Quantitative characterization of the turnover path of red soil aggregate in the splash process

Abstract: A soil aggregate is a cluster of primary soil particles that strongly adhere to each other. In view of the direct target of raindrop impact, the breakdown, formation, and stability of aggregates have been the key indicators of soil quality and functioning, particularly for the development of splash erosion. It is a high demand for a physical model of soil erosion associated with ecological degradation. However, little attention has been focused on transformation paths between aggregates with different sizes during the splash erosion. This study aims to quantify the mutual turnover path of red soil aggregates with different particle sizes during the splash erosion process under the various rainfall characteristics parameters. The quaternary red clay was first collected from Xian'ning in Hubei Province of China. A series of laboratory simulated rainfall experiments were then carried out with the controlled rainfall intensity (90 and 30 mm/h) and rain duration (20, 40, and 60 min). A quantitative characterization was also performed on the transformation of soil aggregates within different particle sizes (2-5 mm, large macroaggregates; 0.25-<2 mm, small macroaggregates; 0.053-<0.25 mm, microaggregates; and <0.053 mm, silt and clay fractions) under the Rare Earth Element (REE) concentration. The tested soil aggregates were labelled by the REE tracing associated with the wet sieving method. Among them, the REE was determined via the inductively coupled plasma mass spectrometry (ICP-MS). The splash erosion experiments were conducted in the improved Morgan's splash pan. The mixture of labelled aggregates was filled in the horizontal pan for the same bulk density measured in the field. The results showed that the recovery rate was more than 90% in the labeled aggregates after the REE wet-mixing process, indicating the stable tracing performance. The turnover pathway of soil aggregates presented a dynamic tendency in the direction of fragmentation. Specifically, the turnover rate from small to microaggregates first reduced and then increased under the rain intensity of 30 mm/h, whereas, that increased and then decreased under 90 mm/h, both of which no path of large macroaggregates formation with the enrichment of microaggregates. The aggregates broke faster and reached a relatively stable state with an accumulated breakdown rate of 14%, indicating the minimal accumulated aggregation rate under heavy rainfall conditions. By contrast, the soil at 30 mm/h suffered a lasting and mild breakdown process with about half of macroaggregate under the accumulation of the kinetic energy of rainfall. The correlation analysis demonstrated that the small agglomerates behaved with a significant correlation with the cumulative fragmentation rate and agglomeration rate during the entire splash corrosion process (P<0.01), and the turnover paths were not significantly correlated between the aggregates whose diameters were >0.25 mm and others (P>0.05), while the aggregates whose diameters were <0.25 mm could affect the no-adjacent aggregates step by step (P<0.05). Consequently, the erosion-induced dynamics of the soil physical structure model can be largely enriched to combine the mixed transformation paths in opposite directions of different aggregates in the splash erosion.