Abstract
Soil bioturbation is associated with the production of soil macropores that influence numerous ecological functions such as those associated with water infiltration and the generation of runoff water. This impact is especially important on sloping lands in the tropics that are highly susceptible to erosion. In this study, we questioned the influence of soil biodiversity on soil macropore properties (> 20 mm3) and saturated hydraulic conductivity (Ksat) on sloping land in northern Vietnam. Biostructures found at the soil surface (casts, sheetings and soil excavated on the ground) were used to identify areas colonized either by earthworms, termites or dung beetles, respectively. The influence of soil macrofauna on Ksat was measured in situ using the Beerkan method below bioturbated zones and compared to the surrounding soil without visible biostructures at the soil surface. Undisturbed soil columns were afterwards sampled and scanned by X-ray computed tomography (X-CT). Properties of macropores below each biostructure depicted a large variability, revealing the complexity of the macropore network. Further, galleries made by termites, dung beetles and earthworms were manually isolated from the rest of macroporosity. Galleries made by beetles, termites and earthworms were clearly differentiated on the basis of their diameter, verticality, sphericity, tortuosity, length and number of branches and the fraction of galleries in the top part of the column and. Ksat was most increased by dung beetles (45-fold), then by termites (30-fold) and to a lesser extent by earthworms (16-fold). Relationships between total macropore properties and Ksat showed that the most important properties explaining Ksat were (i) the volume of percolating macropores, (ii) the diameter, (iii) the critical macropore diameter and (iv) the number of macropores. In conclusion, this study confirmed not only the interest in using X-CT for the quantification of macroporosity but also the absence of a clear relationship between aboveground biostructures and macropore properties and functional impacts.
Highlights
Soil structure regulates many key ecological processes in soils, such as those influencing the habitat of soil organisms, the growth of roots, the protection of carbon, the release of mineral nutrients or the infiltration and diffusion of water in soil
Ants were in all the treatments and in large numbers, especially in the Ctrl treatment, they could not be clearly associated with any specific galleries
Termites belonged to soil-feeding termites in Ctrl, while they belonged to the fungus-growing termite taxon in termite sheetings (TS)
Summary
Soil structure regulates many key ecological processes in soils, such as those influencing the habitat of soil organisms, the growth of roots, the protection of carbon, the release of mineral nutrients or the infiltration and diffusion of water in soil. From the solid-phase perspective, the dynamic of soil structure is considered through the organization and dynamic of soil aggregates. This perspective is useful for understanding the habitat of microbes and the dynamics of carbon and nutrients in soil (e.g., Six et al, 2004). The pore-phase perspective considers soil architecture through its voids and the properties of the soil pore network (Young et al, 2001), in particular their influence on the water dynamic (e.g., Beven and Germann, 1982; Jarvis, 2007; Luo et al, 2010). The interest in X-ray CT relies on its description of the pore size distribution, connectivity, continuity, tortuosity and length, which are all considered to influence soil hydraulic properties (Perret et al, 1999; Vogel, 2000; Pierret et al, 2002)
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