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 (SMAGGSTAB) 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 SMAGGSTAB 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 SMAGGSTAB was established.
Highlights
Agricultural management practices associated with tillage have been shown to impact soil aggregate size characteristics and stability through the mechanized weakening and breaking of the aggregate structure, i.e., organized groupings of different size soil grains and organic materialGeosciences 2018, 8, 470; doi:10.3390/geosciences8120470 www.mdpi.com/journal/geosciencesGeosciences 2018, 8, 470 with defined void spacing [1,2,3,4]
The coefficient of variation for the measured values of dry mean weight diameter (DMWD) and SMAGGSTAB ranged between 2.2% and 26.3%, with the lower mean variability values reported for Contour Ridge Tillage-Low Intensity (CRT-LI), and higher mean variability values for Contour Tillage-High Intensity (CT-HI) (Table 2)
Within intensively managed systems, it may not be sufficient for DMWD and SMAGGSTAB to be examined at soilscape scale, but rather at the landscape scale where the role of connectivity and transport can be adequately evaluated
Summary
Agricultural management practices associated with tillage have been shown to impact soil aggregate size characteristics and stability through the mechanized weakening and breaking of the aggregate structure, i.e., organized groupings of different size soil grains and organic materialGeosciences 2018, 8, 470; doi:10.3390/geosciences8120470 www.mdpi.com/journal/geosciencesGeosciences 2018, 8, 470 with defined void spacing [1,2,3,4]. Agricultural management practices associated with tillage have been shown to impact soil aggregate size characteristics and stability through the mechanized weakening and breaking of the aggregate structure, i.e., organized groupings of different size soil grains and organic material. The tillage disturbances can shift the fraction of the aggregate size distribution towards the lower size range of 0.25–2.0 mm, hereafter referred to as the SMall macroAGGregate (SMAGG) [7] based on the hierarchical classification of Oades and Waters [5]. Kinetic energy is transferred from falling raindrops into the soil aggregate structure upon impact [13,14,15]. When the applied raindrop kinetic energy exceeds the internal molecular bond strength of the soil aggregate, the aggregate structure fails [16]
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