Elsevier

Geoderma

Volume 404, 15 December 2021, 115304
Geoderma

Mechanisms and processes affecting aggregate stability and saturated hydraulic conductivity of top and sublayers in semi-arid soils

https://doi.org/10.1016/j.geoderma.2021.115304Get rights and content

Highlights

  • The saturated hydraulic conductivity differed between sub and top layer soils.

  • The aggregate bulk density is greater in sublayer than in top layer soil.

  • Greater mechanical strength of sublayer aggregates increased their Ks.

Abstract

Soils are turned over during agronomic and environmental activities, such that the sublayer becomes the topsoil. Because subsoils have been subjected to high pressures over long periods in the field, their activities and functionality can be changed when they are shifted to the top layer. The present work objective was to investigated the mechanisms and processes affecting the structural stability, and consequently saturated hydraulic conductivity (Ks), of a semi-arid soils, which were taken from < 0.3 m depth (top soils) and > 0.3 m depth (sublayers soils). Disturbed soil samples, with similar aggregate-size distribution and bulk densities, were packed in columns, prewetted with saline solution (SS), and then their Ks values were determined during consecutive leaching with SS and deionized water (DI). The Ks values of the various soils under SS leaching differed due to slaking and swelling processes that changed the soil structure. The effect of the slaking process on Ks reduction was more significant in the top- than sublayer soils. Soil swelling under SS wetting and leaching caused mainly by penetration of water molecules into capillary pores in the soil that increases the pores volumes, and enlarges the aggregate swelling (matrix-type swelling). The average bulk density, and consequently the structural strength, of the aggregates in the sublayer soils were significantly higher than that of the top soils. This suggested that, under matrix-type swelling, less aggregates would be broken in the sublayer than in the top-layer soils. During leaching of the top and the sublayer soils with DI, the aggregates breakdown and Ks reduction were caused mainly by dispersion and osmotic swelling of the clay fractions in the soil. In this case, a negative relationship between the Ks and SAR values was obtained, regardless of the soil depths.

Introduction

Hydraulic conductivity is a key parameter controlling the movement of water and solutes through the soil profile (Hillel, 2004). A reduction of this parameter could lead to waterlogging, reduction of the infiltration rate and soil aeration, and an increase of surface runoff and soil erosion (Kadu et al., 2003, Ben-Hur, 2008, Shabtai et al., 2014). A soil's saturated hydraulic conductivity (Ks) is strongly governed by soil structure, which affects the soil-pore geometry and quantity (Hillel, 2004). Soil structure, however, is a dynamic property that can be altered by aggregate breakdown (Tisdall and Oades, 1982, Le Bissonnais and Arrouays, 1997, Chenu et al., 2000, Lado et al., 2004, Ben-Hur and Lado, 2008, Barto et al., 2010). For example, intensive land use in recent decades has declined the soil-structure stability, and increased the soil degradation (Nachtergaele et al., 2015).

The amount and decomposition rate of the organic matter (OM) in the soil may affect the size distribution and stability of soil aggregates (Amézketa, 1999, Bronick and Lal, 2005, Mizuta et al., 2015). However, the OM contents in semiarid and arid soils are usually low (Singer, 2007), and therefore, the dominant factor that control the aggregates formation and stability in these soils is the clay fraction. In this case, changes in the attractive or repulsive forces between the clay particles could alter significantly the resistance capacity of the aggregate against destruction forces (Olphen, 1963, Shainberg and Letey, 1984, Lado et al., 2004, Bronick and Lal, 2005, Singer, 2007, Ben-Hur, 2008).

Ghezzehei and Or, 2001, Ben-Hur and Lado, 2008 indicated that soil wetting weakens the adhesion forces inside the aggregates, which, in turn, facilitates their breakdown. Many other studies (Frenkel et al., 1978, Shainberg et al., 1981, Keren, 1989, Edelstein et al., 2010, Buelow et al., 2015) have indicated that rinsing soils that contain expandable clay minerals, such as montmorillonite, with solutions containing low electrolyte concentrations decreases the Ks values of those soils by clay swelling and dispersion mechanisms. In contrast, Greene and Hairsine, 2004, Ben-Hur et al., 2009 found that the Ks reductions could be also a result of aggregate slaking. Slaking occurs during fast wetting of dry soil, when the aggregates are not strong enough to withstand the pressure formed by entrapped air, mechanical impact of running water, rapid heat release and their own differential swelling (Collis-George and Green, 1979). Whereas, the effects of clay dispersion and swelling on Ks values have been well studied and documented only a few studies have investigated the effects of aggregate slaking on Ks during soil leaching.

