The Redistribution of Soil Organic Carbon in Low-gradient Systems Resulting from the Interplay of Aggregate Dynamics and Erosion

Soil organic carbon (SOC) is a central indicator of soil quality. SOC spatial distribution is controlled by aggregate breakdown and transport under effects of soil characteristics, rainfall patterns, topography, and management. Capturing the interplay of aggregate dynamics and erosion requires a novel and holistic approach that explores breakdown, formation, and transport mechanisms across scales. The Intensively Managed Landscape Critical Zone Observatory (IMLCZO) in the US Midwest was designed for studies to understand critical zone processes considering glacial and management legacies. Few studies have examined the influence of glacial-remnant topography on aggregate dynamics, which has unknowingly hindered our understanding of SOC redistribution and advancement of soil management.

This study was conducted in two IMLCZO sites to examine SOC variability concerning aggregate turnover and transport influenced by topographies. The SOC variability was compared between Clear Creek (IA), a hilly and well-connected system, and the Upper Sangamon River (IL), a low-relief and poorly-connected system. Focusing on the Upper Sangamon, aggregate turnover following raindrop impact and wetting-drying cycles were tracked using rare earth elements (REE). This was followed by simulated rainfall studies of interrill erosion in field plots with different residues. Finally, aggregate transport distances after rain events were quantified by coupling REE measurements with transport models.

The SOC exhibited low variability in high-gradient landscapes due to enhanced mobilization and mixing under high runoff. The SOC showed high variability in low-gradient landscapes due to local depressions trapping of sediments and suppressed carbon decomposition under anoxic conditions. Aggregate breakdown dominated in early growing seasons due to raindrop impact. After a drying-wetting cycle, more aggregates were formed because of reduced rain power by plants, clay shrinkage upon drying, and microbial flushes upon rewetting. Fine particles accounted for 97% of lost soil and SOC enrichment increased with decreasing soil loss. No-till with high corn residue was most effective in reducing erosion and removing residue from no-till systems caused high soil loss. Small macroaggregates traveled the longest distances (1.33-117 cm) and extra-large macroaggregates traveled the shortest distances (0.97-36 cm). This research expanded knowledge of aggregate breakdown, formation, and transport in agricultural settings and how these processes vary based on local topography, climate, and management.