Pore space characteristics of soil microaggregates – Possible implications for functioning

Soil microaggregates (SMA) are characterized by a pronounced small-scale structural heterogeneity, with recognizable chemical differences between the aggregate`s interior and its surface. Latter suggests a deterministic spatial pattern with respect to C stabilization, element exchange, and habitat function for microorganisms. Here, a detailed characterization of the pore space is crucial for the understanding of element transfer and microbial colonization in SMA. In our study, the 53-250 µm size fraction of SMA isolated along a soil clay content gradient (19-35%) were investigated in terms of their pore space characteristics. For the visualization of connected “open” pore structures as well as “closed” pores, a modified Hg-porosimetry technique utilizing Wood´s metal was used (WIP). The molten alloy was pressed into accessible connective pores by applying an argon pressure of 55 MPa, filling up pores with a diameter down to ≈20 nm. After solidification of the alloy, polished sections of SMA were analyzed by laser scanning confocal microscopy (Keyence, VK-9700). To image and quantify open and closed pores, grayscale-histograms were segmented and three pore size classes (<10, 10-100, and >100 µm²) were distinguished for open and closed pore systems. Additionally, we scanned 27 samples with high-resolution X-ray tomography (CT, Zeiss Xradia 520 versa) to characterize the 3D pore features at resolutions between 480 and 928 nm. SMA typically consist of two different sections, where particle arrangements are loose or dense. Relatively coarse-sized aggregate-forming materials were observed in sections with loose particle arrangements, where pores appear well connected. To some extent, these coarse aggregate-forming materials are arranged in larger circular structures. In contrast, dense particle arrangements consist primarily of fine aggregate-forming materials. The total porosity of the SMA derived by WIP was highly variable with a maximum of 40 area-%. While CT aggregate volume and CT aggregate surface area did not change with clay content, CT-porosity (vol.-%) increased with increasing clay content. Maximum CT porosity of 27 % was found in the samples with the highest clay content. Maximum pore diameter was similar across all clay contents, but the share of macropores with diameters >10 µm increased with increasing clay content. The Euler number decreased with increasing clay content, which indicates an increased connectivity of the pore space. Another parameter that increased with increasing clay content was the CT aggregate volume / CT internal pore surface area ratio, signifying more accessible surfaces for element exchange and/or C storage. While pores exceeding 100 µm² had the highest share within the open pores, it was the pore system <10 µm² for the closed pores. The proportion of closed pores of total porosity was smaller for the finer SMA sizes within the 53-250 µm fraction, which confirms the CT results (increasing Euler number). Our WIP data reveal that higher shares of clay minerals in SMA cause a narrower pore size distribution with smaller average diameters and increased tortuosity. Consequently, element transport and habitation by microorganisms might be slowed down in smaller, more clay-rich SMA, potentially resulting in larger C conservation within the interior of smaller SMA.