ATMOSPHERIC INPUT OF DUST AND NITROGEN INTO THE NIZZANA SAND DUNE ECOSYSTEM, NORTH-WESTERN NEGEV, ISRAEL

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 tes t site in the southern part of the north-western Negev dune field. The main approach was to investigate the contribution of atmospheric inpu t to the spatial distribution of a dune stabilizing biological crust. Atmospheric input of dust at Nizzana is fairly low and originates, exc ept for singular long-range transport dust storm events, mainly from l ocal sources. Spatial input patterns within the dune field are generat ed by topographically forced disturbance of the local wind field. Net deposition rates are about 6% of bulk dust deposition and roughly corr espond to the overall input pattern. Most effective is the depositiona l balance on the encrusted stable slope units and within higher vegeta tion stands. Surface fine material contents on the encrusted slopes ob viously 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. Ac tive growth of cryptogames in aerodynamically stable environmental uni ts 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 overa ll input of total nitrogen is shown to be a function of regional phase s of soil cultivation combined with strong north-westerly winds during the summer half year. At Nizzana-South both water and nitrogen are li miting factors for plant growth whereas in the northern dune field pot ential nitrogen fixation rates increase exponentially relative to the rainfall gradient, but low atmospheric input of nitrogen still is a li miting factor. (C) 1997 Academic Press Limited.