In pursuit of soil P mineralization: A review and synthesis of radioisotopic labeling techniques

Abstract

Predicting soil phosphorus (P) bioavailability requires an understanding of organic P mineralization (P min ) but assessing P min rates is challenging. Recent advances in radioisotopic P labeling enable estimates of P min rates in soil. However, there is yet no standardization of radioisotopic dilution methods, potentially compromising comparison of results among studies. We conducted a systematic literature search to (1) synthesize the varying procedures among different studies, (2) compare the soil P min results obtained via the two most commonly used approaches, isotope exchangeable kinetics (IEK) and isotope pool dilution (IPD), and (3) address the methodological advantages and limitations of IEK and IPD. We identified and analyzed 98 studies, and found large discrepancies in study-specific radioisotope spike level, soil incubation period and extraction methods used to estimate the soil available P pool. On average, a spike level of 10–20 kBq g −1 soil was used among studies but empirical assessments are required to confirm that this assumed range of spike level is sufficient to measure exchangeable P. We found that incubation duration should be ≤ 14 d because there is an increasing possibility of measuring declining cumulative gross P min values beyond 14 d, even though a reduced cumulative pool with increasing time is theoretically impossible. Gross P min rates were lower by IPD compared to IEK. However, 18 studies reported undetectable gross P min , possibly due to the unverified but widely made assumption that biotic and abiotic process rates are additive. The complexity of the methodological issues may necessitate a more nuanced approach to select a method based on trade-offs. To address the methodological issues identified in this review, a greater quantity and quality of observations need to be collected (e.g., well-designed experiments, data quality assurance such as quenching correction). • Radioisotopic determination of soil P mineralization (P min ) not standardized. • Systematic review reveals discrepancies in radioisotopic spike and incubation period. • Incubation ≤14 d avoids artefacts of apparent declines in cumulative gross P min . • Lower gross P min by isotope pool dilution (IPD) vs isotopic exchange kinetics (IEK). • Trade-offs of IPD vs IEK in time, labor, and additional data on P transformations.

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