One-year-long evaluation of non-rainfall water available to soil biocrusts in the Negev Highlands

Soil biocrusts abound in deserts, but yet little agreement exists regarding their water sources. Most researchers regard non-rainfall water (NRW), particularly dew as an important water source for soil biocrusts, but nevertheless, evidence from the Negev suggests that following the nocturnal soil heat efflux, NRW is drastically reduced. The possible contribution of NRW to 3- to 5-mm-tall lichens, which are less impacted by the warm nocturnal surface temperatures, poses an additional challenge. For this end, NRW obtained by cloths attached to the soil surface and cloths located at a 5-mm height above ground were measured, accompanied by periodic temperature measurements. As for the cyanobacteria and the 1-mm-tall lichens, NRW did not reach the necessary threshold required for the performance of net photosynthesis by cyanobacterial (0.1 mm) or chlorolichens (0.05 mm) but was occasionally reached at 5-mm height above ground. Nevertheless, when the average amount obtained at 1-mm intervals (mimicking the length of the lichen thallus) along the 5-mm height was calculated, it either fell below the necessary threshold or was too low during daytime to compensate for the carbon loss during nighttime respiration. The apparent incapability of NRW to provide sufficient water to sustain the 3- to 5-mm-tall lichens in the Negev Highlands is supported by the confinement of these lichens to north-facing rock terraces (ledges), where they may benefit from additional water through runoff. One may therefore conclude that NRW may not be regarded as a meaningful water source for the Negev biocrusts, whether composed of cyanobacteria or of 1- to 5-mm-tall lichens.

Automated summary

There is little consensus on the water sources of soil biocrusts in deserts. Non-rainfall water (NRW), such as dew, is considered important by most researchers, but evidence from the Negev suggests that NRW is significantly reduced after nocturnal soil heat efflux. The ability of NRW to sustain 3- to 5-mm-tall lichens is challenged by their taller counterparts and the limited availability of NRW during daytime.