Abstract:

This study was conducted to understand the effects of biocrusts （biological soil crusts） in different developmental stages on soil water permeability and water holding capacity in dryland ecosystems. We firstly selected areas with different developmental stages of biocrusts （cyanobacteria， mixture of cyanobacteria and moss， and moss）， as well as the adjacent bare soil， in a watershed named Liudaogou in the northern Loess Plateau of China， as study sites. Next， the soil water permeability of the different treatments was measured by the constant pressure head method and water holding capacity by the Welcox method. Lastly， the saturated hydraulic conductivity and water infiltration parameters （including stable infiltration rate， average infiltration rate， and cumulative infiltration amount） were obtained and analyzed to determine the effects of these biocrusts on soil water permeability and water holding capacity. It was found that the saturated hydraulic conductivity of the cyanobacteria， mixture of cyanobacteria and moss， and moss biocrusts was decreased on average by 59.3%， 62.9%， and 27.6%， respectively， in contrast to that of the 0-10 cm soil. Moreover， the water infiltration parameters of the cyanobacteria， mixture of cyanobacteria and moss， and moss biocrusts was decreased on average by 37.7%， 54.4% and 18.4%， respectively， in comparison to the bare soil. Additionally， the biocrust developmental stage greatly influenced the soil water holding capacity. As compared with bare soil， the field capacity of the cyanobacteria， mixed cyanobacteria and moss， and moss biocrusts was increased changed on average by 0.97， 1.10， and 0.70 times， respectively. Correspondingly， the average volumetric water content in a drainage test （0-120 h） was changed by 1.14， 1.40， and 0.74 times， respectively. All the above results indicate that biocrusts greatly reduced surface soil water permeability and increased water holding capacity as compared with the bare soil， regardless of the different developmental stages. The biocrust comprising a mixture of cyanobacteria and moss had greater impacts on soil water conductivity and water holding capacity than the cyanobacteria biocrusts and moss biocrusts， which are possibly caused by the differences in the community structure of non-vascular plants and microbes of the biocrusts. In conclusion， the soil water permeability of biocrusts initially decreases and then increases along their developmental status from cyanobacteria to mosses in dryland ecosystems， but their soil water holding capacity increases initially and later decreases with biocrust developmental stage.

Key words: saturated hydraulic conductivity, field capacity, cyanobacteria crust, moss crust, mixture of cyanobacteria and moss crust