Arid Dune Ecosystems pp 319-336
Nitrogen Input Pathways into Sand Dunes: Biological Fixation and Atmospheric Nitrogen Deposition
In arid and semiarid regions, water availability is considered to be the controlling factor for the productivity and pattern of vegetation. The total biotic and abiotic N pool size of desert ecosystems is lower than in most other ecosystems (Skujins 1981). Several studies have found that even in arid lands, nutrients are critical for plant growth and successions (McLendon and Redente 1992). After good rainy years, nitrogen can become the limiting factor (Trumble and Woodroofe 1954) whereas added nitrogen increased productivity in several experiments in dry areas (Ettershank et al. 1978; Ludwig 1987). The main N input pathways into the ecosystems are atmospheric deposition in wet, dry and gaseous forms, and the biological fixation of atmospheric nitrogen N2. Biological fixation is carried out by free-living bacteria, Fabaceae—Rhizobium symbiosis and associative symbiontic free-living cyanobacteria, as well as by cyanobacteria in lichens. Another N source is by non-leguminous nitrogen-fixing species; particularly shrubs and trees play a major role in these ecosystems (Schulze et al. 1991; Valladares et al. 2002).
In this paper, we present field measurements of biological N2 fixation (BNF) obtained by the natural 15N abundance method, and use these to estimate the annual nitrogen input by the soil crusts and R. raetam. We follow a novel approach for the natural 15N abundance technique, by using the non-N2-fixing lichens Squamarina lentigeria and S. cartilaginea (=S. crassa) as reference in order to determine N2 fixation by the biological crust in situ in the Negev desert. N input by BNF of atmospheric nitrogen is compared with atmospheric nitrogen deposition.
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