Characterizing soil pore structure using nitrogen adsorption, mercury intrusion porosimetry, and synchrotron-radiation-based X-ray computed microtomography techniques

Soils have a wide range of pore size distribution (PSD) from nanometer to micrometer scale. The detailed characterization of soil pore structure in a wide pore size range is important for understanding the soil processes. In this study, three different techniques are used to quantitatively describe the soil pore characteristics in a wide pore size range and to evaluate whether different pedogenic processes affect porosity and PSD of the soils. The four different types of soils: black soil (BS, Udic Agriboroll), Shajiang black soil (SBS, Aquic Pelludert), paddy soil (PS, Aeric Endoaqualf), and latosolic red soil (LRS, Typic Kandiudults) were selected to represent the most important soil types in China. A combination technique of nitrogen adsorption isotherm (NAI), mercury intrusion porosimetry (MIP), and synchrotron-radiation-based X-ray computed microtomography (SR-mCT) was used to describe the pore structure characteristics including the porosity, PSD, and pore geometry in the soils. The NAI method revealed larger differences in the Brunauer-Emmett-Teller (BET) surface area and 0.002-0.15 mu m pore volume for the studied soils. The latosolic red soil (LRS) has the largest volume of 0.002-0.15 mu m pore, while BS has the smallest. The PSD determined by MIP exhibited that BS and paddy soil (PS) had multimodal peaks, indicating the existence of a more heterogeneous pore system. The PSD from LRS and SBS exhibited a single, sharply defined peak at a pore diameter from 0.02 to 0.04 and 0.01 to 0.06 mu m, respectively. The volume of the > 3.7 mu m pores determined by the SR-mCT method was in the order of BS (18.2 %) > PS (10.6 %) > LRS (7.2 %) > SBS (5.4 %). The pore shape measured by SR-mCT has an obvious difference in pore system for different soils. Regular pores were more frequent in BS than in SBS, PS, and LRS. Conversely, lower percentage of irregular pores was found in the BS. The combination of NAI, MIP, and SR-mCT techniques can quantify the porosity and PSD of soils over a wide range of pores. For the overlapping pore region, the pore volume determined by the MIP and SR-mCT agreed well. Our results indicated that pedogenic processes can greatly influence the soil pore structure both in terms of PSD and pore shape.