Estimating soil porosity and pore size distribution changes due to wetting-drying cycles by morphometric image analysis
Introduction
Soils in natural conditions such as those found in areas under native forest are generally characterized by complex pore arrangements, mainly in the top layers, where the flora and the fauna have a direct impact. Soils submitted to different management practices tend to present modifications in their porous spaces due to the action of the machinery used to prepare it and the crop introduction (Bodner et al., 2013a, 2013b; Boyle and Powers, 2013).
Contrasting management practices can cause continuous changes in the physical properties of the soil, thus resulting in modifications mainly in its porous system with impacts in soil functionality and sustainability (de Andrade Bonetti et al. 2017; de Moraes et al., 2016; Bavoso et al., 2012). After these changes, the action of natural or artificial processes can contribute to soil recovering its functional and structural characteristics. According to de Moraes et al. (2016) and de Andrade Bonetti et al. (2017), soils under long-term no-tillage or with high contents of organic matter have a great ability to return to its original state or as close as possible to it. However, Bavoso et al. (2012) stated that the soil capacity to recover its quality is strongly dynamic and influenced by several different factors.
Wetting and drying (W-D) cycles represent an important process of modification of the soil structure and, consequently, of its porous system. Several physical properties, such as soil bulk density, total porosity, macroporosity, pore size, and shape distributions, are affected by the W-D cycles (Diel and Vogel, 2019; Yiping et al., 2016; Bodner et al., 2013a; Pires et al., 2009, 2008). Soils usually are submitted to W-D cycles as the effect of irrigation practices and the natural water regime processes (e.g., rain and evapotranspiration). Also, measurements of water retention properties such as the soil water retention curve involve the application of several W-D cycles to the soil samples, depending on the method employed (Pires et al., 2009; Bacchi et al., 1998). Jayanth et al. (2012) and Al-Kayssi (2016) reported that water retention properties are strongly influenced and can even be altered by intensive W-D cycles. Thereby, the application of W-D cycles can cause different responses in the porous system of soils under contrasting management systems during water retention properties evaluation, with impacts on the quality of -the acquired data. Thus, the use of techniques such as computed microtomography (μCT) makes it possible to complement the information regarding such modifications at a different scale of measurement, in general, few micrometers.
Methodologies that apply image analysis such as μCT have been widely used in the micromorphological analysis of porous systems such as soils (Pires et al., 2020, 2017a; Diel and Vogel, 2019; Galdos et al., 2019; Borges and Pires, 2012). This technique has enabled the quantification and estimation of image-based porosity, pore size distribution curves, and the characterization of the porous system based on porous functionality (Ferreira et al., 2019; Borges and Pires, 2012; Peth et al., 2008; Vogel and Roth, 2001; Capowiez et al., 1998).
The size of a soil pore determines its role in water transport and retention characteristics. The distribution of pores based on their sizes (i.e., the percentage of pores related to its equivalent diameter) defines what is known as the soil pore size distribution (PSD). The PSD is frequently used to follow and investigate modifications caused by climatic or anthropogenic interventions in the soil porous system (Chandrasekhar et al., 2019). It essentially allows the distinction between the distribution of micro and macro pores and somehow reflects the heterogeneity of the soil porous system. Heterogeneous porous media are frequently characterized by having pores distributed in a wide range of characteristic diameters and vice versa.
This kind of analysis is relevant due to the importance of the PSD considering the physical quality of any porous system. Image analysis methods based on three-dimensional (3-D) μCT data also enabled the proposition of models for the study of distribution, pore morphology, movement and fluid retention (liquid/gas) inside the soil, and porous system complexity (tortuosity and connectivity), among other studies (Diel and Vogel, 2019; Calistru and Jităreanu, 2015; Pires et al., 2013; Yang et al., 2014; Yang et al., 2009).
Researchers worldwide have worked on computer simulations based on μCT images aiming at quantifying the physical properties of different porous media (Yang et al., 2014; Khan et al., 2012; Garboczi and Bentz, 1990). The algorithms used in such simulations are usually based on experimental techniques, for example, Mercury intrusion porosimetry (MIP) (Dong et al., 2018; Do et al., 2013; Yang et al., 2009; Garboczi and Bentz, 1990). One of the advantages of these simulations is the preservation of the structure of the samples investigated, enabling the collection of valuable morphological information and the observation of the physical properties of such media (Dong et al., 2018; Calistru and Jităreanu, 2015; Khan et al., 2012; Yang et al., 2009; Peth et al., 2008).
The soil porous media is a dynamic system influenced by natural or artificial external processes. Among them, wetting and drying cycles are responsible for important changes in the soil structure. Laboratory and field studies have demonstrated that W-D cycles can greatly influence the soil bulk density and, consequently, the soil total porosity. Some studies have reported that the first W-D cycle usually causes the most important modifications on the soil structure (Kong et al., 2018; Tripathy et al., 2002; Leij et al., 2002). Nonetheless, studies that evaluate changes in the porous system of soils under contrasting management systems submitted to W-D cycles at the micrometric scale and in a 3-D perspective are still scarce in the literature.
Thus, this work aims to help researches to fill this gap by analyzing the modifications that W-D cycles can introduce in the structure of core samples submitted to contrasting management practices at the microscopic level of investigation. This was achieved by analyzing PSDs generated by computer simulations performed on 3-D μCT images. The computer program developed by Yang et al. (2009) was used to obtain the PSD. The soil pores were analyzed regarding their characteristic sizes and the modifications due to W-D cycles could be determined. Therefore, the work sought to relate the modifications observed in the porous system regarding the functionality of the soil pores and their size distribution.
Section snippets
Soil sampling
Soil samples were collected from areas submitted to different management practices located in an experimental station belonging to the “Instituto Agronômico do Paraná” (Agronomic Institute of Parana) at Ponta Grossa, in the southern region of Brazil (25° 09’ S, 50° 09’ W, 875 m a.s.l). The soil studied was a Rhodic Hapludox according to the Soil Survey Staff (2013). The soil was classified as a clay texture with 578 g kg-1 clay, 280 g kg-1 silt, and 142 g kg-1 sand. The management practices
Results and discussion
The PSDs of the samples for the different management practices investigated and for the soil under secondary forest submitted to the W-D cycles are shown in Fig. 2.
The application of W-D cycles caused distinct modifications to both the PSD characteristic amplitudes and the most frequent equivalent diameter of the pores in the different management practices under study. Regarding F and MT, the 12 W-D cycles resulted in a reduction in the amplitude associated with the characteristic pore diameter
Conclusions
The algorithm based on Mercury Intrusion Porosimetry showed to be efficient to quantify the changes caused by W-D cycles in the porous system of the soil samples submitted to different managements. PSDs indicated that the intense application of the cycles produced a decrease in the accessed porosity in the soil core samples under CT and F and an increase under NT and MT.
Such results evidence that the soil cores, even having the same textural characteristics, will respond differently when
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
Luiz F. Pires would like to acknowledge the financial support provided by the Brazilian National Council for Scientific and Technological Development (CNPq)and the Coordination for the Improvement of Higher Education Personnel (Capes)through the Grants 303726/2015-6 (Productivity in Research) and 88881.119578/2016-01 (Visiting Scholar). Special thanks to Prof. Sacha Mooney from the Division of Agricultural and Environmental Sciences, School of Biosciences, University of Nottingham, Sutton
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