Nitrogen fixation in macro- and microphytic patches in the Negev desert

Nitrogen fixation is thought to be a major input to the N budget of deserts, and it is generally considered to be carried out by cyanobacteria in the soil microphytic crust, not by free-living heterotrophic bacteria. We have compared N fixation in Negev desert microphytic soil crusts and macrophytic patch soils. We evaluated four different types of crusts in two sites, which vary in their rainfall amount from cyanobacterial-dominated crusts in the dry (<100 mm annual rainfall) area to mixed communities of cyanobacteria, lichens and mosses in the wetter (up to 200 mm annual rainfall) area. We also evaluated five different soil-litter size classes of material from macrophytic patch soils which represented materials in differential states of decomposition. We observed higher rates of fixation in soil from macrophytic patches than in soil crust material. These results suggest that free-living N fixers in macrophytic patches are important to the Negev N budget if desert patchiness is taken into consideration. For example, where macrophytic patches cover 25% of the soil surface, these patches may contribute approximately 40% of the total N fixation in the desert landscape. This contribution is regulated by natural and human factors that influence the extent of macrophytic patches, e.g. average annual rainfall, (over)grazing and restoration activities.