The enigmatic enrichment of potassium and magnesium in runoff and floodwater in the Negev: Do biocrusts hold the key?

Regardless of lithology and plant cover, chemical composition of floodwater in the Negev show a consistent enrichment in K and Mg ions, which could not have been explained by the rock or clay minerals or (due to the scarcity of plants) by plant decomposition. Hypothesizing that rock-dwelling (lithobionts) or soil (loess)-dwelling biocrusts may shed light on the phenomena, we conducted sprinkling experiments in the Negev Highlands. Sprinkling was conducted on 4 types of lithobionts: cyanobacteria which inhabit the south-facing bedrock (ENC), epilithic lichens, inhabiting the inclined (EPI) and the flat (EPI) north-facing bedrocks, and endolithic lichens (ENL) inhabiting south-facing boulders. Additional sprinkling took place on two types of soil biocrusts, a mixed crust composed of cyanobacteria, lichens and mosses at the north-facing footslope and a cyanobacterial crust at the more xeric south-facing footslope. The runoff water (of 5 and for 4 plots for each lithobiont and soil biocrust type, respectively) was analyzed for the ionic composition of Na, K, Mg, Ca, NH, Cl, SO, and NO, whereas HCO was calculated. In comparison to rainwater, all habitats (except for K in ENL) showed high enrichment ratios (ERs) in K and Mg, which, unlike the high ERs of the other ions (such as SO that may stem from gypsum dissolution), could not have been explained by the rock lithology, clay or dust composition. It is suggested that following wetting, K, serving for osmoregulation of cells, is released by the crust organisms, being thus responsible for K enrichment, while chlorophyll degradation is responsible for Mg enrichment. It is suggested that rock- and soil-dwelling microorganisms may explain K and Mg enrichment in runoff and floodwater and subsequently in groundwater of the Negev, and possibly in other arid zones worldwide, affecting in turn the quality of irrigation and drinking water.