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Application of sodium alginate in induced biological soil crusts: enhancing the sand stabilization in the early stage Chengrong Peng, Jiaoli Zheng, Shun Huang, Shuangshuang Li, Dunhai Li, Mingyu Cheng & Yongding Liu Journal of Applied Phycology ISSN 0921-8971 Volume 29 Number 3 J Appl Phycol (2017) 29:1421-1428 DOI 10.1007/s10811-017-1061-2

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Author's personal copy J Appl Phycol (2017) 29:1421–1428 DOI 10.1007/s10811-017-1061-2

Application of sodium alginate in induced biological soil crusts: enhancing the sand stabilization in the early stage Chengrong Peng 1 & Jiaoli Zheng 1,2 & Shun Huang 1 & Shuangshuang Li 1,2 & Dunhai Li 1 & Mingyu Cheng 3 & Yongding Liu 1

Received: 10 July 2016 / Revised and accepted: 10 January 2017 / Published online: 25 January 2017 # Springer Science+Business Media Dordrecht 2017

Abstract Induced biological soil crust (IBSC) technology has proved to be an effective means for speeding up the recovery of biological soil crusts (BSC) in arid and semi-arid regions. This study aims at improving the IBSC technology by using sodium alginate (SA) due to its sand-stabilizing ability in the early development stage of IBSCs. Results showed that SA can easily form a thin film on the surface of soil and can significantly enhance the compressive strength of the topsoil. More importantly, no negative effects of SA on the development and physiological activity of IBSCs were observed, and SA could facilitate the colonization and growth of cyanobacteria on sand. Moreover, the application of SA was much cheaper than the straw checkerboard barriers which are widely used in desertification control. This study suggests that SA can promote and accelerate the formation of BSCs; thus, it can be applied in IBSC technology to enhance the sandstabilizing property of BSCs in the early stage.

Keywords Biological soil crusts . Ecological restoration . Sand stabilization . Sodium alginate . Water retention Electronic supplementary material The online version of this article (doi:10.1007/s10811-017-1061-2) contains supplementary material, which is available to authorized users. * Dunhai Li [email protected]

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Key Laboratory of Algal Biology, Institute of Hydrobiology, Chinese Academy of Sciences, No. 7 Donghu South Road, Wu chang District, Wuhan 430072, China

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University of Chinese Academy of Sciences, Beijing 100049, China

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Qinghai Bei Xiang Municipal Investment Corporation, Xining 810000, China

Introduction Land desertification is a global ecological and environmental issue, threatening nearly a third of the world’s land surface with reducing the availability of fertile land, declining agricultural and animal husbandry productivity, and aggravating natural disasters (Geist 2005; Reynolds et al. 2007; Rao et al. 2012; Wu et al. 2013; Miao et al. 2015; Mukherjee and Chakraborty 2015). Desertification is often related to sand movement and encroachment into oases and desert margins (Hellden 2003), posing threats to various types of life in arid and semi-arid regions; consequently, desertification has played a significant role in human history, contributing to the collapse of several large empires (Lowdermilk 1939; Whitford 2002; Geist 2005;). China is one of the most seriously desertified countries in the world. There was 2.63 million km2 of desertified land by the end of 2009, which accounted for 27.3% of the national territory (Yan et al. 2015). Great efforts have been made to counter this situation over the past decades; however, only a small proportion of the desertified land has been improved (Fan and Zhou 2001). Desertification is still the most critical issue in retarding the development of China, especially in north and northwest China. Since the first soil alga was isolated from a soil collection in the late twentieth century (Starks et al. 1981), soil algae have been recognized as an important component of soil microbial community. Generally, cyanobacteria, microalgae, lichens, bryophytes, bacteria, and microfungi in soil can form a highly specialized community named biological soil crusts (BSCs) due to the growth of the sheath-forming and exopolysaccharide-excreting filamentous cyanobacteria (West 1990; Belnap and Gardner 1993; Mazor et al. 1996). BSCs exist in arid and semi-arid ecosystems worldwide, and they can survive extreme conditions. Besides, BSCs play a

