Soil phosphorus status and N2 fixation rates of leguminous trees: consequences for estimated rates and transfer in agroforestry systems

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Soil phosphorus status and N2 fixation rates of
leguminous trees consequences for estimated
rates and transfer in agroforestry systems

• Marney E. Isaac, Department of Physical and
  Environmental Sciences,
• University of Toronto, Canada, marney.isaac_at_utoron
  to.ca
• Jean-Michel Harmand, UMR EcoSols, CIRAD,
  Montpellier, France

Phosphorus in Soils and Plants PSP5 2014
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Nutrient use efficiency
Global cereal yield

• Considerable amounts of N and P fertilizers have
  been misused in agroecosystems, with profound
  alteration to biogeochemical cycles.
• Rising interest in alternative systems from
  researchers, development organisations,
  governments, and farmers in recent years as a
  revival of pro-environmental practices.

Nitrogen efficiency use
Tilman et al. 2002 Nature
Year
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Nutrient pathways under intercropping

• To reduce excess fertilizer use, plant-mediated
  nutrient supply through
• N2 fixation
• recycling and transfer of fixed N
• mobilization of soil P
• may be important processes for the nutrient
  economy of low-input intercropped agroecosystems

Woody legume
Annual legume
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What we know

• Benefits to N and P acquisition are observed in
  annual intercrops
• Reported stimulated N2 fixation attributed to low
  soil N (Schipanski et al. 2010)
• Reported stimulated P acquisition for annuals
  intercropped with legumes attributed to
  rhizosphere soil acidification (Li et al. 2007)
• Soil P may limit N2 fixation (Nygren et al. 2012)
• Little information on leguminous tree performance
  and activity in tree-based intercropping systems
  (Munroe and Isaac 2014)

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Here, we ask
How do soil fertility and agroecosystem function
co-vary?

• Specifically
• What is the effect of soil P status on biological
  N2 fixation of leguminous trees?
• Does soil P status stimulate plant to plant
  transfer of fixed N?

?
?
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Model species/systemSemi-arid legume tree
intercropping system

• Acacia senegal, an important leguminous species
  in semi-arid environments, has shown promise as
  an agroforestry tree
• Multi-functional produces gum arabic, fodder and
  firewood as well as replenishes soil fertility
  through N2 fixation and N recycling

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High P 42.5 8.58 mg kg-1 Low P 13.8 6.27
mg kg-1
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The natural abundance method
d15N X
d15N R
d15N B
Ndfa (?15NR-?15Nx)/(?15NR-?15NB) x 100 R
reference plant (0 fixation) X sample plant B
plant in N-free solution (100 fixation)
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In-situ edaphic and ontogenetic effects on N2
fixation rates
d15N Ndfa Ndfa
High P High P High P
Reference (Balanites aegyptiaca) 8.4 1.2 a 8.4 1.2 a
Juvenile Acacia 4.0 0.1 c 4.0 0.1 c 39 1.4 a
Mature Acacia 4.8 0.3 b 4.8 0.3 b 33 2.7 b
Low P Low P Low P
Reference (Balanites aegyptiaca) 8.8 1.2 a 8.8 1.2 a
Juvenile Acacia 8.0 1.1 a 8.0 1.1 a 0
Mature Acacia 8.7 0.8 a 8.7 0.8 a 0
Isaac, ME, Harmand, JM, Lesueur, D., Lelon, J.
2011. Forest Ecology and Management 261 582-588
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Soil P-derived differential N2 fixation effects
on soil N
Potentially mineralizable soil N
Total soil N ()
Low P
High P
Low P
High P
?
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Controlled P addition effectson N2 fixation rates
Step 1 Cultivation
B value (100 fix) R value (0 fix)
Acacia senegal in sand culture with Low (200
µmol of P seedling-1) Mid (400 µmol of P
seedling-1) High (600 µmol of P seedling-1)
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(Residual) P effects on Ndfa
Ndfa
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(Residual) P effects on Ndfa
Ndfa
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Step 2 Intercropping trials
Part a Labeling acacia leaves with 15N urea,
for isotopic dilution method
Part b Intercropping cultivated wheat (Triticum
turgidum durum) with the variable P and labeled
acacia in the pots
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Ntransfer
d15N A
d15N X
d15N R

• Ntrans (?15NR- ?15NX) / (?15NR -?15NA)
• ?15NR refers to values from wheat pre-contact
• ?15NX refers to values from wheat post-contact
• ?15NA refers to values from acacia

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Does P status affect fixed N transfer?
Acacia cultivated under fixed N transferred to wheat
Low P 4.19 4.50
Mid P 1.64 0.10
High P 14.04 8.97

• Strong isotopic evidence of biologically
  significant N transfer
• after 21 days of contact between the two species
  under high P additions.

Isaac, ME, Hinsinger, P, Harmand, JM. 2012.
Science of the Total Environment 434 71-78
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Does P status affect fixed N transfer?
Acacia cultivated under fixed N transferred to wheat Shoot N content (mg individual-1)
Low P 4.19 4.50 71.4 5.0
Mid P 1.64 0.10 74.1 4.5
High P 14.04 8.97 78.4 5.4

• Strong isotopic evidence of biologically
  significant N transfer
• after 21 days of contact between the two species
  under high P additions.

Isaac, ME, Hinsinger, P, Harmand, JM. 2012.
Science of the Total Environment 434 71-78
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How do soil fertility and agroecosystem function
co-vary?
Yes soil resources stimulate agroecosystem
function
No facilitation is present under available
resources
Fixed N transfer
Soil P gradient
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Our data indicates that

• Conceptually
• i) Sufficient P supply, not limited P supply, is
  associated with higher N2 fixation rates and
  facilitation for biologically meaningful N
  transfer
• Methodologically
• ii) Controlled conditions reflect estimated
  trends of Ndfa in the field across soil P
  gradients but the magnitude may differ
• Practically
• iii) Plant-mediated nutrient supply is active and
  potentially reliable in low-input agroecosystems

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Acknowledgements

• This research was funded by
• CIRAD, INRA, the ACACIAGUM project (EC FP6
  contract 032233, http//inco-acaciagum.cirad.fr/)
  
• the Natural Science and Engineering Research
  Council of Canada
• With support from the Kenya Forestry Research
  Institute
• Merci beaucoup