Intraseasonal soil macroaggregate dynamics in two contrasting field soils using labeled tracer spheres

Several studies have hypothesized that increased turnover of soil aggregates promotes soil organic matter losses tinder cultivation; while others suggest that organic matter protection requires occlusion into aggregates. However, few direct observations of aggregate dynamics are reported in the literature. A 2-yr field study was performed to observe active organic C dynamics and soil macroaggregate dynamics in two contrasting soils. Dysprosium-labelled tracer spheres were applied to field plots to observe soil macroaggregate dynamics, while CO2-evolution during 10-d laboratory incubations was used to measure active C dynamics. Results of biochemical analyses showed higher active C turnover in the low C soil, suggesting a lower proportion of incoming organic matter was protected when compared with the high C soil. No net aggregation or degradation was determined over the long-term, suggesting the soil was at steady-state. However, aggregation followed a cyclical pattern reset by the over-winter period and tillage. Tracer incorporation into large macroaggregates was observed within 9 d after tillage, reaching a maximum of 40 to 60% tracer incorporation into >1-mm aggregates after 72 d. A rapid approach to equilibrium within the study period reflected rapid dynamics of macroaggregates in both soils studied. Slower macroaggregate dynamics in the high C soil were attributed to sustained aggregate stability and resiliency at the end of the growing season. Based on observations of macroaggregate dynamics in soils with contrasting active organic C dynamics, we suggest that rapid macroaggregate turnover not only results in the exposure of labile organic matter but also provides a mechanism for the occlusion and physical protection of particulate organic matter.