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significant role in soil stabilization (Belnap and Gillette 1997; Chaudhary et al. 2009), boosting desert soil formation (Cabala et al. 2011) and improving the nutrient conditions by fixing atmospheric nitrogen and carbon dioxide (Hawkes 2003; Housman et al. 2006). BSCs in arid and semi-arid regions are of great help in resisting desertification (Li et al. 2003; Wu et al. 2013). Desertified land in the natural environment can recover from desertification without disturbances such as human activities and trampling of animals (Belnap and Eldridge 2001; Bowker 2007; Zhao et al. 2010). Although the self-recovery of desertified land happens all the time, vascular plant colonization, which occurs after BSCs improve the topsoil environment, can take a long time (Lan et al. 2014). Algal crusts (ACs) usually appear at the initial phase of BSCs, their growth and development provide suitable substances for the following succession of BSCs (Wu et al. 2013). On the basis of this, a practical solution using induced biological soil crusts (IBSCs) and straw checkerboard barriers as a complementary measure was established, which has accelerated the process of land restoration in the desertified land at the south fringe of the Hobq Desert in China over the past 14 years (Hu et al. 2002; Chen et al. 2006; Xie et al. 2007; Wang et al. 2009; Lan et al. 2010; Rao et al. 2012; Wu et al. 2013). This solution has made certain achievements in Hobq Desert desertification control, but there are still some problems to be solved. In the initial phase of IBSCs, the crusts are far from forming a stable structure to stabilize the soil. In this period, IBSCs are fragile and weakly attached to the soil surface, particularly on soils with high proportion of sand particles (Read et al. 2011), thus easily to be destroyed by wind, rain, hoofed animals, etc. (Eldridge 1998; Hu et al. 2002; Read et al. 2011). Although the straw checkerboard barriers can increase soil surface roughness and reduce wind erosion (De Vos 1996), the cost is estimated to be more than US$0.47 per meter (straw checkerboard side length, Fig. S1) in Hobq Desert region, and their wind-blocking effect depends significantly on the size of the barriers. Besides, the application of straw checkerboard barriers requires too much material (straw, stick, etc.) and labor force, which are difficult to be obtained in the arid and semiarid regions. To improve the ability against wind or water erosion, some polymers such as urea-formaldehyde with H2SO4 as cross-linker agent (Yang et al. 2007), polymerized vinyl acetate (Liu et al. 2012), alkyl acrylates (Lahalih and Ahmed 1998), polyvinyl acetate, acrylic polymer, and styrene-butadine latex (Siddiqi and Moore 1981) were introduced. The application of these polymers showed their cementing ability to aggregate sand particles and form chemical crusts on the desertified land. However, these solutions mainly focus on the immobilization of the shifting sands; no study has noticed the effects of these polymers on the restoration of the desertified land. Sodium alginate (SA) is a natural polysaccharide which is widely used in foods and medical industries. It has favorable

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film-forming properties and is often applied in coating and biopolymer film (Chan et al. 2006). Besides, SA films are hydrophilic matrices through which water molecules can easily pass. Buttars et al. (1998) conducted inoculation experiments using alginate pelletized cyanobacteria. Their results demonstrated that Microcoleus vaginatus could survive pelletization and successfully escaped from crushed alginate pellets. Inoculation of disturbed soils with pelletized cyanobacteria is a plausible mean to increase recovery rates. However, the method is too complex and not suitable for large-scale application, especially as the residual calcium chloride can be deleterious to the cyanobacteria and will contribute to the conductivity of the soil. Based on the properties of SA, the main purpose of this study was to evaluate the direct application of SA as a sand-stabilizing agent in IBSC technology. Specifically, this study investigated (a) the feasibility of the application of SA as an assistant agent in IBSC technology, (b) the performance of SA on sand-stabilizing, and (c) the effects of SA on the colonization of ACs.

