Evaporation characteristics of biocrust layer and its covered soil in arid and semi-arid regions

Biocrust (biological soil crust) is one special type of the most important surface layer cover in the arid and semi-arid regions. There is also a strong impact on the water balance of surface soil. The biocrusts can rapidly fill the degraded patches to stabilize on the surface soil. Moreover, the biocrusts can also change the physicochemical properties of the surface soil, such as the bulk density, total porosity, field capacity, and organic matter content. This study aims to clarify the influence of the biocrust layer (0-2 cm) and its covered soil (0-15 cm) on evaporation in the Loess Plateau. A comparison was also made on the evaporation among the bare sand, cyanobacteria crusts (cyano-crusts), and moss crusts. Indoor simulated experiments were carried out to in-situ monitor the soil moisture. The results showed that there was a great variation in the effects of the different types of biocrusts on soil evaporation. The cyano-crust layer promoted evaporation, whereas, the moss crust layer inhibited it. Specifically, the biocrust layer has significantly enhanced the evaporation at the low intensity of evaporation (150 W). The average evaporation rates of the cyano-crust layer (0.28 mm/h) and moss crust layer (0.30 mm/h) were 12.0% and 20.0% higher than those of the bare sand (0.25 mm/h), respectively. Furthermore, the average evaporation rate of the cyano-crust layer was similar to that of the bare sand at high evaporation intensity (275 W). While the moss crust layer (0.54 mm/h) was 12.9% lower than that on the bare sand (0.62 mm/h). On the undisturbed soil columns, the average evaporation rate of cyano-crusts (0.18 mm/h) was 1.2 times higher than that of bare sand (0.15 mm/h) under low soil moisture. While the average evaporation rate of moss crusts (0.19 mm/h) increased by 26.7%, compared with the bare sand (0.15 mm/h). The cyano-crusts (0.25 mm/h) and moss crusts (0.30 mm/h) facilitated the evaporation with an average evaporation rate of 8.7% and 30.4% higher than those of bare sand (0.23 mm/h) under high soil moisture. While the moss crusts shared the significant stage behavior. The evaporation rate first decreased by 12.2% in stages 1 and 2, compared with the bare sand. However, the evaporation rate of the moss crusts in stage 3 was 37.8% higher than that of the bare sand. Correspondingly, the soil moisture is also monitored at different depths of layers. The biocrusts (especially the moss crusts) significantly increased the soil moisture from 0 to 10 cm. But the soil moisture at 20 cm was significantly lower than that of bare sand. The soil moisture of cyano-crusts was 11.3% lower than that of bare sand at 0-20 cm. Whereas the soil moisture of moss crusts was 43.9% higher than that of bare sand at 0-10 cm on average. Nevertheless, the soil moisture of moss crusts was 31.5% less than that of bare sand at a depth of 20 cm. In conclusion, the evaporation rate of the surface soil covered by biocrusts was higher than that of the bare sand. The loss of surface soil water was accelerated significantly, even though the biocrust layer shared an evaporation-inhibiting. The finding can provide a strong reference for the soil water balance on the surface soil evaporation in arid and semi-arid regions.