OXYGEN ISOTOPE IN PHOSPHATE: CAN IT WORK IN THE SOIL/PLANT SYSTEM?
description
Transcript of OXYGEN ISOTOPE IN PHOSPHATE: CAN IT WORK IN THE SOIL/PLANT SYSTEM?
OXYGEN ISOTOPE IN PHOSPHATE: CAN IT WORK IN THE SOIL/PLANT SYSTEM?F.Tamburini, SM. Bernasconi, V. Pfahler, E. Frossard
Why 18O-PO4 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 18O-PO4 signatures (e.g. mineral fertilizer vs manure).
At conditions found in soils, only biologically-driven processes can change the 18O-PO4 signature.
How does this work?
P
16O
18O
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.
f – i = ln(x)
Theory - 2
④ Intracellular phosphatases promote a T-dependent equilibrium between PO4 and H2O
T (°C) = 111.4 – 4.3(18OPO4 – 18OH2O)
⑤ PO4 released by extracellular phosphatases will partly inherit O from the original molecule and partly exchange and fractionate O with H2O.
f = x(i) + (1-x)(18OH2O + )18O-PO4 init.
18O-H2O T °C (fract. factor)
18O-PO4 fin.
+15 ‰ -2 ‰ 15 °C - +20.4 ‰
+15 ‰ -2 ‰ - -30 ‰ (Apase)
+3.3 ‰
18O-PO4 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
??? ‰
[PO43-] PO4
3- used
18O-PO4
initial
18O-PO4 final
(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 ‰
Organisms preferentially take up the lighter isotopologue.f – i = ln(x)
for E. coli = -3 ‰ (Blake et al., 2005)
Case study 2- Soil development
Case study 2- Soil development
Pase data (2007) from E. Bünnemann
Apatite signature OM signature
T-dependent equilibriumbiological cycling
Imprint from extracell.enzymes
18O-PO4 in apatite~ +6‰
18O-PO4 atT-equilibrium
+11.5‰ __ +15‰
18O-PO4 in plant> +20‰
Case study 2- Soil development
Resin-P 07.2010
Resin-P 09.2007
HCl-P
Case study 3 – Source tracing
18O-PO4 in manures+11‰ __ +13‰
18O-PO4 at T-equilibrium+13.5‰ __ +14.8‰
18O-PO4 in soils (res-P)+18‰ __ +19‰
18O-PO4 in plant residues> +20‰
Case study 3 – Source tracing
Wrapping up
The answer is YES
The use of 18O-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 18O-PO4 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:Developments in the understanding of processes in the P cycle:new concepts from the use of isotopic tracersnew concepts from the use of isotopic tracers
Developments in the understanding of processes in the P cycle:Developments in the understanding of processes in the P cycle:new concepts from the use of isotopic tracersnew concepts from the use of isotopic tracers