Material and methods Texture of the sand The sand used in this study was collected from the shifting sand dunes (40° 22′ N, 109° 50′ E) on the southern fringe of Hobq Desert, Dalate County of Inner Mongolia, China. The texture classes of the sand were analyzed according to the Unified Soil Classification System (ASTM D2487). The texture and chemical properties of the sand are presented in Table 1. The dominant components of the sand were grains with size between 0.075 and 0.425 mm, and the sand was poor and weakly alkaline. Cyanobacteria cultivation and IBSC inoculation The cyanobacteria M. vaginatus and Scytonema javanicum were isolated from the desert BSCs in the Inner Mongolia of China and maintained in our laboratory. They were cultivated in Table 1

Texture and chemical properties of experimental sand

Property pH Total N (g kg−1 dry soil) Total P (g kg−1 dry soil) Organic matter (g kg−1 dry soil) Silt loam (0.005–0.075 mm) Fine sand (0.075–0.425 mm) Medium sand (0.425–2 mm)

8.97 ± 0.04 0.35 ± 0.02 0.10 ± 0.003 0.45 ± 0.009 16.5% 78.33% 0.76%

The chemical data are presented as mean ± standard deviation (n = 3)

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BG-11 medium and BG-110 medium (BG-11 medium without NaNO3) in 10-L serum bottles, respectively. The cultures were subjected to a 12:12 light/dark cycle with an irradiance of 70 μmol photons m−2 s−1 in a temperature-controlled room (26 ± 1 °C). Sterile filtered air was pumped into the bottles at a rate of approximately 30 mL s−1 to avoid microalgae sedimentation and carbon dioxide limitation. The cultures were harvested when the two strains achieved exponential growth phase. The M. vaginatus was crushed by a blender before inoculation, and the S. javanicum was directly applied in the experiments. Pure sodium alginate (SA) was purchased from Sinopharm Chemical Reagent Co., Ltd., China, and was prepared as stock solution (10 g L−1). Autoclaved sand was divided equally into 24 Petri dishes (diameter = 15 cm), and the surface was carefully smoothed prior to the experiments. About 500 g sand was put into each Petri dish, and the height of the sand was 1.6 cm in average, resulting bulk density was 1.77 g cm−3 and the porosity was 33.27%. Microcoleus vaginatus and S. javanicum were mixed with a ratio of 4:3 (w/w) and resuspended in SA solution with different concentrations. The mixture then was inoculated onto the sand surface in Petri dishes with a sprayer to a final algal biomass of 0.5 g m−2 (dry weight), and the final dosages of SA were 0 (control, without SA), 0.5, 1.0, and 2.0 g m−2. Each treatment was performed in six replicates. The Petri dishes were cultured in an incubator with 12:12 light/dark cycle at 30/15 °C, and 70 μmol photons m−2 s−1 irradiance in the light cycle. During the study period, the Petri dishes were watered every day with a sprayer at 08:00 and 17:00 with watering 0.1 mL cm−2 each time. The Petri dishes were sprayed with BG-11 medium at 08:00 with 0.1 mL cm−2 instead of water every 5 days. The inoculation experiments lasted for 50 days; after that, the Petri dishes were still kept in the incubator, until the compressive strengths of IBSCs were determined after 90 days. The compressive strength measurement The compressive strength was used to evaluate the sandstabilizing property. In order to evaluate the short-term effects of SA dosage on the compressive strength, different dosages of SA (ranging from 0 to 5 g m−2) with the algal mixture mentioned above (0.5 g m−2 dry weight) were sprayed on the sand surface in the Petri dishes. Then, the Petri dishes were air-dried for 48 h, and the compressive strength was measured using a force gauge (FGJ-5, SHIMPO, Japan). The peak values of pressure were recorded when the probe penetrated the surface of IBSCs (Fig. S2). During the study period, the compressive strength of the inoculated Petri dishes was measured every 10 days to evaluate the long-term effects of SA. We also measured 7-year-old IBSCs (algal crusts) which were collected from Hobq Desert in China for reference. Each measurement was repeated three times.

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Algal biomass measurement Areal chlorophyll a (Chl-a) content of IBSCs was measured every 5 days and was used to represent the biomass of IBSCs. For algal crust sampling, IBSCs were covered by a plastic plate with a hole (area is 1 cm2) and cut by a scalpel through the hole; then, the samples were ground in 10 mL 100% acetone with mortar and pestle. The extracts were stored in the dark for 18 h at 4 °C, centrifuged at 7690×g for 10 min, and the supernatant were measured at 384, 490, and 663 nm with a spectrophotometer. At least three crusts were sampled for measurements each time. The Chl-a contents were calculated according to trichromatic equation (Brenowitz and Castenholz 1997; Garcia Pichel and Castenholz 1991): Chl‐a ¼

