Reviewing and analyzing shrinkage of peat and other organic soils in relation to selected soil properties

Abstract

Peat and other organic soils (e.g., organo‐mineral soils) show distinctive volume changes through desiccation and wetting. Important processes behind volume changes are shrinkage and swelling. There is a long history of studies on shrinkage which were conducted under different schemes for soil descriptions, nomenclatures and parameters, measurement approaches, and terminologies. To date, these studies have not been harmonized in order to compare or predict shrinkage from different soil properties, for example, bulk density or substrate composition. This, however, is necessary to prevent biases in the determination of volume‐based soil properties or for the interpretation of elevation measurements in peatlands, in order to predict carbon dioxide emissions or uptake caused by microbial decomposition or peat formation. This study gives a comprehensive overview of shrinkage studies carried out in the last 100 years. Terminology and approaches are systematically classified. In part I, the concepts for shrinkage characteristics, measurement methods, and model approaches are summarized. Part II is a meta‐analysis of shrinkage studies on peat and other organic soils amended by own measurement data obtained by a three‐dimensional structured light scanner. The results show that maximum shrinkage has a wide range from 11% to 93% and is strongly affected by common soil properties (botanical composition, degree of decomposition, soil organic carbon, and bulk density). Showing a stronger correlation, bulk density was a better predictor than soil organic carbon, but maximum shrinkage showed a large spread over all types of peat and other organic soils and ranges of bulk density and soil organic carbon.

On the accuracy of saturation estimation from electrical measurements of soils with high swelling clay content

“…Archie's laws are empirical relationships established on consolidated sedimentary rocks that are often used to estimate soil water content, saturation, and hydraulic conductivity. In the case of soils with swelling clays, ignoring changes in soil volume will result in erroneous volume-based soil properties (Seidel et al, 2023) and the saturation index (I r ) estimation; thus, incorrect estimation of water content. In our experiment, soil shrinkage during natural evaporation resulted in heterogeneous patterns of electrical conductivity reduction (Figure 3), although we assume that pore-fluid loss was happening in a uniform manner based on our visual observations (Figure 2) of uniform shrinkage.…”

“…Such characteristics result in high cation exchange capacity (CEC) and variable permeability in these types of soils (Revil & Leroy, 2004). Thus, soils containing swelling clayey minerals would be expected to obey different petrophysical relationships between resistivity and saturation than the "clean" (i.e., devoid of any significant concentrations of clayey minerals or organically rich soils) sandstone specimens that Archie used in his experiments (Seidel et al, 2023). Indeed, a well-known characteristic of clayey soils that is not considered in Archie's laws is their high surface conductivity that results from both high CEC and high specific surface area.…”