Microstructure and biodegradation of long-established Salix psammophila sand barriers on sand dunes
Introduction
Salix psammophila C. Wang & Chang Y. Yang is a typical desert deciduous shrub or small tree that reproduces easily in grasslands. It grows widely in harsh, windy, arid, sandy environments and is mainly distributed in mobile sand dunes, semifixed sand dunes, and lowlands between hills in Northwest China (Gao et al., 2013). Its strong resistance to drought and high temperature make it useful as a barrier to wind erosion, and thus it is used in sand barriers to fix sand and provide a mechanical sand barrier. Various configurations of S. psammophila are set on the sand surface, and the stems are used to control the direction, speed, and structure of the wind–sand flow. In addition, S. psammophila is also a low-cost and long-lasting sand stabilizer and has thus been used in the setting of mechanical sand barriers since the 1980s (Gao et al., 2013). Hence, it has become one of the key sand control models for the scientific and efficient utilization of local resources (Zhang et al., 2019). Numerous studies have reported that due to its high strength and excellent weather resistance, the use of fast-growing S. psammophila as a sand barrier has a significant positive effect on reducing the near-surface wind speed, increasing surface roughness, and increasing vegetation coverage (Xiang-Yu et al., 2008). In addition, the research shows that S. psammophila sand barriers can effectively improve the microenvironment of sand dunes and promote and accelerate the process of vegetation restoration (Gao and Yu, 1996).
If the vegetative parts of S. psammophila are severed or removed, the supply of plant nutrients stops, and the cell tissues die. With time, S. psammophila can become deteriorated via biodegradation, weathering, aging, and other environmental and human factors in the field environment (Gong, 2012, Colom et al., 2003). The damage rate of S. psammophila sand barriers after setting for five years is as high as 80%, which seriously affects the biological benefits (Gong, 2012, Wang et al., 2019). The internal structure and chemical composition of natural biomaterials will change irreversibly, which is manifested in the darkening of its surface color at the initial stage of field use and gradual graying over time. Along with the formation of a rough texture, fine cracks, and openings on its surface, the graying phenomenon on its surface is an apparent phenomenon of weathering, which is the result of changes in the complex chemical composition and microstructure (Blanchette, 2000, Raczkowski, 1980, Eriksson et al., 1990).
The deepening of the cracks accelerates the aging process, which makes the material deteriorate readily, further reduces the mechanical strength of the material, shortens the service life, and severely impacts the application value (Kongyang et al., 2018, Ragnar Seldén et al., 2004). A field investigation showed that S. psammophila sand barriers have to endure inevitable natural factors such as temperature and humidity changes, UV radiation, rain erosion, wind erosion, and microbial corrosion, and the corrosion resistance of a single S. psammophila sand barrier has been found to differ across different environments (Gong, 2012). The effect of fungi or/and weathering on natural biomaterials, such as cell wall components, as well as the associated processes have been clarified to a great extent (Marzo, 2006). The aboveground parts (which we refer to as the “atmosphere-exposed section”) show deterioration, while the belowground parts (“stable sand-buried section”) exhibit corrosion. Although the S. psammophila sand barrier soil dynamic boundary section is affected by the aboveground and belowground environment, its corrosion resistance is obviously better than the above two parts. Natural biomaterials and their chemical constituents undergo a series of chemical processes such as oxidation, hydrolysis, and depolymerization, which depend on the environment and atmosphere (Pedersen et al., 2015). In most desert environments, the decomposition of natural biomaterials and fungi seems to be dominant, while in humid environments, brown rot fungi and white rot fungi are dominant. Brown rot fungi mainly degrade material polysaccharides, leaving behind polymerized lignin, while white rot fungi generally degrade all polymerized natural biomaterials (Fackler and Schwanninger, 2012, Gilani and Schwarze, 2015). In addition, natural biomaterial cell wall components also degrade during weathering. The natural biomaterial is exposed to weathering (i.e., fluctuations in temperature, humidity, and UV-irradiation) and can degrade severely from the surface and inside, even in the absence of microorganisms (Borgin et al., 1975).
