Elsevier

Geoderma

Volume 345, 1 July 2019, Pages 63-71
Geoderma

Impact of wetting and drying cycles on soil structure dynamics

https://doi.org/10.1016/j.geoderma.2019.03.018Get rights and content

Highlights

  • 3D crack dynamics in structured soil during WD cycles observed with X-ray CT

  • Soil structure dynamics measured via structure labeling with garnet particles

  • Soil structure dynamics dependent on, bulk density, SOM and clay content

  • Higher SOM content led to a higher density of cracks with smaller aperture

  • Soil structure dynamics is negligible due to reactivation of old cracks

Abstract

Soil structure is not static but undergoes continuous changes due to a wide range of biotic and abiotic drivers such as bioturbation and the mechanical disturbance by tillage. This continuous alteration of soil structure beyond the pure swelling and shrinking of some stable structure is what we refer to as soil structure dynamics. It has important consequences for carbon turnover in soil as it controls how quickly soil organic matter gets occluded from or exposed to mineralization. So far there are hardly any direct observations of the rate at which soil pores are formed and destroyed.

Here we employ are recently introduced labeling approach for soil structure that measures how quickly the locations of small garnet particles get randomized in soil as a measure for soil structure dynamics. We investigate the effect of desiccation crack dynamics on pore space attributes in general and soils structure turnover in particular using X-ray microtomography for repeated wetting-drying cycles. This is explored for three different soils with a range of soil organic matter content, clay content and different clay mineralogy that were sieved to a certain aggregate size fraction (0.63–2 mm) and repacked at two different bulk density levels.

The total magnitude of desiccation crack formation mainly depended on the clay content and clay mineralogy. Higher soil organic matter content led to a denser crack pattern with smaller aperture. Wetting-drying cycles did not only effect visible macroporosity (>8 μm), but also unresolved mesoporosity. The changes in macroporosity were higher at lower bulk density. Most importantly, repeated wetting-drying cycles did not lead to a randomization of distances between garnet particles and pores. This demonstrates that former failure zones are reactivated during subsequent drying cycles. Hence, wetting-drying resulted in reversible particle displacement and therefore would not have triggered the exposure of occluded carbon that was not already exposed during the previous drying event.

Introduction

Soil structure is an important indicator of the ecological status of soil as it both controls many ecosystem functions of soil and is shaped by them (Rabot et al., 2018; Young, 2004). It defines the pathways for water and nutrient fluxes (Jarvis, 2007; Köhne et al., 2009), shapes microhabitats in soil (Baveye et al., 2018; Bottinelli et al., 2015; Young et al., 2008) and controls the micro-environmental conditions for chemical reaction patterns in soil (Totsche et al., 2018; Wilcke and Kaupenjohann, 1997). The spatial distribution of substrate and structure-mediated pathways of oxygen diffusion exert a major control on aerobic and anaerobic soil respiration (Keiluweit et al., 2017; Kuzyakov and Blagodatskaya, 2015; Smith et al., 2003) and is considered as one of the major factors controlling long-term carbon stabilization in soil (Kravchenko and Guber, 2017; Lehmann and Kleber, 2015).

Soil structure can in the broadest sense be defined as the spatial heterogeneity of the different components or properties of soil (Dexter, 1988). There are two approaches to characterize soil structure (Rabot et al., 2018). One is centered on the structure of solid components and usually focused on aggregates as the building blocks of soil. The other is focused on the pore perspective and explores the spatial arrangement of voids in undisturbed soil. This dichotomy is particularly relevant with respect to soil structure dynamics by biotic and abiotic agents which typically manifests itself through the steady formation and destruction of pores, but not of solid particles. Furthermore, soil structure dynamics might not even be detectable based on standard measures of the pore structure such as porosity and pore size distribution, since these macroscopic measures may stay constant in dynamic equilibrium, when the microscopic destruction and formation of pores are in balance. In order to resolve this problem a structure labeling approach was recently proposed that enables a direct estimation of soil structure dynamics with a combination of X-ray microtomography and a novel structure labelling approach. (Schlüter and Vogel, 2016). Aggregates are coated with small, inert garnet particles. Garnet is an iron-bearing mineral that evokes good X-ray contrast. The garnet particles are in direct contact with inter-aggregate pores after coating, except for those that get occluded in the contact area of adjacent aggregates after repacking. This results on average in shorter distances between garnet particles and nearest pores than the distance of arbitrary soil matrix locations and nearest pores. This represents an analogy to pool dilution experiments to measure carbon turnover with stable isotopes. The “short distance pool” is highly enriched in garnet particles and soil structure dynamics, or soil structure turnover as it was coined in Schlüter and Vogel (2016), will lead to a dilution of this pool by randomization of distances between garnet particles and pores through the formation of new uncoated pores or occlusion of particles through the destruction of old pores Soil structure dynamics measured by this randomization of passively translocated garnet particles has direct consequences for soil carbon turnover as the formation of new pores may expose previously occluded organic matter, which is one main explanation for the so-called Birch effect (Borken and Matzner, 2009; Lopez-Sangil et al., 2018; Navarro-García et al., 2012) Likewise, the destruction of pores may protect organic matter in its vicinity against mineralization (Beylich et al., 2010; Haas et al., 2016).

It was demonstrated that compaction, a typical abiotic structure-changing process does not lead to a randomization of particle locations (Schlüter and Vogel, 2016). The “tracer particle”-pore distances increase through compaction, but this is the same for any location within the soil. Therefore it was hypothesized that other structure-forming processes may be more efficient to induce structure dynamics in the sense of its reorganization with time measured by the randomization of “tracer particle”-pore distances. In this paper, crack dynamics through wetting-drying cycles are investigated as another important abiotic process that is known to modify soil structure.

