Antagonistic effects of drought and sand burial enable the survival of the biocrust moss Bryum argenteum in an arid sandy desert

Biocrust moss is an essential soil surface biocover. It can represent the latest succession stage among the diverse range of surface-dwelling cryptogams (e.g., cyanobacteria, green algae, and lichen, which are also referred to as biocrusts), and it can make a major contribution to soil stability and fertility in many arid sandy desert ecosystems. The soil surface represents a very large ecological niche that is poikilohydric in nature. Biocrust moss is therefore highly susceptible to drought and sand burial, which are two ubiquitous stressors in arid sandy deserts. However, little information is available regarding the mechanism by which biocrust moss can survive and flourish in these habitats when stressed simultaneously by the two stressors. The combined effects of drought and sand burial were evaluated in a field experiment using the predominant biocrust moss, Bryum argenteum Hedw., in the Tengger Desert, China. Drought was simulated by applying distilled water in three artificial rainfall regimes at 8-day intervals in spring and autumn: 4 and 6mm (average rainfall, control), 2 and 3mm (double drought), and 1 and 1.5mm (4-fold drought), respectively. The effect of sand burial was determined by applying six treatments, i.e., sand depths of 0 (control), 0.5, 1, 2, 4, and 10 mm. The four parameters of chlorophyll a content, PSII photochemical efficiency, regeneration potential, and shoot upgrowth were evaluated in the moss. It was found that the combined effects of drought and sand burial did not exacerbate the single negative effects of the four parameters tested. Drought significantly ameliorated the negative effects of deep-sand burial on the retention of chlorophyll a content, PSII photochemical efficiency, and the regeneration potential of B. argenteum. Sand burial diminished and even re-versed the negative effects of drought on the maintenance of chlorophyll a content, PSII photochemical efficiency, and regeneration potential. Although drought and sand burial imposed an additive negative effect on shoot upgrowth, which suggested a trade-off between growth ability and stress tolerance, their mutually antagonistic effect on the physiological vigor of B. argenteum provided an opportunity for the biocrust moss to overcome the two co-occurring stressors. In addition to providing a strong stress tolerance, drought and sand burial may provide an important mechanism for the biodiversity maintenance of biocrust mosses in arid sandy ecosystems.