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