Lime application effects on soil aggregate properties: Use of the mean weight diameter and synchrotron-based X-ray μCT techniques
Introduction
Soils that offer good structure for plant growth and adequate water storage and transport are characterized by the presence of stable aggregates (Hillel, 2004). These stable aggregates are responsible for the presence of inter and intra-aggregate pores inside the soil. Inter-aggregate pores, in turn, may respond differently from intra-aggregate pores to human and natural actions (Kutílek, 2004).
The mean weight diameter (MWD) is the most widely used index in relating aggregate size to stability (Nimmo and Perkins, 2002). Higher MWD indicates the dominance of less erodible, larger aggregates in the soil and, therefore, greater aggregate stability (Piccolo et al., 1997). However, this index does not offer any type of information about the porous space inside the aggregates. An important question is whether different aggregate sizes are associated with distinct soil pore space properties (e.g. porosity, connectivity and tortuosity of pores). Zhou et al. (2017), as an example, recently investigated effects of fertilizers on the bimodality of the soil pore structure by combining porosities of aggregates and core samples determined by computed microtomography. They observed differences only in the inter-aggregate porosity, but not in the intra-aggregate porosity between the different sample sizes.
Individual aggregates are usually denser than a bulk sample (Dexter, 1988; Horn, 1990). Tiny soil aggregates are mainly characterized by the presence of intra-aggregate pores, which have great influence on the water hydrostatics and hydrodynamics (Tisdall and Oades, 1982; Kutílek et al., 2006). Therefore, the characterization of these pores is important from the environmental and physical points of view due to their relevance in other soil physical properties (Wang et al., 2012).
An important tool that can be used to evaluate the porous system of tiny soil aggregates is the X-ray computed microtomography (μCT). Synchrotron-based μCT is especially interesting given the spatial resolution achieved (Wildenschild et al., 2013). μCT has been used to investigate the influence of management systems in the soil structure, the long term vegetation restoration effect on soil aggregate structure and the organic matter distribution in terms of pore networks inside soil aggregates, to cite a few examples (Ngom et al., 2011; Zhou et al., 2012; Peth et al., 2014). Additionally, by applying concepts of mathematical morphology, geometrical characteristics of the intra-aggregate pore space such as its connectivity, elongation, tortuosity, lacunarity and fractal dimension can be evaluated through μCT (Martínez et al., 2015; Tseng et al., 2018; Borges et al., 2018; Pires et al., 2017; Passoni et al., 2015; Chakraborti et al., 2003).
Liming is a soil treatment frequently used to reduce acidity problems, which reportedly affects soil structure, involving for example changes in soil biota and timing of the lime application (Holland et al., 2018). Liming was found to strengthen bonding related to water stability of aggregates (>0.25 mm), which may be attributed to calcium (Ca) ion bridging between organic matter and clay mineral surfaces (Chan and Heenan, 1999). Ferreira et al. (2018a) showed that surface liming changed soil attenuation properties, for γ-ray radiation (59.5 keV and 661.6 keV), as influenced by the increase of Ca contents at the limed soil areas. In addition, there is evidence that aggregate formation, porosity and chemical properties are strongly related (Regelink et al., 2015) but, to this date, there are few scientific contributions concerning effects of soil amendments, including lime, specifically focusing on the intra-aggregate pore space (Zhou et al., 2013; Naveed et al., 2014; Wang et al., 2017).
The aim of this research was to identify the effects of soil surface liming (rates of 0, 10 and 15 t ha−1) on the porous system of soil aggregates (diameters of 2–4 and 1–2 mm) by μCT. Analyses of porosity, connectivity, tortuosity, fractal dimension, and distributions of normalized porosity and number of pores as function of pore volume classes, were performed. Additionally, the mean weight diameter was determined to verify effects on soil aggregation, and XRF was used in an attempt to detect changes in elemental composition of soil aggregates.
Section snippets
Material and methods
The soil samples were collected from a rural site (25°28′S, 50°54′W, 821 m above sea level) located in the SE region of the Paraná State, Brazil. The soil was classified as Dystrudept silty-clay (Soil Survey Staff, 2013; Ferreira et al., 2018a, Ferreira et al., 2018b).
The study was established in May 2012 in a soil under no-till system (NTS). Lime rates of 0 (L0), 10 (L10), and 15 (L15) t ha−1 were applied on the surface without disturbing the soil. The lime used had 285 g kg−1 of CaO, 200 g kg
Results and discussion
From the distribution of MWD values shown in Fig. 4, it is noticed that all means are between 8 mm and 10 mm, which belongs to the largest aggregate size class (c1). A few other studies have determined the MWD for soils under NTS, using a methodology similar to the one adopted here, and showed that NTS promotes indeed macroaggregation. For example, Madari et al. (2005) reported means of 7.9–5.4 mm, in different depths from 0 to 20 cm; Tivet et al. (2013) reported means in the range of
Conclusions
Surface liming improved soil aggregation by increasing the mean weight diameter at shallow depth, layer A (0–10 cm), but not deeper in the soil profile at layer B (10–20 cm). Calcium (Ca) was identified by XRF in aggregates selected from the wet sieving method with equivalent diameters ranging from 0.053 to 2 mm for all lime treatments (L0, L10, and L15) and both layers. However, the percentage of Ca in these aggregates progressively increased with the lime rates at layer A, where aggregates
Acknowledgments
This study was financed in part by the “Coordenação de Aperfeiçoamento de Pessoal de Nível Superior” - Brazil (CAPES) - Finance Code 001. LFP thanks CNPq (“Conselho Nacional de Desenvolvimento Científico e Tecnológico”) and CAPES through Grants 303726/2015-6 (Productivity in Research) and 88881.119578/2016-01 (Visiting Scholar). TRF was supported by CAPES Doctorate Sandwich Program (88881.132800/2016-01). This research used resources of the Brazilian Synchrotron Light Laboratory (LNLS) (under
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