Physiological responses of artificial moss biocrusts to dehydration-rehydration process and heat stress on the Loess Plateau, China

Ex-situ cultivation of biological soil crusts （biocrusts） is a promising technology to produce materials that can induce the recovery of biocrusts in the field for the purposes of preventing soil erosion and improving hydrological function in degraded ecosystems. However, the ability of artificially cultivated biocrusts to survive under adverse field conditions, including drought and heat stresses, is still relatively unknown. Mosses can bolster biocrust resistance to the stresses （e.g., drought and heat） and the resistance may be introduced prior to field cultivation. In this study, we subjected the well-developed artificial moss biocrusts （dominant species of Didjmodon vinealis （Brid.） Zand.） that we cultivated in the phytotron to a dehydration-rehydration experiment and also a heat stress experiment and measured the activities of protective enzymes （including peroxidase （POD）, superoxide dismutase （SOD）, and catalase （CAT）） and the contents of osmoregulatory substances （including soluble proteins and soluble sugars） and malondialdehyde （MDA, an indicator of oxidative stress） in the stem and leaf fragments of mosses. The results showed that, during the dehydration process, the activities of protective enzymes and the contents of osmoregulatory substances and MDA gradually increased with increasing duration of drought stress （over 13 days）. During the rehydration process, values of these parameters decreased rapidly after 1 d of rehydration. The values then showed a gradual decrease for 5 days, approaching to the control levels. Under heat stress （45℃）, the activities of protective enzymes and the content of soluble proteins increased rapidly within 2 h of heat exposure and then decreased gradually with increasing duration of heat exposure. In contrast, the contents of soluble sugars and MDA always increased gradually with increasing duration of heat exposure. This study indicates that artificial moss biocrusts possess a strong drought resistance and this resistance can be enhanced after a gradual dehydration treatment. This study also indicates that artificial moss biocrusts can only resist short-term heat stress （not long-term heat stress）. These findings suggest that short-term heat stress or prolonged drought stress could be used to elevate the resistance of artificial moss biocrusts to adverse conditions prior to field reintroduction.

Osmotic Adjustment of Soil Biocrust Mosses in Response to Desiccation Stress

Biological soil crusts(biocrusts) widely occur in semiarid and arid regions throughout the world and play important roles in many desert ecosystems: protecting soil from wind erosion and detaining nutrient-rich dust and organic carbon. An experiment was conducted in the Shapotou revegetated area of the Tengger Desert, Ningxia Hui Autonomous Region of China to investigate the physiological responses of the dominant biocrust mosses, Bryum argenteum and Didymodon vinealis, to desiccant stress using different osmotic adjustments. B. argenteum and D. vinealis accumulated K+, total soluble sugar, sucrose, trehalose, proline, and glycine betaine during desiccation. The proline content of B. argenteum was about two times higher than that of D. vinealis. The K+and glycine betaine contents in B. argenteum were slightly higher than those in D. vinealis. In contrast, the total soluble sugar, sucrose,and trehalose contents in D. vinealis were about 3 to 5 times higher than those in B. argenteum. With gradual desiccation stress,the Na+content of B. argenteum was low and did not significantly change. On the contrary, the Na+content of D. vinealis sharply increased and reached a very high level of about 10 to 18 times higher than that of B. argenteum, indicating that B. argenteum and D. vinealis gradually adapted to desiccation stress by osmotic substances accumulation to different degrees.