Quantifying water-stable soil aggregate turnover and its implication for soil organic matter dynamics in a model study

Recent studies have concluded that the dynamics of soil structure are central to the understanding of soil organic matter (SOM) cycling and the ensuing soil-water-nutrient relationships. Aggregate turnover directly controls the stabilization and physical protection of SOM. Therefore, quantifying aggregate dynamics will improve our ability to predict SOM behaviour as affected by ecosystem management and global change. We present an approach to directly quantify aggregate dynamics using rare-earth oxides as tracers. A 6-week laboratory incubation was set up to measure aggregate dynamics at different times. We made samples in which each different aggregate size-fraction contained a different tracer. By following the redistribution of these tracers into the other aggregate size-fractions, we could quantify all soil mass transfers between aggregate size-fractions. A comparison with a control soil showed that the tracer did not affect soil respiration or the aggregation process itself. Tracer mixing homogeneity, recovery and immobility were tested and validated. While initially macroaggregate formation occurred rapidly, microaggregate formation occurred more slowly during the experiment. Subsequent aggregate stabilization was more pronounced for the newly formed microaggregates than for the newly formed macroaggregates. Calculated turnover times were smaller for macroaggregates than for microaggregates (i.e. 30 vs. 88 days). Further research is needed to investigate to what extent these results can be extrapolated to the field. Our results confirmed existing qualitative views and concepts on aggregate dynamics in a quantitative way and will be valuable in directly linking aggregate turnover to the stabilization and protection of SOM.