Computed tomography and soil physical measurements of compaction behaviour under strip tillage, mulch tillage and no tillage
Introduction
Soil tillage aims to increase crop yields and at the same time preserve ecological soil functions, like habitat functions and regulatory functions for water and nutrients. In recent decades, an increasing number of practitioners have abandoned traditional tillage methods which turn the soil using a plough (conventional) in favour of conservation-oriented soil tillage (see e.g. Licht and Al-Kaisi, 2005, Nowatzki et al., 2009). The latter does not involve turning the soil with a plough, but instead only loosening it or leaving it completely untilled. Conservation tillage thus covers the soil surface with dead plant material (Gajri et al., 1999). This has both ecological and economic benefits for the soil, such as for example conserving water, preventing soil erosion, preserving economic productivity, reduced investments in machinery and less time spent on seedbed preparation (Carter, 2004, FAO, 1993). There are a variety of conservation tillage systems, which can be roughly divided into no tillage, mulch tillage, strip tillage, ridge tillage and minimum tillage (FAO, 1993). Strip tillage is special in that the soil is divided into a sowing zone and a soil management zone. The sowing zone, which is 5–15 cm wide, is worked mechanically down to a depth of 25 cm in order to optimise the soil and microclimate conditions for crop germination and growth, while the soil management zone is left untilled (Lal, 1983). Strip tillage therefore combines the conventional advantages of no tillage and those of deeper, non-turning primary tillage. It also allows farmers to combine individual working steps, thus reducing the number of times the field is driven over (Nowatzki et al., 2009).
However, any type of tillage affects the physical properties of the soil (Carter, 2004). In particular, there is a higher risk of compaction damage if the machinery used has not been adapted to the site and local conditions (Rücknagel et al., 2012, Koch et al., 2008). Compaction processes mainly affect parameters such as dry bulk density, aggregate stability, pore size distribution, infiltration rate and water conservation (FAO, 1993). This causes a deterioration in nutrient uptake and plant growth, while surface run-off increases (e.g. Paglai and Jones, 2002, Voorhees, 1986).
When investigating compaction effects in agricultural soils, conventional soil mechanical methods such as soil compression tests make it possible to map the compaction process and identify volumetric soil deformation for different initial soil structures. This yields indirect information about functional properties of the internal structure, such as the stress-strain relationship and aggregate density/bulk density ratio (Rücknagel et al., 2007). Typically, there is a lack of direct information about changes to geometric properties and morphologies of the void system. With this in mind, in recent decades non-destructive imaging methods, such as X-ray computed tomography (X-ray CT), have been increasingly used to successfully answer questions about soil physical properties (e.g. Keller et al., 2013, Schlüter et al., 2011, Schlüter et al., 2016). Computed tomography not only detects the spatial distribution of pore geometries and maps their positions precisely, but also enables quantitative image analysis.
Only a few studies have dealt with the combined analysis of structural differences between individual conservation soil tillage systems and compaction effects in those soil tillage systems with the aid of computed tomography scans (e.g. Dal Ferro et al., 2014, Jarvis et al., 2017, Luo et al., 2010). None of these studies considered the strip tillage method. In addition, no links have been established between conventional soil mechanical methods and those involving computed tomography. Using a combination of soil mechanical and computed tomography methods, this study therefore focuses on the influence of the special, two-part soil structure present under strip tillage compared to mulch tillage and no tillage. Specifically, it aims to answer the following questions: (i) Does the strip tillage method create small-scale structural differences within and between the seed rows? (ii) Under strip tillage, how do dry bulk density and aggregate density change as stress increases compared to mulch tillage and no tillage? (iii) To what extent can morphometric parameters, based on X-ray CT, map soil compaction behaviour in strip tillage compared to mulch tillage and no tillage? (iv) Are there correlations between the parameters measured using conventional methods and those measured with X-ray CT? (v) And what implications do the results have for agricultural land use? Overall, this study aims to explore to evaluating the role of the different soil tillage methods in the compaction process.
Section snippets
Trial site
Soil sampling was performed at the strip tillage experiment set up by the International Crop Production Centre in Bernburg-Strenzfeld (Germany, federal state Saxony-Anhalt, 11° 41′ E, 51° 50′ N; 80 m above sea level) in 2012. The average annual temperature is 9.7 °C and average annual precipitation is 511 mm. The soil type is a chernozem (FAO, 1998). The texture of the top soil (0–30 cm) contains 60 g kg−1 sand, 740 g kg−1 silt and 200 g kg−1 clay, constituting a silt loam (USDA, 1997). The total
Dry bulk density
Before the strip tillage trial was set up in 2012, BD at soil depths 2–8 cm and 12–18 cm was 1.15 g cm−3 and 1.36 g cm−3 respectively, regardless of tillage treatment (Table 1). In 2014 and 2015, neither depth displayed any significant differences in BD between mulch tillage and strip tillage WS on the one hand and strip tillage BS and no tillage on the other. By contrast, at both depths and in both years, BD was significantly lower for mulch tillage and strip tillage WS compared to strip tillage
Soil physical condition
Overall, there were intact soil structures for all tillage treatments, depths and years where BD values were always lower than a site-specific, root-limiting BD of 1.55 g cm−3 (Kaufmann et al., 2010) and Ks values were higher than 10 cm d−1 (Werner and Paul, 1999).
The tillage treatments created different soil physical conditions. On the one hand strip tillage WS and mulch tillage, and on the other strip tillage BS and no tillage, each displayed very similar soil structural properties.
Strip
Conclusions
The tillage treatments displayed clear differences in terms of initial structure and compaction behaviour. In addition to higher mechanical precompression stress values under strip tillage BS and no tillage, these also showed higher BDxi and AD throughout almost the entire load range compared to strip tillage WS and mulch tillage. The CT scans and the morphometric parameters also confirmed the mechanically more stable soil structure under strip tillage BS and no tillage, with higher mean
Acknowledgments
The authors would like to thank the “Internationales DLG-Pflanzenbauzentrum Bernburg-Strenzfeld” for allowing them to take the soil samples at the Bernburg site. We gratefully acknowledge the expert technical assistance and guidance of J. M. Köhne and are grateful to V. Hentschel and F. Hickmann for their support in the field and laboratory work.
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