1:02  A663 −0:27  A348 −0:01  A490 V  C Chl‐α S

where CChl-a is the extinction coefficient of Chl-a (L g−1 cm−1), and the value is 92.5; A663, A490, and A384 are absorbance of extracts at 663, 490, and 384 nm, respectively; V is the volume of extracts (mL); and S is the area of the crust sample (cm2). Water-retaining testing The following experiment was designed to evaluate the waterretaining capacity of SA based on its high hydrophilicity. Four hundred fifty grams of sand was put into each Petri dish (diameter = 15 cm). Different amounts of SA solution were sprayed evenly onto the sand in five groups of Petri dishes to the final contents of 0, 0.5, 1.0, 2.0, and 5.0 g m−2, respectively. Then, different amounts of distilled water were sprayed into each Petri dish to make all the dishes have the same initial weight of liquid. The Petri dishes were then were placed in a constant airtemperature room (26 ± 1 °C, air relative humidity 25%). The water content of each Petri dish was measured according to ISO 11465 at the third day. During IBSC experiment, the water content of the crust samples was determined following the same procedures. Each measurement was repeated three times. Chlorophyll fluorescence and photosynthetic rate The chlorophyll fluorescence parameter Fv/Fm represents the maximum photochemical efficiency of photosystem II and reflects the potential photosynthetic activity in BSCs. The effects of SA on the photosynthetic activity of BSCs were evaluated by measuring of chlorophyll fluorescence with a PAM-2100 (Walz, Germany) every 5 days after watering. Before measurement, the IBSCs were dark-adapted for at least 20 min, and a saturating pulse for Chl-a excitation was set at approximately 1500 μmol photons m−2 s−1 for all the measurements. Each measurement was repeated five times for one sample.

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Net photosynthetic rates of IBSCs were measured every 5 days, using an S151 Infrared Gas Analyzer (Qubit Systems, Canada). The CO2 consumption rate was recorded using a Lab Pro interface and Logger Pro 3 version software (Vernier Software & Technology, USA), at 26 °C with light intensity of 70 μmol photons m−2 s−1. The photosynthetic rates were calculated following the manufacturer’s instruction. Scanning electron microscopy In preparation for ultrastructure observation, IBSC samples were collected as described before (BAlgal biomass measurement^ section), then air-dried, gold-sputtered with Hitachi E-1010 (Hitachi, Japan), and observed under a Hitachi S4800 Scanning Electric Microscope (Hitachi, Japan). Statistical analysis The data were analyzed by SPSS 20.0 for Windows (IBM SPSS Inc., USA). The relationships between SA content and compressive strength were regressed. Significant differences between IBSCs samples were determined using one-way ANOVA followed by Tukey’s test.

Results Effects of SA on the compressive strength The compressive strength of the cemented samples with different content of SA is shown in Fig. 1. The sand-stabilizing property of cyanobacteria in the initial phase of IBSCs was significantly increased by SA: the compressive strengths of

treatment groups were all higher than that of the control (0 g m−2). There was an apparent linear relationship between the SA contents and the compressive strengths (R2 = 0.998). The compressive strength of the 7-year-old IBSCs collected from the Hobq Desert of China was 230.3 ± 19.6 kPa. The hardness of the 7-year-old IBSCs was equivalent to an application of 2 g m−2 SA on the soil surface. The long-term effects of SA on the compressive strength of IBSCs were also evaluated (Fig. 2). In the first 12 days, the compressive strength of SA-treated IBSCs was significantly higher than that of the control (p < 0.05); however, the difference became smaller with the growth of cyanobacteria and degradation of SA. After 90 days of growth, the compressive strengths of 1.0 and 2.0 g m−2 groups were significantly higher than that of the control (p < 0.05). Furthermore, there was a significant difference between the 1.0 and 2.0 g m−2 groups (p < 0.05).

