Exploring the resilience and resistance of soil against hydraulic and osmotic stress in unsaturated column experiments

Sudden or extreme changes in the hydraulic and chemical conditions severely alter water flow and chemical interactions in soil. In response, this may cause an internal erosion of pore space as soil constituents are disaggregated, released and transported, which ultimately even shapes soil horizons. The resilience and resistance of soils against hydraulic and osmotic stress determines their susceptibility to internal erosion. However, the impact of single stress events cannot be observed in field experiments due to a multitude of parallel processes and boundary conditions that change simultaneously. In contrast, unsaturated column experiments using undisturbed soil monoliths offer close-to-natural packing conditions while at the same time providing full control over the boundary conditions.

To investigate how susceptible soils are for internal erosion and thus to the release of (in-)organic soil constituents, unsaturated column experiments were performed with undisturbed topsoil monoliths of a Luvisol and a Regosol formed on loess. Hydraulic and osmotic stress events were simulated by irrigation sequences with two drainage events (desiccation; hydraulic stress), two flow interrupts (ponding; hydraulic stress), and two tracer applications (osmotic stress).

After each stress event, an increase in particle concentration was measured in the effluent, most pronounced when the ionic strength of the influent decreases after the tracer breakthrough. Likewise, the release of soil organic matter (OM) responds predominantly to osmotic stress events and OM fluorescence points to the release of plant derived and microbial processed OM. Moreover, the application of X-ray µ-CT imaging on soil monoliths revealed the alteration of soil structure during the experiment. Especially, the position of secondary carbonates and macropores were identified as useful reference points to reveal structural changes such as pore refilling and soil compaction. In this way, we were able to show how the evolution of soil structure in response to the transport of (in-)organic soil constituents relates to specific hydraulic and osmotic events.