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

Abstract

Soil phosphate oxygen isotope analysis (δ18OP) emerges as an effective method to trace the cycling of phosphorus (P) in soils. This study uses δ18OP 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 δ18OP in soil sampled at the site. In the incubations, the δ18OP 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 δ18OP 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 δ18OP 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.

N : P stoichiometry and habitat effects on Mediterranean savanna seasonal root dynamics

“…root diameter). Unlike N ions, P is relatively immobile in soil, so in general P availability promotes primary root growth at the expense of lateral development (Williamson, 2001). Less surface soil exploration is necessary under high availability of immobile P, reducing the need to access P-rich plant residues (Lynch, 2011).…”

Estimates of mean residence times of phosphorus in commonly considered inorganic soil phosphorus pools

“…A precise characterization of P mineral forms present in these inorganic pools is difficult since there is a plethora of mixed compounds and not pure crystalline P forms in soils. However, spectroscopic techniques have been used to confirm that soils with more HCl-P tend to contain more calcium phosphate, and soils with relatively larger NaOH pools contain more P associated with Fe and Al Helfenstein et al, 2018a;Kar et al, 2011;Prietzel et al, 2016;Wu et al, 2014).…”

Fate of N additions in a multiple resource‐limited Mediterranean oak savanna

“…4). Overall, low label recovery in microbial biomass when N and P were added was surprising given that previous research done at the site indicated that soil microbes were co-limited by N and P (Weiner et al 2018). This is supported by statistically lower microbial biomass N concentrations in the N-only plots compared to N + P plots in March 2018, indicating that potentially microbes in N-only plots were growing slower.…”

“…However, because the nutrient addition treatments may have affected communities in both habitats, it is not possible to say what role this played. Overall, low label recovery in microbial biomass when N and P were added was surprising given that previous research done at the site indicated that soil microbes were co-limited by N and P (Weiner et al 2018).…”