Abstract
The soil structure near the surface of agricultural soils changes with season mainly by land management together with climatic and biological factors. Quantitative analysis of post-tillage changes in soil structure and related hydraulic properties are necessary for evaluating and improving models of soil hydrological and transport processes. The objectives of this study were to quantify changes in soil seedbed structure induced by rainfall and drainage and to estimate the eff ;ects of soil texture and SOC on these changes. We collected samples from the harrowed layer of twenty-six fine to coarse textured Swedish mineral soils. Air-dried soil was placed in cylinders (5 cm high, diameter 5 cm) and exposed to simulated rainfall (5 mm h−1 for 4 h) and drainage (−50 cm pressure potential) cycles in the laboratory. We used X-ray tomography to quantify changes in pore networks in a thin surface layer and in the whole cylinder. Infiltration rates at -5 cm pressure potential were measured using a mini disc tension infiltrometer on replicate air-dried samples and on the samples included in the consolidation experiments at the final state. Total imaged specific pore volumes generally decreased from initial to final state and pore size distributions were shifted towards larger proportions of below image resolution pores (< 80 μm). There was a strong positive correlation between clay content and changes (i.e. final state-initial state) in the specific volume of pores <80 μm. Soils with high clay content and soil organic carbon (SOC) content often have strong aggregates that resist changes. Nevertheless, both clay and SOC contents were negatively correlated with the changes in specific imaged pore volume. These results highlight the importance of swelling, which is largely controlled by clay content, for seedbed consolidation. In line with previous studies, when excluding coarse textured soil, the changes in surface porosity were negatively correlated with silt content. Changes in infiltration capacity were not significantly correlated with any basic soil properties. Our results suggest that shrinking-swelling should be a central part in any model for seedbed consolidation.
Highlights
IntroductionThe soil structure (i.e. the spatial arrangement of solids and pores) at and near the soil surface plays an important role for the distribution of rainfall between surface runoff and infiltration
The soil structure at and near the soil surface plays an important role for the distribution of rainfall between surface runoff and infiltration
In the study by Sandin et al (2018) the largest changes in pore networks generally took place close to the bottom of the samples whereas soil structural changes, evaluated by visual inspection, were rather homogeneous with depth in our study. These differences in consolidation be tween the two studies highlight the importance of the wetting process, where rapid wetting and saturation at the bottom of the sample followed by drainage often led to a decrease in volume while slower wetting from the top followed by drainage mainly resulted in swelling and an increase in sample volume
Summary
The soil structure (i.e. the spatial arrangement of solids and pores) at and near the soil surface plays an important role for the distribution of rainfall between surface runoff and infiltration. This, in turn, has a strong influence on water retention, water flows and solute transport in the soil. Soil structure is not constant but changes continuously due to both abiotic and biotic processes (Lin, 2011). Soil tillage may within a few seconds completely change the structure of the top 5− 6 cm during seedbed preparation. The newly created seedbed returns to its pre-tillage condition in a process referred to as soil consolidation. Soil consolidation is the sum of a number of processes including micro-cracking, swelling and shrinking, aggregate coalescence and slaking that may occur one-at-a-time or simultaneously
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