Guillaume LE MER guillaume.le_mer@etu.upmc.fr

N. Bottinelli MF. DIGNAC P. JOUQUET

Y. CAPOWIEZ S. SAMMARTINO

L. CANER A. MAZURIER

O. ROZENBAUM C. RUMPEL

Investigate pore and organic matter spatial organization in earthworm casts through X-Ray microtomography

Organic Mineral Biological Organic matter Mineral particles

Earthworms

+

Earthworm casts climate

SOIL : endogeic system (biogeochemical interface)

+

CONTEXT .

In the actual context of climate change and it’s mitigation, the investigation of (i) the spatial organization and (ii) the internal structure temporal evolution of earthworm casts can provide useful tools and knowledge to reduce massive losses of C from soils.

INTRODUCTION

may reduce OM degradability by protecting OM from mineralization through : • chemical recalcitrance • organomineral interactions • micro-organisms selective filter (formation in

earthworm gut) • particulate organic matter and porosity organization

(OM accessibility and physical protection)

Micro- organisms

(Smith et al., 2015 ; Barthod et al., 2016, 2020 ; Angst et al. 2017, 2019 ; Frouz, 2018 ; Le Mer et al., 2020)

RESEARCH LEVERS . INTRODUCTION

• Quantitative analytical methods are developing rapidly in the field of soil chemistry,

• While new soil physics advanced technologies are often used to provide only pretty images without quantitative functions.

• A recent study tried to characterize and discriminate earthworm cast structures (using microtomography), but without any segmentation between porosity and particulate organic matter (POM).

• To the best of our knowledge, no study has been conducted on the 3D characterization of pore and POM networks in cast of different earthworm species at different dates.

Spatial heterogeneity may impact OM accessibility for decomposers and micro-organisms.

OM

deg

rada

tion

MatrixPorePOM

(Lin, 2012)

(Le Bayon et al., 2020)

(Nunan et al., 2003)

OBJECTIVE . INTRODUCTION

➜ Provide a detailed method to investigate the pore and POM networks in bio-aggregates (cast) produced by different earthworm species.

Objective

➜ Link cast physical organization and CO2 emissions (OM mineralization), measured during 140 days, on casts of 6 earthworm species.

Next step

EARTHWORM FUNCTIONAL DIVERSITY

+ +Organic Matter Mineral particles Microorganisms

Spatial organization variability of earthworm bio-aggregates ?

+- Contrasting impacts on ecosystem services : carbon sequestration, mineralization, water retention,…

PREPARATION STEPS . IMAGE ANALYSIS

Non-Local Mean Filter

SNN Filter

Unsharp Masking Filter

Mask

Histogram

SNN Filter

Unsharp Masking Filter

Mask

Histogram

Differentiated procedure to highlight (i) dark voxels (for pore segmentation) and (ii) dark grey voxels (for POM segmentation) : the method (used filters, parameters and spectrum derivative orders) will be available in our next publication.

Final Pores Images

Raw µCT (9.5 µm voxel)

Final POM Images

(i)

(ii)

OUTPUTS . IMAGE ANALYSIS

➜ Our differentiated approach allows us to produce several pore and POM layers for cast characterization :

Cast Total Porosity Connected Porosity Non-Connected Porosity Internal Porosity External Porosity

Total POM Connected POM Non-Connected POM Internal POM External POM

➜ We measured several parameters such as (i) the pore and POM relative abundance in casts (volume %), (ii) the connectivity of pore and POM and (iii) the distribution of porosity and POM between the internal and the external part of the bio-aggregates.

HETEROGENEOUS PORE DISTRIBUTION . RESULTS

Y pr

ojec

tion

(cm

)

X projection (cm)

2D distribution of pore abundance in a 7 days old cast of Aporectodea nocturna

(pores > 0.0001 mm3)

➜ On our ‘total porosity’ layer, we saved pore coordinates in order to summarize 3D informations and plot pore distribution :

Earthworms may affect the porosity in cast : on this example, there seems to be a polarization of pore repartition between the left and the right side of the cast (maybe due to earthworm cast excretion ?).

➜ X projection was set as the longest cast diameter

PORE AND POM CONTRIBUTION .

