Abstract:In a recultivation area located in Brandenburg, Germany, five types of biocrusts (initial BSC1, developed BSC2 and BSC3, mosses, lichens) and non-crusted mineral substrate were sampled on tertiary sand deposited in [1985][1986] to investigate hydrologic interactions between crust patches. Crust biomass was lowest in the non-crusted substrate, increased to the initial BSC1 and peaked in the developed BSC2, BSC3, the lichens and the mosses. Water infiltration was highest on the substrate, and decreased to BSC2, BSC1 and BSC3. Non-metric multidimensional scaling revealed that the lichens and BSC3 were associated with water soluble nutrients and with pyrite weathering products, thus representing a high nutrient low hydraulic feedback mode. The mosses and BSC2 represented a low nutrient high hydraulic feedback mode. These feedback mechanisms were considered as synergic, consisting of run-off generating (low hydraulic) and run-on receiving (high hydraulic) BSC patches. Three scenarios for BSC succession were proposed.(1) Initial BSCs sealed the surface until they reached a successional stage (represented by BSC1) from which the development into either of the feedback modes was triggered, (2) initial heterogeneities of the mineral substrate controlled the development of the feedback mode, and (3) complex interactions between lichens and mosses occurred at later stages of system development.

In a global meta study, which contained 157 biological soil crust (BSC) samples from all climatic regions worldwide, climatic and soil texture threshold values favoring BSC development were derived. Response variable was the "relative crust biomass", which was computed per literature source as the ratio between each individual crust biomass value of the given study to the study maximum value reported. Four crust types were distinguished: cyanobacterial, lichen, green algal and moss crusts. To quantify threshold conditions at which crust biomass responded to differences in texture and climate, we (I) determined correlations between bioclimatic variables, (II) calculated generalized linear models to determine the effect of typical climatic variables with soil clay content and with study site as a random effect. (III) Thresholds of climatic and texture effects were identified using a regression tree. Three mean annual temperature classes for texture dependent BSC growth limitation were identified: (1) <9• C with a threshold value of 25% silt and clay (limited growth on coarser soils), (2) 9-19• C, where texture did have no influence on relative crust biomass, and (3) >19• C at soils with <4 or >17% silt and clay. Non of the precipitation related bioclimatic variables did influence the relative crust biomass significantly. Because biocrust development is limited under certain climatic and soil texture conditions, it is suggested to consider soil texture in biogeochemical modeling of cryptogamic ground covers.

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