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5 Swiss Geoscience Meeting, Geneva 2007

Tracing origin, distribution and fate of phosphate in
terrestrial environments with oxygen isotopes
Tamburini Federica*, Bernasconi Stefano M.** & Frossard Emmanuel*

*Institut f. Pflanzenwissenschaften, ETH, CH-8315 Lindau
**Geologische Institut, ETH Zürich, Ch-8092 Zürich

The eutrophication of surface waters is mainly the consequence of excessive
inputs and successive losses of nutrients from agricultural systems. This
problem is felt everywhere in Europe and in Switzerland, where water quality of
rivers and lakes is compromised by the overload of nutrients lost from
grasslands and agricultural fields. In many cases, the reduction of nutrient
inputs to the surface waters have decreased through the construction of
sewage treatment plants, but the restoration of surface and subsurface waters
to mesotrophic or oligotrophic conditions is not yet achieved.
The European COST action 869 (Mitigation options for nutrient reduction in
surface water and ground waters) has the primary objective to evaluate different
options for reducing nutrient loss to surface and ground waters by studying,
among other topics, the temporal dynamics of nutrients loss.

Phosphorus is the nutrient that most commonly determines the trophic status of
the environment, and is generally considered the main responsible for
eutrophication of inland water bodies. Because it is applied to agricultural fields
in a variety of forms, such as cow and pig manure, organic, mineral and
conventional fertilizers, phosphate may follow diverse paths in the soil,
distributing to mineral and organic phases, which present different and well-
defined reactivity.

The fact that phosphorus has only one stable isotope precludes studies dealing
with stable isotopic fractionation. This kind of approach could help in
deciphering the various steps of the phosphorus cycle in the natural
environment and in tracing and quantifying the contribution of different sources
of phosphate to a final pool.

In a pioneering study Longinelli showed that at low temperature, the oxygen
isotope composition of oxygen present in phosphate groups is related to
temperature (Longinelli, 1965). More recent studies have shown that the
phosphate group only exchanges isotopes with ambient water as a result of
biological activity, and showed that intra- and extra-cellular enzymatic activity is
responsible for oxygen isotope fractionation in phosphate (Blake et al., 2005;
Colman et al., 2005). These studies indicate that oxygen isotopes have a good
potential to identify different phosphate sources and transformations related to
biological activity.

After developing and optimizing a method for extracting phosphate from soils,
plants and fertilizers, and preparing the samples to be analyzed for oxygen
isotopes, we will characterize 1. the isotopic signature of the primary sources, 2.
the origin of phosphate present in soils, and 3. the proportion of phosphate in
5 Swiss Geoscience Meeting, Geneva 2007

the soils and 4. in the water bodies directly derived from fertilizers, with a
particular interest in animal manure. The obtained results will be then used to
assess the effect of different phosphorus inputs to soils and the risk of
phosphorus losses to water bodies.

Blake R.E., Alt J.C. & Martini A.M. 2001: Oxygen isotope ratios of PO4: An
inorganic indicator of enzymatic activity and P metabolism and a new biomarker
in the search of life. PNAS 98(5), 2148-2153.
Colman A.S., Blake R.E., Karl D.M., Fogel M.L. & Turekian K. K. 2005: Marine
phosphate oxygen isotopes and organic matter remineralization in the oceans.
PNAS 102(37), 13023-13028.
Longinellil A. 1965: Oxygen isotopic composition of orthophosphate from shells
of living marine organisms. Nature 207, 716-719.

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