Structure and pore system in differently managed clayey surface soil as described by micromorphology and image analysis

Abstract

Water infiltration is a crucial process for functioning of vegetated buffer zones (BZ). Structure of a clayey surface soil of three differently managed BZs, i.e., 1) natural with no treatment, 2) harvested once a year and 3) grazed by cattle was investigated in this study. Soil macro pores (> 50 μm) were characterized by qualitative description and quantitative image analysis of soil thin sections in order to assess their capability to water infiltration. Less than 10% of the macro pores consisted of rounded and irregular pores smaller than 300 μm indicating root activity. Instead, macro porosity was clearly dominated by elongated pores characterized by irregularity, expressing the complexity of the pore system. This pore pattern appeared in thin sections as weak or moderate ped separation suggesting good water infiltration when initially dry. Partial accommodation of pores may result in decrease of hydraulic conductivity, as these pores tend to close upon wetting and swelling. In the grazed site a platy structure was observed due to hoof pressure, which may further impair the hydraulic properties of soil. Moisture and temperature related processes (shrink–swell, freeze–thaw, and water saturation) are thought to be conducive to the aggregation and rearrangement of soil structure around the year, resulting in a complex pore system with low intra-aggregate porosity. In addition, wet periods typical of boreal soils result in clay dispersion and formation of aggregate-related pedofeatures of dense infillings, described as fine clay intrusions.

Introduction

Surface soil structure controls the infiltration of water into the deeper soil horizons. Poor water infiltration makes soil prone to surface runoff and, consequently, to erosion. Vegetated buffer zones (BZs) reduce erosion and nutrient transport from agricultural areas to water bodies. The functioning of the BZ is partly based on retarding the movement of water, which is conducive to sedimentation of soil particles and reduces the transport of particle bound phosphorus (Hoffmann et al., 2009). Therefore, soil structure of a BZ should allow adequate infiltration in order to decrease overland water flow. The pore system of the thin surface layer is most critical, because it is exposed to various stresses affecting its structural properties.

Field observations of soil properties and determination of bulk density and porosity using undisturbed core samples are the first steps in the investigation of soil structure. Also soil micromorphology has been used qualitatively for decades to study soil processes and genesis as well as soil structure, as it appears in nature (Brewer, 1976, Kubiena, 1938). In this approach, characteristics of solids, pores and various other soil features are observed in thin sections and described using terminology developed for this purpose (see e.g. Stoops, 2003). Computer aided digital image analysis of thin sections is an efficient tool to quantitatively study differences in soil structure under various management practices or treatments (Murphy et al., 1977, Ringrose-Voase, 1991). For example, changes in pore number, pore size distribution and/or pore shape pattern have been used to compare effects of conventional and zero tillage (Pagliai et al., 1984, Vandenbygaard et al., 1999), influence of earthworms (Vandenbygaart et al., 2000) and application of waste organic matter on soil structure (Pagliai and Antisari, 1992). Image analysis of thin sections as well as computed tomography and magnetic resonance images are modern tools that improve the understanding of soil physical properties and processes (e.g. Kodešová, 2009).

Automatically detected or visually observed pore characteristics are attributed to soil structure and processes acting in soil. As an example, generation of rounded pores (channels) is associated with biological activity, plant roots and burrowing animals. Shrinkage of drying soil generates planar voids (i.e. cracks due to normal stress release), and ice lens formation in freezing soil may generate elongated voids and platy or lenticular structure (Vandenbygaard et al., 1999). In aggregated soils, granules and crumbs tend to form packing voids that are winding, elongated and interconnected, whereas voids between the blocky peds or plates are planar flat voids. Massive structure or planar pores oriented parallel to soil surface may indicate compaction (Bullock and Murphy, 1980). The pore type is essential for root growth, water transport, and air exchange (Bouma et al., 1977, Pagliai et al., 2004, Vandenbygaard et al., 1999). Especially elongated continuous pores contribute to these properties (Pagliai et al., 2004). Although the relationship between pore type and its functionality is well established, characterization of pore systems under various soil types and climatic conditions is yet to be done.

Soils of Finland are characterized by boreal climate and cryic soil temperature regime. The annual precipitation is rather low (around 550 mm), but because the clay soils of the country commonly have a very low hydraulic conductivity in the subsoil they have plenty of redox concentrations and an aquic soil moisture regime, and often also stagnic soil color pattern (FAO, 2006). According to Soil Taxonomy they are thus classified as Cryaquepts (Yli-Halla and Mokma, 2001). The soil is wet several months annually and compaction is a major threat to soil quality during agricultural operations (Alakukku, 1996), and due to cattle trampling (Pietola et al., 2005). However, the topsoil experiences several wetting–drying cycles annually, caused by drought in summer and frost in winter, commonly to the depth of 50–100 cm (Yli-Halla et al., 2009). Clay mineralogy, consisting of mica and vermiculite, is conducive to shrink–swell phenomenon, with coefficients of linear extensibility (COLE) high enough to justify for the Vertic qualifier (FAO, 2006, Yli-Halla et al., 2009). These soil characteristics subject the soil to pedoturbation and structural alterations.

Numerous papers have documented physical properties of boreal clayey soils in terms of bulk density, saturated hydraulic conductivity and total and macro porosity (e.g. Alakukku, 1996, Aura, 1983, Pietola et al., 2005). In those studies, importance of macro porosity on water transport properties and plant growth has been recognized. Messing and Jarvis (1990) reported high seasonal variation in hydraulic conductivity due to structural changes of clay soils in Sweden. Clayey surface soils of Finland are prone to plastic deformation after winter (Räty et al., 2010a). The first shrinkage cycles in spring may cause irreversible changes in inter-aggregate pore system (Rasa et al., 2009). However, studies containing detailed characterization of soil pore system are less abundant. Sveistrup et al. (2005) studied morphological and physical properties of Norwegian silt/silt loam soils and addressed the importance of freezing and desiccation on structure formation of those soils. Yli-Halla et al. (2009) presented a detailed description of a clay soil of Finland, including micromorphology of vertically oriented thin sections, but concentrating on pedogenesis with the emphasis on subsoil horizons. Qualitative and quantitative characterization of the top soil pore system, most vulnerable to natural and human induced alterations, is still lacking.

The aim of this paper was to document soil properties, especially the complexity of the pore system, of three differently managed vegetated BZs on clay soil. To complete information received from undisturbed core samples, quantitative data of pore size, pore number, and shape were obtained using image analysis of soil thin sections. In addition, the micromorphology of thin sections was qualitatively described. This study adds to the scarce information of structural properties of boreal clay soils. More specifically, these results help to assess the effects of different management practices on surface soil properties and functioning of vegetated BZs.