OXYGEN ISOTOPE IN
PHOSPHATE: CAN IT WORK
IN THE SOIL/PLANT SYSTEM?
F.Tamburini, SM. Bernasconi, V. Pfahler, E. Frossard
Why
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18
d O-PO
4 in
soils?
Stable isotopes are used to identify biogeochemical and
physical processes and trace sources. They also allow to
study long-term evolution of signals and are not dangerous for
the environment.
P has only one stable isotope (31P)…
But phosphate has 4 oxygen atoms. This is the only stable
isotope approach to study P cycling.
Different sources have distinctive d18O-PO4 signatures (e.g.
mineral fertilizer vs manure).
At conditions found in soils, only biologically-driven processes
can change the d18O-PO4 signature.
How does this work?
P
18O
16O
Theory - 1
① There is little fractionation associated to inorganic
processes such as adsorption, precipitation and
dissolution.
② Inorganic hydrolysis of condensed phosphates
promotes incorporation of water oxygen w/out
any fractionation.
③ Organisms preferentially take up the lighter
isotopologue.
df – di = e ln(x)
Theory - 2
④ Intracellular phosphatases promote a T-dependent
equilibrium between PO4 and H2O
T (°C) = 111.4 – 4.3(d18OPO4 – d18OH2O)
⑤ PO4 released by extracellular phosphatases will
partly inherit O from the original molecule and
partly exchange and fractionate O with H2O.
df = x(di) + (1-x)(d18OH2O + e)
d18O-PO4 init.
d18O-H2O
T °C
e (fract. factor)
d18O-PO4 fin.
+15 ‰
-2 ‰
15 °C
-
+20.4 ‰
+15 ‰
-2 ‰
-
-30 ‰ (Apase)
+3.3 ‰
18
d O-PO
4
in the soil/plant system
Preparation and analysis
TCEA/IRMS
Tamburini et al., EJSS (early view)
Case study 1 – Plant uptake
PDC-20 Verena Pfahler et al.
Effects of plant uptake on the δ18O signature of phosphate
??? ‰
Organisms preferentially take up the lighter isotopologue.
df – di = e ln(x)
e for E. coli = -3 ‰ (Blake et al., 2005)
[PO43-]
PO43used
d18O-PO4
initial
d18O-PO4
final
e (fract.
factor)
0.5 mM
(4 mmoles)
45%
+12.4 ‰
+11.3 ‰
-
0.05 mM
(0.4 mmoles)
98%
+12.4 ‰
+17.7 ‰
-2.5 ‰
0.02 mM
(0.16 mmoles)
96%
+12.4 ‰
+21.9 ‰
-3.1 ‰
Case study 2- Soil development
BigLink Project
2007-2010
Damma glacier forefield
(Switzerland)
Case study 2- Soil development
Pase data (2007) from E. Bünnemann
Apatite signature
T-dependent equilibrium
biological cycling
OM signature
Imprint from extracell.
enzymes
Case study 2- Soil development
Resin-P 07.2010
Resin-P 09.2007
HCl-P
d18O-PO4 in plant
> +20‰
d18O-PO4 at
T-equilibrium
+11.5‰ __ +15‰
d18O-PO4 in apatite
~ +6‰
Case study 3 – Source tracing
COST Action 869
Prediction of
contributing areas
for P-losses from
agricultural land
(Claudia Hahn)
Baldeggersee
(Switzerland)
Case study 3 – Source tracing
d18O-PO4 in plant residues
> +20‰
d18O-PO4 in soils (res-P)
+18‰ __ +19‰
d18O-PO4 at T-equilibrium
+13.5‰ __ +14.8‰
d18O-PO4 in manures
+11‰ __ +13‰
Wrapping up
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The answer is YES
The use of d18O-PO4 in the soil/plant system is really
promising, but it is still in its infancy.
The developed conceptual models are giving a good
prediction on what to expect.
As for other isotopic systems, the “good” use of d18OPO4 to understand the dynamics of P in soils is bound to
our knowledge of the individual fractionation processes
and of the complex interplay between them.
2012 in Ascona
Developments in the understanding of processes in the P cycle:
new concepts from the use of isotopic tracers