Atmospheric deposition to a desert ecosystem and its implications for management

The Negev Desert in Israel is a mosaic of macrophytic patches, consisting of shrubs and annual plants growing in a soil mound, and microphytic patches, consisting of algae, cyanobacteria, bacteria, mosses, and lichens growing on a soil crust. Accumulation of soil is often a limiting resource in this ecosystem, so the loose soil mound in the macrophytic patch permits relatively high production of annual plants. To determine whether atmospheric deposition was responsible for soil mound formation, we measured particle deposition in macrophytic and microphytic patches. Our results show that fine particles, consisting mostly of mineral dust, are deposited at similar rates in macrophytic and microphytic patches. However, coarse particles, which are primarily composed of organic material from plant detritus, are deposited at greater rates in macrophytic compared to microphytic patches. The coarse particles have higher concentrations of carbon and nitrogen than the fine particles. Further experimentation showed that much of the runoff water generated from microphytic patches is captured by macrophytic patches downslope. Calculations based on the measured deposition rates suggest that a typical soil mound would require at least 180-290 yr to develop. Since the presence of the soil mound is crucial for annual plant germination and growth, and since annual plants are the major source of productivity in the system, these results imply that the system will be slow to recover from disturbances that reduce the abundance of macrophytic patches, such as livestock grazing and firewood cutting.