Water repellency and pore clogging at early successional stages of microbiotic crusts on inland dunes, Brandenburg, NE Germany

Biological soil crusts play a key role for hydrological processes in many open landscapes. They seal the topsoil and generate surface run-off. Utilization of the mineral substrate at early stages of microbiotic crusts was investigated using scanning electron microscopy (SEM), water repellency indices were measured using the ethanol/water microinfiltrometer method, and steady state water flow was determined on the dry crusts and after 300, 600, 1200 and 1800 s of wetting, thus allowing to follow pore clogging through swelling of extracellular polymeric substances (EPS). It was found that water repellency increases with initial crust development where filamentous cyanobacteria and filamentous green algae were dominating, but decreases as coccal algae, bryophytes and fungi associated with bryophytes emerged. Swelling of EPS took place immediately after wetting, and its influence on steady state water flow was most pronounced when filamentous cyanobacteria and algae partially filled in the matrix pores and enmeshed sand grains, still leaving micropore channels available for free water infiltration, but prior to appearance of coccal algae, bryophytes and associated fungi which formed a dense cover on the surface. It was concluded that a new phase of crust succession was accompanied by easier wettability but slower infiltration. Transition from hydrophobicity to pore clogging as ruling mechanisms causing water run-off may occur during wetting of individual biological crusts, but also during crust succession over time.