Essential nutritional ions show enrichment (K+, Mg2+) and depletion (NO3-) in runoff water generated by biocrusts covering arid sand dunes under natural rain conditions

The role of runoff in providing nutrients to runon loci in deserts was not extensively explored. Here we report 2 years of measurements of the chemical composition of rain and runoff (three events for each year, which correspond to the long-term runoff events) in plots that were constructed over four biocrust types in the Negev Desert, Israel. The enrichment ratios showed high variability, being high for K+ (3.7), moderately high for NH4+ (1.6) and slightly high for Mg2+ and SO42- (1.2). It was low for Cl- and NO3- (0.5), moderately lower for Ca2+ (0.7) and slightly lower for Na+ and HCO3- (0.8). When examined per rain event, significant higher concentrations were found for K+ and Mg2+ while NO3- exhibited significantly lower concentration. The high enrichment of K+ and the enrichment of Mg2+ may point to a biogenic origin. While K+ enrichment is suggested to result from K+ excretion by the cyanobacteria, bacteria, and possibly by the mosses following cell wetting and the K+ role in cell osmoregulation, decomposition and erosion of the chlorophyll pigment may result in Mg2+ release. On the other hand, the data point out that despite the crust capability to fix nitrogen and thus to provide its own needs for nitrogen, NO3- was depleted from the runoff water, a phenomena that may be explained by the crust preference to utilise available low-cost nitrogen provided by rain. Due to runoff accumulation at small depressions within the interdune and at the dune-interdune interface, runoff may contribute additional amount of nutrients to these habitats. For the dune-interdune interface it may account for an addition of 273.8% and 35.3% of the total potassium and nitrogen, respectively. The addition of water and nutrients may have important contribution to the growth of the moss-dominated biocrusts and the shrubs at the dune-interdune interface, being responsible for the formation of 'mantles and islands of fertility' at the dune-interdune interface. It also points to the possible role that biocrusts may play in agroforestry practices.