Estimating soil phosphorus mineralization and immobilization: a comparison of isotope pool dilution vs and isotopic exchange kinetics frameworks

Quantifying soil phosphorus (P) fluxes is necessary to guide agroecosystem P management and understand ecosystem P cycling, but challenging. Recent developments in P radioisotope labeling of soils enable such assessments, but lack of method standardization hampers confidence in quantified values and comparisons across studies. We compared magnitudes of soil P fluxes between the two most commonly used approaches under isotopic exchange kinetics (IEK) and isotopic pool dilution (IPD) frameworks, specifically for gross organic P mineralization (P min ), microbial inorganic P immobilization (P immob ) and net P min , as well as total P influx integrating both abiotic processes and gross P min , using 18 soils from five long-term experiments with varying soil properties under diverse land use and management practices (e.g., fertilization). We found highly variable estimates by up to two orders of magnitude difference calculated by IEK- vs IPD-based approaches. In general, the IPD-based approach provided more conservative results than the IEK-based approach; however, no soil properties were found to be associated with this conservativeness of IPD- relative to IEK-based approach. Global regressions across all sites showed that P flux magnitudes determined under IEK- versus IPD-framework were poorly correlated (R 2 = 0.06 to 0.24), but when grouped by different site had greater agreement, especially for gross P min estimates (up to R 2 = 0.96). However, across all sites, total P influx estimated using IEK- and IPD-based frameworks were strongly and positively correlated (R 2 ≈ 0.7), suggesting a convergence of total P influx despite fundamentally different conceptual and methodological approaches. Since agroecosystem and methodological variables tested in this study (e.g., IEK- vs IPD-derived approach, incubation duration, land management) impacted magnitudes of P fluxes, standardization of the methodology assaying soil P cycling fluxes will be challenging and stands to benefit from understanding underlying mechanisms of P transformation. To address violated assumptions in IPD- and IEK-based approaches, there is a need to identify effective soil sterilization methods to furnish the abiotic control and to evaluate the necessity of pre-incubation (i.e., comparing the tracer’s kinetics in non-preincubated vs preincubated soils) and to reconsider the concept of ‘basal soil P min ’ under the IEK framework.