Texture and degree of compactness effect on the pore size distribution in weathered tropical soils

Highlights

• Impact of degree of compactness on pore size distribution is soil texture-dependent.

• Reductions in the macroporosity are mitigated by silt plus clay content.

• Mesopores were predominant in soils with lower silt plus clay content.

• Only macro- and micropores were observed in soils with high silt plus clay content.

• Microaggregation play a role on the pore size distribution of weathered soils.

Abstract

Soil pores can be functionally classified into macro-, meso- and micropores. The volume of these pores is highly affected by constitutive and state components of soils, such as particle size distribution and degree of compactness. In this study, we aimed to examine the impact of soil texture and degree of compactness on total porosity, and distribution of macro-, meso- and micropores. Undisturbed soil cores were taken in eight differently textured soils cultivated with sugarcane and equilibrated at matric potentials of − 30 and − 100 hPa for calculation of macropores (effective pore diameter – EPD > 100 µm), mesopores (EPD 100–30 µm) and micropores (EPD < 30 µm). The volume of each pore category was modelled using additive spline functions of soil particle size and degree of compactness. Soil organic carbon and mean weight diameter of soil aggregate (MWD) were also measured to support the findings. Total porosity and macropores were dependent on the degree of compactness and silt plus clay content, whereas silt plus clay content significantly affected meso- and micropores. Total porosity and macroporosity decreased with increasing compactness, mainly for soils with silt plus clay content lower than ~500 g kg⁻¹; especially for the macropores, these decreases were substantially reduced for further increase in silt plus clay content (> 500 g kg⁻¹) due to the higher MWD verified for these soils. Mesopores were predominant in soils with lower silt plus clay content, while their volume was practically null in soils with silt plus clay content higher than 500 g kg⁻¹, for which only macro- and micropores were observed. In conclusion, the increased degree of compactness may reduce both total porosity and macroporosity in a process whose magnitude depends on soil texture, whereas meso- and micropores have major changes associated only with particle size distribution. However, macropores showed to be also sensitive to soil aggregation rather than properly changes in total soil volume (e.g., degree of compactness).

Introduction

Soil porosity drives many of the soil functions, including water and air fluxes, which are important for regulating gas exchanges between the soil and the atmosphere as well as the movement and retention of water in the soil (Beven and Germann, 1982, Asare et al., 1999, Berisso et al., 2012). These processes are essential for maintaining aeration and water availability for the life of soil organisms and for root growth (Pandey et al., 2021, de Lima et al., 2020b).

The complex relationship between pore geometry and water retention has led to a number of indirect ways of classifying pore size space (Luxmoore, 1981, Beven and Germann, 1982). Koorevaar et al. (1983) divide the porous system into macropores, mesopores and micropores, where they classify macropores (> 100 µm diameter) as those drained at matric potential of − 30 hPa and micropores (< 30 µm diameter) as those with water retained at the matric potential of − 100 hPa, respectively, while mesopores (100–30 µm diameter) is the difference between soil water content at − 30 and − 100 hPa. According to Koorevaar et al. (1983), macropores would be functionally related to water conduction during flooding and ponding rain, consequently affecting aeration and drainage, whereas mesopores would be effective in conducting water also after the macropores have become empty. Finally, the remaining soil solution is retained or moves very slowly through the micropores, whereas part of this water may be taken up by plant roots.

The volume of these pores is highly affected by constitutive and state components of soils, such as soil particle size distribution and degree of compactness. Soil particles consist of mineral and organic components, whose distribution is known to affect the state of soil aggregation (Six et al., 2004, Martinez and de Souza, 2020). Under tropical soils, typically oxide-rich soils, aggregation is strongly affected by iron and aluminium oxides, which has a particular influence on pore size distribution compared to temperate soils (Tomasella et al., 2000; Six et al., 2004; Carducci et al., 2012). Both mineral and organic components affect the state of compactness, which is also sensitive to tillage practices and field traffic (Reichert et al., 2009, Berisso et al., 2012, de Lima et al., 2017). In addition to changing the total pore volume, soil compaction could cause a rearrangement in pore size distribution during stress transmission by agricultural machinery (Berisso et al., 2012, Mossadeghi‐Björklund et al., 2019), such that either large or capillary pores may be affected (Schjønning et al., 2015) in a process whose magnitude depends on soil particle size distribution and mineralogy, which are predominantly distinct between tropical and temperate soils.

The determination of a defined state of compactness is not easy to assess across a range of soil textures because, as previously mentioned, total porosity is dependent on both the state of compactness and texture (Håkansson, 1990, Håkansson and Lipiec, 2000). Using soil bulk density as a degree of compactness indicator would not be satisfactory because compacted soils could be found with bulk density at around 1.30 Mg m⁻³ for clayey soils and uncompacted for sandy ones (Reichert et al., 2009; Keller and Håkansson, 2010). Knowledge of the state of compactness for a range of soil textures can therefore be determined relative to soil bulk density through the degree of compactness, which is calculated by the relationship between soil bulk density and reference soil bulk density of a given soil (Håkansson, 1990, Håkansson and Lipiec, 2000, de Lima et al., 2017).

Although it is well known that soil texture and compaction affect pore size distribution (e.g., Håkansson, 1990; Håkansson and Lipiec, 2000), the magnitude of these changes across a large range of textures combined with different degrees of compactness has not been much investigated in tropical soils. Furthermore, it is still not clear which pore fractions would be significantly impacted by compaction in oxide dominated soils, for which the presence of abundant micro-peds makes these soils resemble coarse-textured soils, reflecting the behaviour of a soil with the presence of macropores (Tawornpruek et al., 2005).

Some studies report that compaction essentially reduces macropores (Alakukku, 1996, Colombi et al., 2017, Feng et al., 2019), while it would only cause minor variations in microporosity (Alakukku, 1996, de Lima et al., 2020a). For mesopores, only a few studies have reported changes due to compaction (e.g. Mossadeghi‐Björklund et al., 2019). Thus, we hypothesised that the increase in the degree of compactness would cause significant changes in macro-, meso- and micropores regardless of their textural composition in oxide-rich tropical soils. In this context, this study was designed to examine the impact of a wide range of differently textured soils and degree of compactness on soil total porosity, and distribution of macro-, meso- and micropores in oxide dominated tropical soils.