A BREAK IN THE NITROGEN-CYCLE IN ARIDLANDS - EVIDENCE FROM DELTA-N-15OF SOILS

We examined the content and isotopic composition of nitrogen within so ils of a juniper woodland and found that a cryptobiotic crust composed of cyanobacteria, lichens, and mosses was the predominant source of n itrogen for this ecosystem. Disturbance of the crust has resulted in c onsiderable spatial variability in soil nitrogen content and isotopic composition; inter-canopy soils were significantly depleted in nitroge n and had greater abundance of N-15 compared to intra-canopy soils. Va riations in the N-15/N-14 ratio for inter- and intra-canopy locations followed similar Rayleigh distillation curves, indicating that the gre ater N-15/N-14 ratios for inter-canopy soils were due to relatively gr eater net nitrogen loss. Coverage of cryptobiotic crusts has been redu ced by anthropogenic activities during the past century, and our resul ts suggest that destruction of the cryptobiotic crust may ultimately r esult in ecosystem degradation through elimination of the predominant source of nitrogen input.