 20 30 401

1.5

2

2.5

Oxygen partial pressure (kPa)

I0/I

( )1

2

0 1][1

-

úû

ùêë

é-+

+= f

OK

f

I

I

SV

I0 Fluorescence signal in oxygen free water

I Fluorescence signalf Quenchable fraction of the dye

KSV Stern-Volmer constant

→ Neutron radiography to map water content distribution:

• The Beer-Lambert law describes the attenuation of neutrons:

where is the detected neutron flux after an incident neutron flux,, passes material of thickness with a neutron attenuation

coefficient of

I=I *exp(- d)0 S

I

I d0

S

Figure 4: Experimental setup for neutron radiography (left), interaction of neutrons (centre) andsensitivity to atomic numbers compared to x-rays (right), diagrams from Paul Scherrer Institute

Highly resolved imaging at the soil – plant root interface:A combination of fluorescence imaging and neutron radiography

N. Rudolph ; S. Nagl ; E. Lehmann ; S.E. Oswald1 2 3 1

1. Earth and Environmental Science, University Potsdam, Potsdam, Germany 2. Institute of Analytical Chemistry, University Leipzig, Leipzig, Germany 3.Paul Scherrer, Villigen, Switzerland

Results• Three vital parameters (oxygen, pH, water content) can be directly visualized by only one combined imaging experimental setup• Additionally the root structure and the root volume can be measured by segmenting the images

Acknowledegments

We wish to greatfully acknowledge Mr P.Vontobel from Paul Scherrer Institute (PSI), MrA. Darra, Mrs E. Bauer and Mrs T. Greiner,University Potsdam and Dr. A. Moradi, UCDavis, USA, and University Potsdam for theirsupport of the project. It was funded by theDFG-SPP 1315 “Biogeochemical Interfaces inSoils”

→ Dynamics of pH/ H O2

1. Root surface pH is afunction of space and time!

2. Water content has a strong impact on thesphere of influence regarding root exudation

Does rootage and root order play a role concerning exudation behavoir?

Conclusion

→ Dynamics of O / H O2 2

Figure 9: Time series ofwater contentdistribution (upper row)in a container with alayer of finer grainedquartz sand andcorresponding oxygenconcentrations (mg/l,lower row)(rudolph etal., 2012a)

Figure 11: Time series of O2 development (right) and O2 concentration asfunction of axial distance from root centre (left, profil from black line)

Figure 10: Time series of pH dynamics of narrow-leafed lupine (centre) and pH distribution around rootsof different ages as a function of distance from root centre (young vs old left, tap root vs lateral right)

Can roots act as both? Oxygen sink (respiration) andsource (preferential path) depending on bulk soilproperties and aeration?

O2

[mg/l]Day 2, 11pm Day 3, 2 am Day 4, 6 am

Axial distance from root centre [mm]

Water content distribution

pH distribution

So

il p

H

Wa

ter

co

nte

nt

[cm

/ c

m]

33

Source Collimator Object Detector

☺ We showed that dynamics in the rhizosphere are highly variable in space and time:

Young roots tend to acidify their vicinity whereas old roots tend to alkalize it

Root surface pH with most acidic point 0.56 - 3.36 mm behind root tip

Roots enhances oxygen transport under water saturated condition because they

act as preferential pathways

Root respiration forms oxygen deficits which increased from local, patchy

depletion zone close to the roots to a diffuse pattern over a larger area

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