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

Abstract

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 (δ), 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 δ was observed.δ captured roughness effects on connectivity along the flowpaths: highest connectivity was noted for parallel-ridge-till flowpaths, where δ ranged from 0-8.2 Pa, and lowest connectivity for contour-ridge-till flowpaths, where δ 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 δ. Finally, a linear relationship between DMWD and SMAGG STAB was established.

The impact of tillage row orientation on physical and chemical sediment enrichment

“…The SFA of macroaggregates in the in‐situ soil were found to be larger in the CRT site compared to PRT. This could be attributed to the effects of tillage‐induced roughness from contours which promotes deposition of coarser fractions in furrow sections (An & Liu, 2017) and disrupts hydraulic connectivity of downslope flowpaths, thereby decreasing the breakdown of aggregate fractions during transport (Wacha et al., 2018). For the CRT plot, the SFA of <0.25‐mm sediment fractions were significantly higher than the in‐situ soil, hitting a punctuated maximum of 0.78 after 100 min of rainfall, before decreasing to an ending value of 0.64.…”

“…The effects of tillage‐induced roughness on the transport of sediment size fractions and associated C and N were investigated using rainfall simulators within two experimental hillslopes (Wacha et al., 2018). The hillslopes were similar in terms of gradient and texture but differed in tillage row orientation with respect to the dominant flowpath direction (Figure 1).…”

“…However, these studies have not examined the effects of tillage orientation with respect to the dominant flow-pathway on the preferential transport of sediment fractions. Wacha et al (2018) reported significant differences in surface aggregate characteristics along flow-pathways due to the effects of oriented roughness on hydraulic connectivity of the hillslope. Roughness elements facilitate discontinuity of flow networks (Gomi, Sidle, Ueno, Miyata, & Kosugi, 2008), thereby impacting transport capacity, sediment sorting, and SOC fluxes (Papanicolaou et al, 2015).…”

“…It has also been well documented that tillage can alter the size distribution and stability of aggregates (Alvaro‐Fuentes et al., 2008; Amezketa, 1999; Ciric et al., 2012; Devine, Markewitz, Hendrix, & Coleman, 2014; Wang, Zhang, & Zhang, 2015; Yang & Wander, 1998). Tillage orientation causes preferential sorting of sediment size fractions, which can be enriched in SOC (Ampontuah, Robinson, & Nortcliff, 2006; Wacha et al., 2018). The inherent variability in size distribution and nutrient composition among transported fractions, combined with highly active flow‐pathways and tillage‐induced roughness features can prompt higher degrees of spatial heterogeneity in terms of SOC fluxes, especially within the soil active layer (Nadeu et al., 2011; Nie et al., 2015).…”

Cropping pattern changes diminish agroecosystem services in North and South Dakota, USA

“…Tillage events further fragment soil structure by breaking apart soil aggregates and exposing organic material to oxidation processes (Hatfield, Wacha, & Dold, ). With decreased stability, soil aggregates may collapse under hydrologic force, thus clogging pore spaces and dampening infiltration (Wacha et al., ). These conditions can increase the intensity and frequency of overland flow events, as well as contribute to increased erosion.…”