The Role of Hydraulic Connectivity and Management on Soil Aggregate Size and Stability in the Clear Creek Watershed, Iowa

The role of tillage practices on soil aggregate properties has been mainly addressed at the pedon scale (i.e., soilscape scale) by treating landscape elements as disconnected. However, there is observed heterogeneity in aggregate properties along flowpaths, suggesting that landscape scale hydraulic processes are also important. This study examines this supposition using field, laboratory and modeling analysis to assess aggregate size and stability along flowpaths under different management conditions: (1) tillage-induced abrasion effects on aggregate size were evaluated with the dry mean weight diameter (DMWD); (2) raindrop impact effects were evaluated with small macroaggregate stability (SMAGG(STAB)) using rainfall simulators; and (3) these aggregate proxies were studied in the context of connectivity through the excess bed shear stress (delta), quantified using a physically-based landscape model. DMWD and SMAGG(STAB) decreased along the flowpaths for all managements, and a negative correspondence between the proxies and delta was observed. delta captured roughness effects on connectivity along the flowpaths: highest connectivity was noted for parallel-ridge-till flowpaths, where delta ranged from 0-8.2 Pa, and lowest connectivity for contour-ridge-till flowpaths, where delta ranged from 0-1.1 Pa. High tillage intensity likely led to an increase in aggregate susceptibility to hydraulic forcing, reflected in the higher gradients of aggregate size and stability trendlines with respect to delta. Finally, a linear relationship between DMWD and SMAGG(STAB) was established.