Bimodal Soil Pore Structure Investigated by a Combined Soil Water Retention Curve and X‐Ray Computed Tomography Approach

Abstract

Core Ideas Soil water retention curves of the paddy soil were bested fitted with bimodal models. Computed‐tomography imaging and retention curves generated similar bimodal pore structure. Combined use of organic and inorganic fertilizers increased structural porosity. Textural porosity was not affected by fertilization treatments. Well‐structured soils are considered to have bimodal pore structure, including textural pores between particles and structural pores between aggregates. Pore structure can be inferred from the soil water retention curve (SWRC) but our understanding of the 3‐D pore geometry that regulates this curve is limited. This study investigated the pore structure of a paddy soil under different fertilization regimes by both SWRC and X‐ray micro‐computed tomography (micro‐CT) imaging with the aim of comparing the two methods. Undisturbed soil aggregates and cores were collected from the surface layer of long‐term unfertilized (CK), inorganically fertilized (NPK), and organically and inorganically fertilized (NPKOM) paddy soils. Aggregates and cores were scanned with micro‐CT and pore structures were analyzed. The SWRCs were measured on the same CT‐scanned cores. Three unimodal models, three bimodal models, and one trimodal model were evaluated for fitting the SWRC and to derive soil pore size distribution (PSD). Results showed the SWRC of the paddy soil were best fitted with the bimodal lognormal (BLN) and double‐exponential (DE) models, with the derived PSD showing distinct bimodality. The micro‐CT images revealed the hierarchy structure of the paddy soil and a bimodal pattern in the PSDs. The structural porosities from BLN, DE models and CT imaging were consistent and correlated with the natural logarithm of saturated hydraulic conductivity. Long‐term application of NPKOM increased structural porosity though no significant differences were recorded in the textural porosity compared with the NPK and CK treatment, while the latter two showed a near identical pore structure. These results demonstrated the benefits of both approaches especially for soil structures with a bimodal pore structure such as the well‐structured paddy soil.