Effects of SA on the growth of IBSCs With the growth of inoculated cyanobacteria, the Chl-a contents of IBSCs increased rapidly within the initial 30 days and then slowed down. The use of SA promoted the growth of cyanobacteria, that is, the higher the dose of SA, the higher the content of Chl-a in the IBSCs (Fig. 3). On the tenth day of the experiment, the Chl-a contents of the 1.0 and 2.0 g m−2 groups were significantly higher than the control (p < 0.05). There was also a significant difference between the 1.0 and 2.0 g m−2 groups (p < 0.05). The Chl-a contents of the 1.0 and 2.0 g m−2 groups were significantly higher than that of the control from the 30th and 20th day (p < 0.05), respectively, and this

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Fig. 2 The development of compressive strength of IBSCs under different SA content with the initial cyanobacterial biomass of 0.5 g m−2. Asterisk represents a significant increment (p < 0.05) compared with the control group. Error bars indicate the standard deviation (n = 3)

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Fig. 3 Chl-a content of IBSCs variation under different SA content with the initial cyanobacterial biomass of 0.5 g m−2. Asterisk represents a significant increment (p < 0.05) compared with the control group. Error bars indicate the standard deviation (n = 3)

situation lasted to the end of the experiments. The difference of Chl-a contents between the 1.0 and 2.0 g m−2 groups became less with the development of IBSCs.

Fig. 5 The chlorophyll fluorescence parameter Fv/Fm of IBSCs under different treatments during the study period. Error bars indicate the standard deviation (n = 5)

treatments, and then, the situation was reversed. The water content of 2.0 g m−2 group was significantly higher than that of the control (p < 0.05) at the 30th, 35th, and 50th days.

Effects of SA on the water-retaining capacity of IBSCs

Effects of SA on the chlorophyll fluorescence and photosynthesis

The water-retaining capacity of SA in sand without cyanobacteria inoculation was tested. The water content of sand decreased over time (Fig. 4a), and there were no significant differences of water contents among all the groups (p > 0.05) at each test time. Although SA had no effects on the water-retaining capacity of sand, it could affect the waterretaining capacity of IBSCs (Fig. 4b). In the first 10 days, the water content of control was higher than those of the SA

In the early stage, the Fv/Fm values of IBSCs were obviously affected by SA (Fig. 5), especially in the first 5 days of the experiment, the Fv/Fm values of treatments were significantly higher than that of the control (p < 0.05). However, with the development of IBSCs, the Fv/Fm values increased and there was no difference between the control and the treatments. With the development of IBSCs, the net photosynthesis increased gradually (Fig. 6). In the first 10 days, there was no

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Discussion

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Sodium alginate alone could easily form a thin film on the surface of sand, which could effectively fix sand particles (Fig. 7a). In IBSCs without SA, the cyanobacterial filaments also could fix sand particles (Fig. 7b); however, the growth of cyanobacteria was very slow and in the early stage of development, the cyanobacteria were not enough to cement the majority of sand particles. When SA and cyanobacteria inoculated together, they could form a tight film on the sand

BSCs are able to survive in nearly all environments, especially widely distributed in arid and semi-arid regions, and play important roles in improving topsoil environment, enhancing soil stability, reducing erosion, and increasing nutrient accumulation (Harper and Belnap 2001; Maestre et al. 2011; Lan et al. 2014). Extracellular polysaccharides (EPSs) are fundamental to this adaptability allowing cyanobacterial cells to adapt to and adjust the microenvironment of the soil (Mager and Thomas 2011; Colica et al. 2014, 2015 ). Polysaccharides were the main component of EPSs, and previous studies showed that the polysaccharides in EPSs were a group of high-molecular-weight polymers (Hu et al. 2003; Chen et al. 2014). The sand-stabilizing property of BSCs in the early stage was mainly depended on the cyanobacteria filaments and the excretion of EPSs into the surrounding soil (Hu et al. 2003). Cyanobacteria inoculation has been proved to be an effective method for the restoration of BSCs in arid and semi-arid regions (Wang et al. 2009); however, the unavoidable problem is that IBSCs by this method are very fragile in the early development stage due to the slow growth of cyanobacteria filaments and low accumulation of EPSs. In this study, SA was used to simulate the function of EPSs to enhance the sand-stabilizing property in the early development stage of IBSCs, and the results indicated that the application of SA in IBSCs was feasible in the microcosm experiments. SA can easily form a film on the sand surface and significantly enhance its compressive strength. It was widely used in the production of coating materials and biopolymers, and sometimes, SA was used in the formation of calcium alginate with calcium chloride (Buttars et al. 1998; Rhim 2004). In this study, SA solution was directly sprayed onto the sand surface, and the film could be naturally formed without other

