Following the Turnover of Soil Bioavailable Phosphate in Mediterranean Savanna by Oxygen Stable Isotopes

Soil phosphate oxygen isotope analysis (δ¹⁸O_P) emerges as an effective method to trace the cycling of phosphorus (P) in soils. This study uses δ¹⁸O_P measurements to learn how the nutrient status (P and N) affects the biological turnover rates of P in the soil of a Mediterranean holm oak Savanna. Such ecosystems cover >3 × 10⁶ ha at the Iberian Peninsula. The analysis was part of a large-scale nutrient manipulation experiment, where N and P were added. We followed the changes in δ¹⁸O values of soil bioavailable P during incubation of soils with a pulse of P and in addition measured the δ¹⁸O_P in soil sampled at the site. In the incubations, the δ¹⁸O_P values changed from the original value of the added P and approached a steady state of 16.3‰, which is 3.8‰ higher than the isotopic equilibrium with water. The steady state was higher with ¹⁸O-enriched incubation media water. The change in δ¹⁸O_P values was more pronounced under trees, indicating a faster microbial P turnover rate. Incubation of soils fertilized with either P or N showed faster P turnover rate than control, implying N and P colimitation. Soil samples from P-fertilized plots displayed higher δ¹⁸O_P than the fertilizer, rather than the expected decrease toward steady-state values, found at the control and N plots. The microbial P turnover rates during incubations were slower than the rates reported for lowland tropical forest with lower bioavailable P concentrations but resemble ecosystems with similar concentrations.