『Abstract
Phosphorus (P) availability limits productivity in many ecosystems
worldwide. As a result, improved understanding of P cycling through
soil and plants is much desirable. The use of the oxygen isotopes
associated to phosphate can be used to study the cycle of P in
terrestrial systems. However, changes with time in the oxygen
isotopes associated to available P have not yet been evaluated
under field conditions. Here we present the variations in available-P
oxygen isotopes, based on resin extractions, in a semi-arid site
that included plots in which the amount of rainfall reaching the
soil was modified. In addition, the oxygen isotopes in the less
dynamic fraction which is extractable by HCl, were also measured.
The δ18O of the HCl-extractable phosphate shows no
seasonal pattern and corresponds to the average value of the available
phosphate of 16.5‰. This value is in the expected range for equilibration
with soil water at the prevailing temperatures in the site. The
δ18O values of resin-extractable P showed a range of
14.5-19.1‰(SMOW), and evidence of seasonal variability, as well
as variability induced by rainfall manipulation experiments. We
present a framework for analyzing the isotopic ratios in soil
phosphate and explain the variability as mainly driven by phosphate
equilibration with soil water, and by the isotopic effects associated
with extracellular mineralization. Additional isotopic effects
result from fractionation in uptake, and the input to the soil
of phosphate equilibrated in leaves. These results suggest that
the δ18O of resin-extractable P is an interesting marker
for the rate of biological P transformations in soil systems.』
1. Introduction
2. Methods
2.1. Study sites
2.2. Analytical methods
3. Results
4. Discussion
4.1. Phosphate concentrations
4.2. Oxygen isotopes variations
4.2.1. Expected oxygen isotopic effects in the terrestrial phosphate
cycle
4.2.2. Interpreting the measured δ18OP
values
5. Conclusions
Acknowledgments
References