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Atmospheric input of nutrients, mainly in the form of particulates, is widely accepted as a major but still poorly known contribution to the nutrient budgets of arid ecosystems. As part of integrated ecosystem studies, dust and nitrogen deposition was monitored at the Nizzana test site in the southern part of the north-western Negev dune field. The main approach was to investigate the contribution of atmospheric input to the spatial distribution of a dune stabilizing biological crust. Atmospheric input of dust at Nizzana is fairly low and originates, except for singular long-range transport dust storm events, mainly from local sources. Spatial input patterns within the dune field are generated by topographically forced disturbance of the local wind field. Net deposition rates are about 6% of bulk dust deposition and roughly correspond to the overall input pattern. Most effective is the depositional balance on the encrusted stable slope units and within higher vegetation stands. Surface fine material contents on the encrusted slopes obviously are in a state of equilibrium with net dust deposition rates. However, the spatial pattern of both bulk and net deposition does not coincide with the distribution and thickness of the biogenic crust. Active growth of cryptogames in aerodynamically stable environmental units and differences in the microhydrological budget relative to aspect and slope angle may play a much more important role than fine material input. The biological crusts do not act as a trap for dust. The overall input of total nitrogen is shown to be a function of regional phases of soil cultivation combined with strong north-westerly winds during the summer half year. At Nizzana-South both water and nitrogen are limiting factors for plant growth whereas in the northern dune field potential nitrogen fixation rates increase exponentially relative to the rainfall gradient, but low atmospheric input of nitrogen still is a limiting factor. |
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