The chemical, biological and physical properties of soils differ along the soil profile, especially in arid and semiarid regions. For example, for most of the soil types in Israel, the sublayers contain more clay, and have higher salinity and sodicity and lower levels of OM than the top layers (Singer, 2007). Moreover, in most cases, the sublayers are also more compacted and dense than the top layers because of the physical stress imposed by the weight of the upper soil layer on the sublayer (Sheoran et al., 2010, Shabtai et al., 2014, Ball et al., 2015).

However, most of the studies and measurements in the laboratory on aggregate stability and soil Ks have been conducted on soil samples taken from the top layer of a field, usually from depths of 0–0.2 m (Mace and Amrhein, 2001, Lado et al., 2004, Ouyang et al., 2013, Buelow et al., 2015). In other studies, Ks values of topsoil have been measured under external pressure conditions (Jo et al., 2001, Callaghan et al., 2014), to mimic the conditions existing in deep soils in the field.

In the course of many agronomic activities, such as certain types of deep tillage, soils from the sublayer are brought close to the soil surface. Moreover, a global increase in infrastructure construction has led to accumulation of large amounts of waste soils, which were excavated from sublayers (under 0.3 m), when the topsoil (0–0.3 m) is used for rehabilitation purposes in the construction sites. For example, in 2016, the amount of waste soil in the European Union (28 countries) was estimated at 494 × 106 tons (https://ec.europa.eu/eurostat). Recently, it has been suggested to transport the waste soils from the construction sites and adding it as a top layer on degraded agricultural lands to increase the thickness of the soil profile or the land surface level (Kaufmann et al., 2009, van Jaarsveld et al., 2016, Han et al., 2018, Tanner et al., 2018).

We hypothesized that, when subsoils, which have been subjected to high pressures over a long period in the field, will be used as top soils, the effects of their chemical and physical properties on soil structure and Ks values under wetting and leaching conditions may differ from those observed for “native” topsoil. The objective of the present study was to investigate the differences in the mechanisms and processes that affect the structural stability and Ks values of soils from top layers and sublayers, under wetting and leaching conditions. Loess soils from a semiarid region were used in this study because of their high sensitivity to soil-structure changes.

Section snippets

Soil samples

Soil samples were collected from different locations at rural, plain area in the northern Negev, Israel (Fig. 1), a semiarid region with average maximum temperatures of 35 °C in the summer and 8.1 °C in the winter, and average annual rainfall of 200 mm, falling mainly from October to April. The soil type in this region is loessial brown soil (Calcisols) generated mainly by loess deposits (Roskin et al., 2014), and with montmorillonite as the dominant clay type (Singer, 2007).

Top-layer soil

The studied soil samples

The studied soils, which were sampled from the top and sub-layers, varied widely with their chemical and physical properties (Table 1). The dry conditions in the sampling area were the main causes for the low organic matter contents in the soil samples, and atmospheric deposition of dust enrich with salty minerals (Singer, 2007) were, most likely, the sources for the high variation in the salinity and sodicity values in the soil samples (Table 1).

Saturated hydraulic conductivity

The average Ks values of the different soil

Conclusions

  • 1.

    Wetting and leaching of the studied soil samples with SS changed their structure, due to slaking and swelling processes, which led to reduced Ks. These changes in soil structure and Ks reduction differed in the soils from the top and sublayers; the effect of the slaking process on Ks reduction was stronger in the soils from the top layer than in those from the sublayer.

  • 2.

    Under soil wetting and leaching with SS, the soil swelling is caused mainly by penetration of water molecules into the

Funding

This study was supported by the Israeli Ministry of Agriculture and Rural Development (grant no. 44-01-0003).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Haim Tenaw, Roee Katzir and Jenny Sayegh for their assistance with the hydraulic-conductivity experiments and aggregate-destruction measures, Elazar Volk and Alon Ronen for their assistance with the analysis of the Agisoft Metashape software data and Ehud Roitman, the site engineer for the Road 31 widening project, for his collaboration and assistance in supplying the soil samples used in this study.

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