The physical–mechanical properties of natural biomaterial tend to decrease with age, which has a negative effect on its service life. Therefore, it is important to evaluate the influence of the aging process on natural biomaterial properties. The density and mass loss rate of natural biomaterials are closely related to their dimensional stability. Frequent expansion and contraction caused by water fluctuations and uneven water distribution can lead to changes in the microscopic and macroscopic features of the cell walls (Ben-Gui et al., 2007, Wang et al., 2020). Therefore, it is of fundamental importance to evaluate the changes in density and mass loss rate properties, establish the correlations with structural changes in the cell wall components of natural biomaterials, and propose better preservation and protection methods.
Salix psammophila is mainly composed of cellulose (a skeleton material providing strength), hemicellulose (a binding material providing shear resistance), lignin (a filling material providing hardness), and a small amount of extract, which all play an important role in determining the physical–mechanical properties of the S. psammophila sand barrier. However, detailed information regarding the changes in S. psammophila sand barrier cell-wall components is lacking, i.e., the molecular macroscopic structures, molecular interactions, and component distribution. Imaging techniques such as Fourier-transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) have been used in the in situ characterization of chemical and chemical structure changes in detail (Traoré et al., 2016, Ahvenainen et al., 2016, Bhuiyan et al., 2000). The aging process of the S. psammophila sand barrier may lead to changes in crystallinity, which will affect the modulus of rupture, dimensional stability, and other valuable physical and mechanical properties of biomaterials (Hwang et al., 2016). FT-IR microscopy can obtain cellular spatial resolution information for the location of degradation regions in biomaterials (Fackler and Schwanninger, 2012). In this way, the important spectral bands resulting from degradation can be identified (Jinrui et al., 2009).
In this paper, a long-established S. psammophila sand barrier was used to investigate the degradation of the atmosphere-exposed section, stable sand-buried section, and atmosphere–sand dynamic interface. We carefully studied the changes in the microstructure, chemical properties, physical properties, and mechanical properties of the S. psammophila sand barrier. Using newly-developed observation instruments and characterization means in the field of materials science, we revealed the mechanisms whereby the microstructure and chemical composition influence the physical and mechanical properties of the S. psammophila sand barrier during the aging process. Our findings provide a theoretical basis and data support for the application of sand barrier pretreatment in production practice as well as for sustainably improving the resource utilization rate and ecological environment of Northwest China.
Section snippets
Samples
The test materials were collected by field investigation in November 2019 in Hangjin Banner, Ordos, Inner Mongolia (1084121E, 402934N). S. psammophila forest land is located in a continental monsoon climate, with an altitude of 1016 m and an average annual precipitation of 186 mm. There is sufficient sunshine and a large temperature difference between day and night, with an annual average temperature of 6.3 °C, extreme maximum temperature of 38.1 °C, extreme minimum temperature of
Degree of deterioration and physical and mechanical properties
After an extended period of setting in the natural environment, the S. psammophila sand barriers are subjected to severe natural factors, such as temperature and humidity changes, UV radiation, rain erosion, wind erosion, and microbial corrosion, which lead to different degrees of deterioration of the sand barriers and serious degradation of their performance. As shown in Fig. 2a, after five years of setting, the S. psammophila sand barrier exhibited obvious differences depending on the
Conclusions
With the increase in the setting period of sand barriers, the AES and SSBS sections deteriorated and decayed, respectively. The mass loss rate was SSBSAESASDI, which also exhibited different degrees of decrease in mechanical properties such as MOR, ultimately resulting in lodging and damage and the loss of the function of the S. psammophila sand barrier in preventing wind–sand movement and fixing quicksand. Microstructural changes showed that after eight years of setting, the vessel and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The research was supported by the National Natural Science Foundation of China (41861044), “The enhancement mechanism of atmospheric-sand dynamic process on Salix Sand Barrier resistance”.
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