Naturally occurring Acacia spirorbis in New Caledonia: a top-notch nitrogen fixing species through extremely contrasted soils.

Bryan VINCENT

Philippe JOURAND

Farid JUILLOT

Marc DUCOUSSO

Antoine GALIANA

New Caledonia = Hotspot of biodiversity

A: Dry forests B: Ultramafic ecosystems C: Calcareous islands D: Mangrove

Pacific archipelago 1800 km East of Australia Coral reef

Rehabilitation of post-mining soils

strong polymetallic toxicity = extremes conditions

imported Eucalyptus and Australian Acacias failed to survived

focus on endemic species adapted to such environments

• phyllodenous Acacia species resulting from natural dispersal of Australia

• endemic to New Caledonia

• naturally occurring over contrasted soils

• symbioses with AMF, ECM and rhizobia = often mentioned as helpers in plant adaptation

Acacia spirorbis subsp. spirorbis Labill. (1825)

Rhizobia ECM AMF

What is the symbiotic N2-fixing potential of A. spirorbis ? Is the N2 fixation efficiency impacted by soil properties ?

Study sites

Distribution of the study sites according to their soil properties

Metalliferous soils

Siliceous soils

Calcareous soils

Confirmation of 3 contrasted soil categories hosting natural A. spirorbis populations

Assessing the symbiotic N2 fixation with the natural abundance of 15N

[15N/14N]air = 0.3663 %

14N 15N

14N

15N

14N

15N

14N Inspired from Peoples et al., 1989

SOIL

AIR

No

n-N

2-f

ixin

g sp

eci

es

N2-f

ixin

g le

gum

e

nodules

Formula of δ15N and %Ndfa

• δ15N (‰) = 1000 x %15𝑁

𝑠𝑎𝑚𝑝𝑙𝑒 − 0.3663

0.3663

– If δ15Nlegume close to 0 = similar to air = N2 fixation

– If δ15Nlegume close to δ15Nnon-fix = no N2 fixation

• %Ndfa = 100 x (δ15Nnon−fix − δ15Nlegume)

(δ15Nnon−fix − β)

PM

S P

MS

TIE

SRP

P

MS TI

E PN

2

PN

2

PIN

P

N2

P

MB

SR

P

EAS PM

S TIE

PM

B

PN

2

PIN

V

RT

EAS

PIN

W

AN

EAS

VR

T W

AN

EA

S W

AN

WA

N

PIN

EA

S W

AN

V

RT

-6,00

-4,00

-2,00

0,00

2,00

4,00

6,00

8,00

10,00δ

15N

(‰

)

A. spirorbis

D. viscosa

Melaleuca sp.

M. citrifolia

P. nummularoides

Sannantha sp.

Scaevola sp.

W. indica

Results of δ15N for A. spirorbis and the non-N2-fixing reference species

large range of δ15N for the reference species the δ15N of A. spirorbis is centered around 0 ‰ = similar to atm

Proportion of N fixed by A. spirorbis on metalliferous soils

Site Non-N2-fixing reference species Acacia spirorbis

Name δ15Nnf (‰) δ15Nf (‰) %Ndfa

PN2

Dodonaea viscosa -1.68 ± 0.55

+0.35 ± 0.97

100

Melaleuca quinquenervia -1.25 ± 1.05 100

Sannantha sp. -1.15 ± 0.51 100

PIN

Dodonaea viscosa -1.16 ± 0.72

+0.45 ± 0.79

100

Melaleuca quinquenervia +4.43 ± 0.36 74

Wikstroemia indica +1.46 ± 0.22 42

TIE Melaleuca quinquenervia -2.09 ± 0.07

+0.09 ± 0.09 100

Sannantha sp. -3.28 ± 1.37 100

Proportion of N fixed by A. spirorbis on calcareous soils

Site Non-N2-fixing reference species Acacia spirorbis

Name δ15Nnf (‰) δ15Nf (‰) %Ndfa

EAS

Morinda citrifolia +4.86 ± 1.77

-0.88 ± 0.74

99

Psychotria nummularoides +2.06 ± 0.72 98

Scaevola sp. +2.97 ± 0.59 99

Wikstroemia indica +2.94 ± 1.18 99

WAN

Morinda citrifolia +7.05 ± 1.47

+3.13 ± 0.54

49

Psychotria nummularoides +4.10 ± 0.75 19

Scaevola sp. +2.97 ± 0.14 0

Wikstroemia indica +2.28 ± 1.27 0

Proportion of N fixed by A. spirorbis on siliceous soils

Site Non-N2-fixing reference species Acacia spirorbis

Name δ15Nnf (‰) δ15Nf (‰) %Ndfa

PMB Melaleuca quinquenervia -1.09 ± 0.22 +0.21 ± 0.11 100

PMS

Melaleuca quinquenervia -2.71 ± 1.11

-0.20 ± 0.83

100

Sannantha sp. -4.81 ± 0.42 100

Wikstroemia indica -4.83 ± 0.09 100

SRP Dodonaea viscosa -3.22 ± 0.03 -1.04 ± 0.42 95

VRT Melaleuca quinquenervia +5.17 ± 0.23

+0.50 ± 0.29 76

Wikstroemia indica +2.97 ± 0.95 63

What is the N2-fixing potential of A. spirorbis ? • very high %Ndfa compared to other legumes

o Acacia mangium 50% (Ivory Coast)

o A. melanoxylon 43 % (Australia)

o A. mucronata 48 % (Australia)

o Calliandra calothyrsus 50 % (Australia)

o A. spirorbis 90 % (New Caledonia) = top-notch

Is the N2 fixation affected by contrasted soils ? • very effective on most of the sites = independent from soil N ?

• A. spirorbis = currently tested as a nurse plant in post-mining soils

Nitrogen fixation plays a key role in A. spirorbis adaptation to soils conditions

Thank you ! Question time ?

Study sites characteristics

Soil properties of the study sites

Element contents are expressed in mg.kg-1

Nitrogen contents and δ15N of the soil, reference species and A. spirorbis