Detection of soil water in macropores of undisturbed soil using microfocus X-ray tube computerized tomography (μCT)

Abstract

Minimal information has been garnered regarding the spatial distribution of soil water in relation to pores and the soil matrix. Destructive layer-by-layer reconstructions derived from polished section methodology exclude any data of water in the soil. In contrast, microfocus X-ray tube computerized tomography generates images of the internal structure of the soil with a resolution down to 1 μm, at the same time creating a visual image of the spatial distribution of water in undisturbed soils. As X-rays pass through the soil, some radiation is absorbed, some is scattered, and some is transmitted. Using advanced microfocus computerized tomography (μCT) which ensures controlled and stable output intensity for X-ray emissions and thus a constant focal spot size and spatial resolution, the resulting pattern of radiation detects to a 0.5% contrast difference. While 2D X-ray imaging is sufficient in many cases, 3D images derived from X-ray irradiation of a soil sample can reveal complex inner structures in more comprehensive format, providing information on the causal connection of water and soil structure. Using the X-ray Feinfocus Y.FOX System and related programmes, two- and three-dimensional images of two different soils (Haplic Luvisol and Stagnic Anthrosol) at field capacity (pF 1.8) have been produced which show films of water which are associated with the pore surfaces. The mean thickness of the water films was 10.6 μm in the Stagnic Anthrosol and 3.0 μm in the Haplic Luvisol. These results were unexpected in pores >50 μm since at field capacity only the adhesive water should be present which would create water films in the range of nanometres. Myriads of colloidal dispersed nanoparticles, detected with dark field microscopy and SEM, seem to be the source of the adhesion and cohesion, causing micro-rheological effects which lead to water films of up to 30 μm in pores. Additionally, nanoparticles correlated to the clay content (fine clay) appear to conglomerate in the water films, presumably forming surface protuberances on the films of varying extent.