Soil Phosphate Stable
Oxygen Isotopes across Rainfall
and Bedrock Gradients
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Abstract
The stable oxygen isotope compositions of soil phosphate
(δ<sup>18</sup>O<sub>p</sub>) were suggested recently to be
a tracer of
phosphorus cycling in soils and plants. Here we present a survey of
bioavailable (resin-extractable or resin-P) inorganic phosphate δ<sup>18</sup>O<sub>p</sub> across natural and experimental rainfall gradients,
and across soil formed on sedimentary and igneous bedrock. In addition,
we analyzed the soil HCl-extractable inorganic δ<sup>18</sup>O<sub>p</sub>, which mainly represents calcium-bound inorganic phosphate.
The resin-P values were in the range 14.5–21.2‰. A similar
range, 15.6–21.3‰, was found for the HCl-extractable
inorganic δ<sup>18</sup>O<sub>p</sub>, with the exception of
samples from a soil of igneous origin that show lower values, 8.2–10.9‰,
which indicate that a large fraction of the inorganic phosphate in
this soil is still in the form of a primary mineral. The available-P
δ<sup>18</sup>O<sub>p</sub> values are considerably higher than
the values we calculated for extracellular hydrolysis of organic phosphate,
based on the known fractionation from lab experiments. However, these
values are close to the values expected for enzymatic-mediated phosphate
equilibration with soil–water. The possible processes that
can explain this observation are (1) extracellular equilibration of
the inorganic phosphate in the soil; (2) fractionations in the soil
are different than the ones measured at the lab; (3) effect of fractionation
during uptake; and (4) a flux of intercellular-equilibrated inorganic
phosphate from the soil microbiota, which is considerably larger than
the flux of hydrolyzed organic-P