Rare Earth Oxides for Labeling an Andisol Aggregate Turnover: Optimization, Verification, and Distribution

Rare earth element (REE) oxides as tracers to determine the complex soil aggregate formation, stabilization, and breakdown processes could be limited by recovery rate. In this study, the impacts of REE oxide types, labeling methods, and sieving methods on the recovery rates of REE oxides were analyzed. Our objectives were (i) to optimize the combination of labeling and sieving methods for REE oxides as tracers for Andisols, (ii) to verify the feasibility of REE oxides as tracers for labeling aggregates of Andisol isolated by dry and wet sieving, and (iii) to examine the distribution of the REE oxides in soil aggregates. Dry-sieved and wet-sieved Andisol aggregates were labeled with REE oxides using six labeling methods. After optimizing the labeling method, the labeled soil samples were first classified into four aggregate size fractions, namely < 0.053, 0.053–0.25, 0.25–2, and 2–5 mm, and then, the recovery rates of REE oxides in all soil fractions following the dry and wet sieving were calculated. The results showed a uniform distribution of REE oxides in all the dry-sieved aggregate fractions using the wet labeling methods (with and without incubation). Moreover, the recovery rates of REE oxides after dry sieving were significantly higher than those after the dry labeling method ( p < 0.05), with range values of 83.11–86.04% and 59.23–77.73%, respectively. In contrast, the REE oxides were labeled more homogeneously in the wet-sieved soil aggregates using the wet labeling method, showing a recovery rate range of REE oxides of 66.03–69.47%, which is significantly lower ( p < 0.05) than that obtained without incubation (85.78–87.98%). On the other hand, a significant linear relationship was observed between the predicted and measured aggregate size proportions, more particularly for the 0.053–0.25- and 0.25–2-mm aggregate fractions. This finding theoretically demonstrates the effectiveness of the REE oxide labeling method for investigating the aggregate dynamics in Andisols that have unique mineralogy and a distinct aggregate hierarchy. In addition, this study highlighted the most efficient combined labeling and sieving methods for REE oxide tracers for Andisols.