Rare earth oxides for labelling soil aggregate turnover: Impacts of soil properties, labelling method and aggregate structure

Rare earth oxides (REOs) as tracers for determining complex processes of soil aggregate formation, stabilization and breakdown could be limited by recovery rate. In this study, the impacts of soil properties, labelling methods and pre-existing aggregate structure on the recovery rate of REOs were analyzed. Our objectives were (i) to optimize the method of REOs as tracers for labelling aggregates; (ii) to investigate the effects of soil properties on the recovery rate of REOs; and (iii) to verify the feasibility of REOs as tracers for studying aggregate turnover. Natural and artificial soil aggregates with a difference in pore structure were labelled with REOs by either a wet or a dry labelling method. After incubating the labelled and repacked soil, the soil samples were separated into four aggregate size fractions (<0.053 mm, 0.053–0.25 mm, 0.25–2 mm and 2–5 mm), and the recovery of REOs in the fractions before and after wet sieving was calculated. Our results show that for natural aggregates the REO distributed more evenly in different aggregate fractions, and the wet sieving recovery of REO was significantly greater following the wet labelling method relative to the dry labelling method (105% vs 84%) (P < 0.05). In contrast, for artificial aggregates the REOs were labelled more homogenously following the dry labelling method, and the wet sieving recovery of REOs following the dry labelling method (98%) was significantly higher than following the wet labelling method (87%–92%) (P < 0.05). However, less pore structure heterogeneity was observed in artificial aggregates relative to natural aggregates. Soil properties (SOC, TN, pH, field capacity, and soil texture) and types of REO had no effect on wet sieving recovery of REOs in natural and artificial aggregates, except for the Gd ₂ O ₃ in artificial aggregates. A significant linear relationship was observed between predicted and measured aggregate size distributions, especially for larger size fractions. This finding theoretically supports that REOs effectively track the life cycle of soil aggregates. Our results indicate that natural aggregates following the wet labelling method fit better with field conditions.