Crack dynamics mainly depend on the clay content and its mineralogy, as some minerals (i.e. kaolinite or illite) have low to no swelling potential, while this is high for others (i.e. smectite, vermiculite). A second important factor is the heterogeneity in the assembly of soil particles, as more heterogeneous soil matrices tend to crack more easily, e.g. (Fiès and Bruand, 1998; Wang et al., 2018). The aperture and width of cracks are governed by the initial water content, the drying intensity and the antecedent moisture regime. Other important factors are bulk density, the content of soil organic matter (SOM), particulate organic matter (POM), and sesquioxides (Peng et al., 2007; Tang et al., 2011; Zhang et al., 2016). However, studies regarding crack dynamics in soil often evaluate two-dimensional crack patterns in drying soil suspensions and are seldom carried out in intact, structured soils as three-dimensional crack patterns are hard to investigate in opaque media. Here, X-ray tomography provides an opportunity to study crack dynamics through non-invasive imaging. The objectives of this paper are two-fold. First, the capacity of repeated wetting-drying cycles to induce soil structure changes in general and soil structure dynamics in terms of particle-pore distances in particular are investigated. Second, the effect of important soil properties like texture, bulk density, soil organic matter content and clay mineralogy on soil structure changes through wetting-drying cycles is explored.

Section snippets

Materials and methods

We examined three top soils from Germany, which differ in texture, organic matter content and clay mineralogy. The luvisol from Bad Rotthalmünster (RM) has both low clay and low SOM content (Kögel-Knabner et al., 2008), the chernozem from Bad Lauchstädt (BL) has low clay and medium SOM content (Altermann et al., 2005), and the gleysol soil from Giessen (GI) has both high clay and SOM content (Jürgen Böttcher, personal communication) (Table 1). The BL and RM soil are not only similar in terms of

Visual assessment

At the low bulk density (Bd1), inter-aggregate pores are clearly visible in the wet stage (W2) (Fig. 3). This means that a large part of the garnet particles is in direct contact with connected pores. The structural changes through drying (W2 → D2) can be recognized very well in the segmented images. The difference in connected porosity between the two moisture levels seems to increase in the order RM < BL < GI. The proportion of pores classified as occluded is relatively small.

At the high bulk

Impact of soil properties on pore space dynamics

The three soils were chosen such that the impact of texture, SOM content and clay mineralogy on structure dynamics during desiccation and rewetting could be investigated. The initial pore structure prior to the first wetting (W1) is roughly the same for all soils due to identical sieving and packing. The main difference between the luvisol (RM) and the chernozem (BL) is a higher SOM content in the BL soil which entailed a larger increase in macroporosity through drying especially at the low

Conclusions

The magnitude of crack dynamics due to repeated wetting-drying cycles depended on a number of soil properties. The magnitude in macroporosity changes was largest in the clay-rich soil and was on average larger at the lower bulk density (1.22 g/cm3) than at the higher bulk density (1.48 g/cm3). A higher soil organic matter content led to a higher density of cracks with smaller aperture. In none of the investigated soils did the repeated wetting-drying cycles lead to a randomization of distances

Acknowledgments

We are grateful to the editor and two anonymous reviewers for their helpful comments. This study was partially funded by the Deutsche Forschungsgemeinschaft through the research unit DFG-FOR 2337: Denitrification in Agricultural Soils: Integrated Control and Modelling at Various Scales (DASIM). We thank Jürgen Böttcher (Leibniz University Hannover), for sharing texture and organic matter data and Reinhold Jahn, Klaus Kaiser and Sonia Banze (Martin Luther University Halle-Wittenberg) for sharing

References (42)

  • J. Pöhlitz et al.

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

    Soil Tillage Res.

    (2018)
  • E. Rabot et al.

    Soil structure as an indicator of soil functions: a review

    Geoderma

    (2018)
  • S. Schlüter et al.

    Long-term effects of conventional and reduced tillage on soil structure, soil ecological and soil hydraulic properties

    Geoderma

    (2018)
  • C.-S. Tang et al.

    Desiccation and cracking behaviour of clay layer from slurry state under wetting–drying cycles

    Geoderma

    (2011)
  • C. Wang et al.

    Geometric and fractal analysis of dynamic cracking patterns subjected to wetting-drying cycles

    Soil Tillage Res.

    (2017)
  • C. Wang et al.

    Effects of straw incorporation on desiccation cracking patterns and horizontal flow in cracked clay loam

    Soil Tillage Res.

    (2018)
  • I.M. Young et al.

    Chapter 4 Microbial Distribution in Soils: Physics and Scaling

    (2008)
  • Z.B. Zhang et al.

    Puddling intensity, sesquioxides, and soil organic carbon impacts on crack patterns of two paddy soils

    Geoderma

    (2016)
  • M. Altermann et al.

    Chernozem—soil of the year 2005

    J. Plant Nutr. Soil Sci.

    (2005)
  • P.C. Baveye et al.

    Emergent properties of microbial activity in heterogeneous soil microenvironments: different research approaches are slowly converging, yet major challenges remain

    Front. Microbiol.

    (2018)
  • W. Borken et al.

    Reappraisal of drying and wetting effects on C and N mineralization and fluxes in soils

    Glob. Chang. Biol.

    (2009)
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