Computed tomography and soil physical measurements of compaction behaviour under strip tillage, mulch tillage and no tillage

- Strip tillage created two locally different soil structures.
- Strip tillage led to both high bulk density and saturated conductivity between seed rows.
- Macroporosity and pore connectivity were higher in tilled than in untilled treatments.
- Under strip tillage, precompression stress was higher between than within seed rows.

In recent years, there has been an increased application of conservation-oriented tillage techniques, where instead of being turned the soil is only loosened or not tilled at all. Strip tillage, a special form of conservation tillage, results in small-scale structural differences, since tillage is performed only within the seed row, while the soil between seed rows is not tilled. However, tillage always impacts upon physical soil properties and processes.A combined application of conventional soil mechanical methods and X-ray computed tomography (X-ray CT) is employed here in order to investigate small-scale structural differences in a chernozem (texture 0-30  cm: silt loam) located in central Germany under strip tillage (within and between seed rows) compared to no tillage and mulch tillage. Apart from recording changes over time (years: 2012, 2014, 2015) to dry bulk density and saturated conductivity at soil depths 2-8 and 12-18 cm, stress-strain tests were conducted to map mechanical behaviour for a load range of 5-550 kPa at a soil depth of 12-18 cm (year 2015). Mechanical precompression stress was determined from the stress-dry bulk density curves. In addition, computed tomography scans were created followed by quantitative image analysis of the morphometric parameters mean macropore diameter, macroporosity, connectivity and anisotropy of the same soil samples.For strip tillage between seed rows and no tillage, a significant increase in dry bulk density was observed over time compared to strip tillage within the seed row and mulch tillage. This was more pronounced at a soil depth of 2-8 cm than at 12-18 cm. Despite higher dry bulk density, strip tillage between the seed row displayed also an increasing saturated conductivity compared to strip tillage within the seed row and mulch tillage. The computed tomography scans showed that the macropores became more compressed and soil aggregates were pushed together as mechanical stress increased, with the aggregate arrangement being transformed down into a coherent soil mass. The soil mechanical and morphometric parameters supported each other in terms of what they revealed about the mechanical properties of the soil structures. For instance, in the strip tillage between seed rows and no tillage treatments, the lack of soil tillage not only resulted in higher dry bulk densities, but also higher aggregate densities, mechanical precompression stress values, mean macropore diameters as well as lower macroporosity and connectivity values compared to mulch tillage and strip tillage within the seed row. The computed tomography parameters are therefore highly suitable for providing Supplementary information about the compaction process. Overall, this study showed that strip tillage combines the advantages of no tillage and a deeper, soil conservation-oriented primary tillage because, on a small scale, it creates two distinct soil structures which are beneficial in terms of optimal plant growth as well as mechanical resistance by driving over the soil.