Shrinkage curve features to characterize soil structure quality of agricultural soils with contrasting texture and soil organic content

Soil structure is essential to most soil functions. An intact network of soil structural pores allows fast drainage and aeration under wet conditions and ensures water retention in small pores within aggregates under dry conditions. The measurement of shrinkage curves (the relationship between the reduction of soil volume and the loss of water) has the advantage to quantify soil structural features at all scales, ranging from macropores to the interstitial voids of clay minerals. In this study, we measured the shrinkage curves of 24 agricultural soil samples of contrasting texture (ranging from 16% to 43% clay content) and soil organic content (SOC) (ranging from 1.16% to 7.3% SOC), and compared the results with visual evaluation of soil structures (CoreVESS) and X-ray imaging. In addition to the typical ‘S-shaped’ shrinkage curves reported in literature as described in the model of Braudeau et al. (1999) (including the S-shape with an additional linear drop of soil volume in the wet range), several samples showed a ‘double S-shape’ with a well-defined bimodal function or a ‘J-shaped’ curve. Samples with a ‘double-S-shape’ had better soil structural quality as quantified by CoreVESS and were found in samples with high SOC content. These samples also had a large macropore volume according to the X-ray images. The curve in the wet range of the commonly reported ‘S-shape’ reveals the stability of the structures and aggregate arrangements that do not shrink with the onset of water loss and capillary suction and is said to characterize hydrostructural stability. However, the curve in the wet range which is typical for S-shaped curves is lost in samples with poor structure quality (high CoreVESS scores). These poorly structured samples are usually characterized by a linear domain in the wet range, followed by a gradual stabilization of the soil volume at the dry end, thus displaying what we called a  ‘J-shaped’ curve. These curves denote a loss of hydrostructural stability.  Based on the dataset we want to define conditions for the presence of these new forms of shrinkage curves. The different shapes (‘double-S-shape’, ’S-shape’ and ‘J-shape’) could then be used as indicators for soil functions and soil health.