Chemical composition of dew and rain in an extreme desert (Negev): Cobbles serve as sink for nutrients

Dew is important source of water in the Negev Desert, yet little is known about the chemical composition of dew and consequently about its possible role in supplying nutrients to the ecosystem. Dew was collected in Sede Boqer (SB) in the central Negev Highlands and in Nizzana (NIZ) in the western Negev Desert (P = 95 mm) during the fall of 1992 and analyzed for their main ion constituents. In addition, rain was also collected and analyzed. Dew composition was mainly of the Ca–SO4 type and showed high total dissolved ions (TDIs) ranging between 88 and 758 mg/l, which decreased with an increase in dew precipitation. Contrarily, it increased with time since the previous dew event indicating that dissolution of dust particles may have a substantial contribution to the chemical composition of dew. When the ion concentration of dew per millimeter was compared to that of rain, all ions showed a substantial enrichment, of 1.5–7.7. High dew to rain ratios characterized the terrestrial ions and especially the major nutrients N (5.4–6.1), K (3.9–7.6), and P (3.5). When the annual contribution of dew nutrients was calculated for SB, total potential contribution was 2.5, 1.4 and 3.0 times that of rain for N, P, and K, respectively, albeit its low annual amount (∼38 mm). The findings highlight the role of substrates subjected to enhanced condensation such as cobbles and stones, implying that they may serve as sink for nutrients, and may provide an additional explanation for the relatively high plant cover and high species composition in the Negev Highlands. It also implies that a possible decrease in dew precipitation due to global warming may also result in nutrient depletion and subsequently in a reduction in plant cover.