E. fe

tida

L. ca

stane

us

A. ca

ligino

sa

A. ic

teric

a

A. no

cturn

a

L. te

rrestr

is

7 days old

42 days old

140 days old

PORE

REL

ATIV

E VO

LUM

E (%

)PO

M R

ELAT

IVE

VOLU

ME

(%)

➜ On ours 'total porosity' and ‘total POM' layers, we measured the total volume contribution :•For most species (except L. terrestris and L. castaneus), the pore contribution seems to increase during the 140 days of incubation, with different dynamics between species.

•Conversely, POM contribution seems to decrease through time.

RESULTS

Earthworm species seem to impact in contrasting ways pore and POM abundance in casts. Our next publication will highlight these effects on the (i) spatial distribution and (ii) connectivity in casts.

If you’re interested about our research : Do not hesitate to contact me or to keep you informed about the

next release of our article (Le Mer et al., 2020)

guillaume.le_mer@etu.upmc.fr

Highlights

• 3D imaging and quantification of particulate organic matter and porosity spatial organization

• Proposed method captures the complexity and diversity of pores and organic-matter distribution in earthworm casts

• Overview of several earthworm casts and parameters evolution during the OM consumption by micro-organisms

Next to come

• Link cast physicochemical parameters (thermal stability, pH, OC, N, MIRS signature, Pore and POM spatial

organization) to measured CO2 emissions

• Propose a model to predict measured CO2 emissions with other parameters (such as POM and pores connectivity).

PERSPECTIVES

REFERENCESAngst, G., Mueller, C.W., Prater, I., Angst, Š., Peterse, F., Nierop, K.G.J., 2019. Earthworms act as biochemical reactors to convert labile plant compounds into stabilized soil microbial necromass. Nature Communications Biology 1–7.

Angst, Š., Mueller, C.W., Cajthaml, T., Angst, G., Lhotáková, Z., Bartuška, M., Špaldoňová, A., Frouz, J., 2017. Stabilization of soil organic matter by earthworms is connected with physical protection rather than with chemical changes of organic matter. Geoderma 289, 29–35.

Barthod, J., Rumpel, C., Paradelo, R., Dignac, M.F., 2016. The effects of worms, clay and biochar on CO2 emissions during production and soil application of co-composts. Soil 2, 673–683.

Barthod, J., Dignac, M.-F., Le Mer, G., Bottinelli, N., Watteau, F., Kögel-Knabner, I., Rumpel, C., 2020. How do earthworms affect organic matter decomposition in the presence of clay-sized minerals? Soil Biology and Biochemistry 107730.

Frouz, J., 2018. Effects of soil macro- and mesofauna on litter decomposition and soil organic matter stabilization. Geoderma 332, 161–172.

Le Bayon, R.C., Guenat, C., Schlaepfer, R., Fischer, F., Luiset, A., Schomburg, A., Turberg, P., 2020. Use of X-ray microcomputed tomography for characterizing earthworm-derived belowground soil aggregates. European Journal of Soil Science 1–15.

Le Mer, G., Barthod, J., Dignac, M.-F., Barré, P., Baudin, F., Rumpel, C., 2020. Inferring the impact of earthworms on the stability of organo-mineral associations, by Rock-Eval thermal analysis and 13C NMR spectroscopy. Organic Geochemistry 144, 104016,.

Lin, H., in Hydropedology, 2012, https://doi.org/10.1016/C2009-0-30647-9

Nunan, N., Wu, K., Young, I.M., Crawford, J.W., Ritz, K., 2003. Spatial distribution of bacterial communities and their relationships with the micro-architecture of soil. FEMS Microbiology Ecology 44, 203–215.

Smith, P., Cotrufo, M.F., Rumpel, C., Paustian, K., Kuikman, P.J., Elliott, J.A., McDowell, R., Griffiths, R.I., Asakawa, S., Bustamante, M., House, J.I., Sobocká, J., Harper, R., Pan, G., West, P.C., Gerber, J.S., Clark, J.M., Adhya, T., Scholes, R.J., Scholes, M.C., 2015. Biogeochemical cycles and biodiversity as key drivers of ecosystem services provided by soils. Soil 1, 665–685.

https://doi.org/10.1016/C2009-0-30647-9