Exopolysaccharides may increase biocrust rigidity and induce runoff generation

The potential of biocrusts (biological soil crusts) to generate runoff is a matter of great controversy, especially in arid and semiarid regions. Most authors maintain that runoff in the presence of biocrusts is generated as a result of hydrophobicity, crust structure, texture or surface roughness. Hypothesizing that runoff over biocrusts may result from partial pore clogging, brought on by extracellular polymeric substances (EPS), we compare two crust types from Nizzana in the northwestern Negev, Israel (NIZ(ID) , NIZ(PON)) that readily generate runoff under natural field conditions to a crust type from Nizzanim in the southern coast of Israel (NIMID) that fails to generate runoff. We hypothesize that biocrust-induced EPS of the NIZ crusts will have different biochemical and viscoelastic properties than that of NIM. We conducted sprinkling experiments on the crusts, measured the biochemical properties (thickness, water repellency, compressive strength, water-holding capacity, silt and clay content, roughness) and the adhesion and viscoelastic properties of the crust EPS using quartz crystal microbalance with dissipation monitoring (QCM-D). Both crusts were not water repellent and were characterized by a relatively smooth surface. The NIZ crusts were however characterized by higher water-holding capacity and compressive strength. As far as the EPS is concerned, higher shear viscosity and shear modulus characterized the NIZ crusts, indicating higher crust integrity and rigidity and the formation of durable mineral grain-EPS matrix of the NIZ crusts. The findings point to a possible link between crust rigidity and runoff coefficients providing supporting evident for the role played by EPS in crust formation and runoff generation. Knowledge regarding the potentiality of runoff and hence water redistribution may assist in ecosystem management and agroforestry practices.