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

Predicting soil phosphorus (P) bioavailability requires an understanding of organic P mineralization (P_min) but assessing P_min rates is challenging. Recent advances in the use of 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 two most commonly used approaches, isotopically 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 reduced cumulative rate with additional 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 necessitated a more nuanced approach via an advanced statistical approach (e.g., decision tree) 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).