Kinetics of water sorption, EPS swelling and evaporation in biological soil crusts on reclaimed temperate dunes, Brandenburg, Germany

Biological soil crusts (BSCs) play a crucial role in regulating water infiltration into surface substrates; however, their exact hydrological effects remain a subject of ongoing debate. This study quantitatively investigates the kinetics of water immobilization through sorption and the swelling behavior of extracellular polymeric substances (EPS) during the wetting of dry BSCs on recultivated temperate dunes in Brandenburg. Crust samples dominated by the green alga Zygogonium ericetorum were collected from an inland dune in the Lusatian post-mining area. Following a multi-week drying phase, rehydration experiments were conducted using deuterium oxide (D₂O) to avoid spectral interference with the O–H groups of the EPS. Using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the time-dependent shift of the O–D stretching vibrations was analyzed to determine the sorption kinetics of the biological crusts relative to an undisturbed mineral control. In parallel, gravimetric measurements monitored evaporation, while the EPS composition was characterized via ATR-FTIR spectroscopy. Spectroscopic analysis identified alginate as the primary organic component of the crust EPS. While the uncoated mineral control showed almost immediate water infiltration, the BSC samples exhibited a pronounced plateau in OD vibrations during the first three minutes, indicating an initial phase of free water at the surface. Between the 3rd and 12th minutes, a distinct redshift toward lower wavenumbers followed, reflecting the ongoing functional group reorganization, hydration, and EPS swelling, until thermodynamic equilibrium was reached after approximately 12 minutes. Gravimetric data also revealed a significantly higher and faster evaporation rate for the BSCs compared to the control. These results demonstrate that the alginate and EPS matrix of the biological soil crust acts as a water reservoir that hydrologically seals the surface, drastically limiting vertical infiltration into deeper soil layers while retaining water in a highly accessible surface pool. Furthermore, it is conceivable that this delayed water immobilization by the swelling EPS matrix provides a vital ecological benefit by buffering the crust microbiome against severe hypo-osmotic shock during rapid rewetting. However, direct microbial analyses are required to confirm this effect. By moving beyond conventional visual assessments or indirect macro-hydraulic inferences, this approach provides unprecedented molecular-scale insights into the rapid physicochemical transition from free to matrix-bound water during EPS hydration. To our knowledge, this study represents the first application of time-resolved diffuse reflectance infrared spectroscopy (DRIFTS) combined with deuterium oxide (D₂O) rehydration to directly track water immobilization kinetics in biological soil crusts. By eliminating spectral interference from biopolymer hydroxyl groups, this approach provides unprecedented insights into the rapid physicochemical transition from free to matrix-bound water.