Analysis of porosity and pore connectivity of soil microaggregates by intrusion of the molten alloy Wood´s metal

The limited exchange of dissolved matter with the interior of soil microaggregates (SMA) due to narrow pores of high tortuosity result in spatial biogeochemical gradients from outer towards inner SMA surfaces. The goal of our study was to analyze pore space characteristics in SMA by visualization and quantification of connected “open” versus closed pore structures in each of the pore size classes <10, 10-100 and >100 µm². SMA ranging in size between 20 and 250 µm were isolated along a soil clay content gradient (19-35%) and investigated by using a porosimetry technique with a molten alloy, called Wood´s metal, which solidifies below 78°C. The alloy was pressed into connective pores of SMAs by applying an Ar pressure of 55 MPa, allowing an intrusion of pores with a diameter below ~20 nm. After alloy solidification, polished sections of intruded SMA were analyzed by confocal laser scanning microscopy (Keyence, VK-9700) and the ImageJ software was used to quantify open and closed pores by segmenting grayscale histograms. The SMA typically consisted of two different sections, where particle arrangements are loose or dense. Coarse-sized aggregate-forming materials were observed in sections with loose particle arrangements, where pores appear well connected. Part of the coarse SMAs >100 µm show large circular structures with dense particle arrangements in the edge region. These dense arrangements contain high shares of layer silicates and reveal closed pore structures. The total SMA porosity was up to 40 area-%. The share of closed pores <20 nm in diameter ranged typically from 20 to 40 % of total porosity. Pore systems >100 µm² had the highest share within the open pores, while pore system <10 µm² showed the largest share of closed pores. The proportion of closed pores to total porosity decreased for SMA sizes < 50 µm. Our results show that, in addition to an extended connective pore system, SMA also exhibit sections with high contents of clay minerals, where the pore size distribution is narrow (<20 nm) and tortuosity high. Here, element transport and habitation by microorganisms might be impaired, resulting in larger conservation potential for organic matter.