3D analysis of the soil porous architecture under long term contrasting management systems by X-ray computed tomography

Highlights

• Conventional and minimum tillage areas were studied.

• μCT allowed quantifying size and shape of pores.

• One large connected pore was responsible for around 90% of the porosity.

• All tillage systems presented extremely well connected pores.

• Zero tillage had the smallest porosity while conventional tillage the largest.

Abstract

The development of adequate soil structure is important for achieving good physical status, which influences the sustainability of agricultural areas. Different management systems lead to the development of a wide range of soil pore network characteristics. The objective of this research was to analyze the effect of three contrasting tillage systems (zero-tillage, ZT; reduced tillage, RT; conventional tillage, CT) in the soil porous system of an Oxisol. Samples were collected from the surface layer (0–10 cm). An area under secondary forest (F) was also assessed to provide an undisturbed reference. X-ray Computed Tomography (μCT) scanning of undisturbed soil samples and image analysis were employed for analysis of the pore network. The soil under ZT had the smallest porosity in comparison to the other management systems. The conventionally tilled soil had the largest porosity and the most connected pores. One large connected pore was responsible for around 90% of the porosity of the resolvable pores (>35 μm) studied for all the management systems. Pores of elongated shapes, which enhance water movement through the soil, were the most frequent pores in terms of shape.

Introduction

The use of tillage has been employed for centuries to improve soil structure for enhanced crop development. However, the choice of tillage systems can have a significant impact on soil heath and quality. Sustainable farming systems greatly depend on soil quality (Bünemann et al., 2018). Soil tillage provokes substantial changes in several soil physical properties such as total porosity, bulk density, water retention and infiltration, penetration resistance, pore size distribution, connectivity and tortuosity (Imhoff et al., 2010; Daraghmeh et al., 2009; Blanco-Canqui et al., 2004; Katsvairo et al., 2002).

In Brazil the adoption of minimum tillage systems such as reduced (RT) and zero tillage (ZT) is common. The total Brazilian area used in crop production is around 66 million hectares and there are over 31 million hectares under ZT (FEBRAPDP, 2013). Conventional tillage (CT) is characterized by the disruption of the top soil due to ploughing and harrowing operations employed to turn over and loosen the soil. As a result of these operations, macropores are created and pore continuity is disrupted, which directly affect the water movement (e.g. hydraulic conductivity and infiltration) and retention (Blanco-Canqui et al., 2017; Ogunwole et al., 2015; Cássaro et al., 2011; Imhoff et al., 2010). Minimum tillage systems such as RT and ZT do not usually lead to drastic soil structure changes. These systems, known as conservation techniques, have been utilized as a means of reducing tillage and field costs as well as for conserving soil structure due to reduced disturbance (Aziz et al., 2013; Cavalieri et al., 2009). The residues of the previous crop are left intact and the absence of harrowing in ZT and RT can increase soil organic carbon and aggregate stability, reduce CO₂ emissions and moderate fluxes of water, air and heat through the soil (Aziz et al., 2013; Daraghmeh et al., 2009; Zibilske and Bradford, 2007).

The fluxes of water and air, organic matter decomposition, plant-available water and soil resistance to erosion are directly linked to the architecture of the soil porous system. Mesopores and macropores play an important role in these processes (Imhoff et al., 2010; Fuentes et al., 2004; Cameira et al., 2003). In CT, the soil porous system is affected by operations such as ploughing and harrowing, which can increase porosity and loosen soil (Mangalassery et al., 2014). This operation allows good root growth and air exchange, while the exposition of the soil to rain in tropical regions can sometimes lead to erosion (Alvarez et al., 2009). On the other hand, the activity of earthworms and root decay help to create channels and burrows under RT and ZT, which facilitate drainage and gaseous diffusion (Soto-Gómez et al., 2018; Carducci et al., 2017; Pires et al., 2017; Pierret et al., 2002).

Based on the important functions that mesopores and macropores fulfill for a healthy soil, techniques to image and measure key properties such as X-ray Computed Tomography (μCT) are very important (Tseng et al., 2018; Yang et al., 2018; Ferreira et al., 2018; Pagenkemper et al., 2014). The spatial distribution of pores can be non-destructively imaged at high resolutions and in three dimensions (3D) by μCT (e.g. Galdos et al., 2019; Helliwell et al., 2013; Peth et al., 2008). μCT has been previously applied with success to study the size, shape, number, connectivity, degree of anisotropy, macropore thickness, fractal dimension and tortuosity of the soil porous system (Wang et al., 2016; Dal Ferro et al., 2014; Garbout et al., 2013; Vogel, 1997). This provides vital information to characterize the physical structure of the porous system, which allows a better understanding of key processes (i.e. mass and energy transport, nutrient cycling, root development) within the soil (Hillel, 2004).

Previous studies on evaluating the influence of tillage systems at the μm scale in 3D in tropical soils are still scarce. In Brazil, one of the largest food and agricultural producers of the world, previous studies have characterized the soil porous system at μm to measure the porosity and pore size distribution of Brazilian Oxisols (Vaz et al., 2011), assessed the effect of tillage systems on the percentage of macropores (Beraldo et al., 2014) and explored the spatial and morphological configuration of the pore space of Oxisols under CT (Carducci et al., 2017, 2014). Other studies have determined the influence of ZT on the pore size and shape distribution of macropores (Passoni et al., 2015), tested the capacity of soil recovering under different management strategies (Marchini et al., 2015) and measured the impact of ZT and CT on the pore size and shape distribution and water retention (Pires et al., 2017). Recent work has analyzed the soil structure utilizing the geometrical parameters of the soil porous system (Tseng et al., 2018), considered the influence of liming on the structure of aggregates under ZT (Ferreira et al., 2018) and revealed the structural development associated with long term (>30 years) ZT (Galdos et al., 2019).

The objective of this particular research was to apply the X-ray Computed Tomography technique to evaluate, in 3D and at the μm scale, the morphological properties of an Oxisol under contrasting soil management systems. Experimental areas under long term zero-tillage and reduced and conventional tillage systems were investigated. Samples were collected at the soil surface layer (0–10 cm).