Fig. 7 Ultrastructure of top soil under different treatment during the study period. a The surface of sand was only sprayed with SA and cultured for 5 days. The arrow means the SA film; b the surface of sand was only sprayed with cyanobacteria mixture (0.5 g m−2) and

cultured for 30 days. The arrow points at the cyanobacteria filaments; c the surface of sand was sprayed with cyanobacteria mixture (0.5 g m−2) and SA (1 g m−2) and cultured for 30 days. The arrow indicates the crusts composed of cyanobacteria and SA

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Fig. 6 Net photosynthesis of IBSCs under different treatment and control during the study period. Asterisk represents a significant increment (p < 0.05) compared with the control group. Error bars indicate the standard deviation (n = 3)

significant difference of photosynthetic rates between the treatments and the control (p > 0.05). The net photosynthetic rates of the 1.0 and 2.0 g m−2 groups were significantly higher than that of the control from the 25th day onwards, and the photosynthetic rate of the 2.0 g m−2 SA-treated IBSCs was significantly higher than those of the 0.5 and 1.0 g m−2 SAtreated IBSCs (p < 0.05) thereafter.

The ultrastructure of IBSCs

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supplementary measures (Fig. 7a). It has been reported that the more sandy a soil, the recovered BSCs on the top soil would be more fragile (Read et al. 2011). In our microcosm experiments, the sand proportion was more than 99% (Table 1), the compressive strength of SA-treated soils were significantly higher than that of the control (Fig. 2), and they were SA dose dependent (Fig. 1). Besides, the application of SA had long-term effects on the compressive strength of IBSCs (Fig. 2). The higher compressive strength very likely also causes a decreased erodibility; this was very important for the development of IBSCs in the early stage. As already pointed out, the compressive strength of the 7-year-old field BSC sample (230 kPa) was equivalent of 2 g m−2 SA on the soil surface. Moreover, the compressive strengths of the 8-yearold crusts were 355 ± 18.1 and 1060 ± 41.3 kPa at sunny and shady sides of dunes, respectively (Lan et al. 2014). Therefore, in the process of induced recovery of BSCs, it is very effective and feasible to increase the compressive strength of IBSCs by using SA. It is worth noting that our results represent the absolute best-case condition, but IBSC organism in the field will have to cope with much harsher condition and thus, are likely to grow slower/weaker. Secondly, measuring the compressive strength in the lab is not the same as measuring the stability of the crusts in the field. Consequently, these results are not 100% transferable to field conditions and that it remains to be seen whether or not SA will also form this very strong film in the field. Although SA is produced using natural polysaccharides, its possible negative effects in application in IBSC technology should be considered, especially as it forms a film on the soil surface, which changes the material and energy exchange of the interface between soil and air in the early stage of IBSCs. Our results showed that SA has no negative effects on and even promotes the growth of cyanobacteria, that is, SA benefits the recovery of IBSCs. Chlorophyll fluorescence can be used to evaluate the resistance of plants to environmental stress. In our results, after 5 days of inoculation, the Fv/Fm values of SA-treated IBSCs were higher than that of the control, indicating that the addition of SA had created a favorable microenvironment for the colonization of cyanobacteria (Fig. 4). Thereafter, there was no significant difference in Fv/Fm values between the SA treatment groups and the control group, implying that the cyanobacteria had recovered from the disturbance. The SA-treated IBSCs also showed higher photosynthetic rates than the control (Fig. 5), and also suggesting that SA had promoted the growth of cyanobacteria. The change of Chl-a content also indicated that SA could promote the growth of cyanobacteria, that is, the cyanobacterial biomass of SAtreated groups was significantly higher than that of the control group. During the study period, a mature stage of IBSCs was observed in all the treatments (i.e., the Chl-a content no longer increased significantly); this phenomenon was consistent with another study (Xie et al. 2007).

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Although SA alone did not affect the water content of the soil (Fig. 4a), IBSCs with SA showed a higher average water content than the control (Fig. 4b). It is believed that cyanobacteria could enhance the hydraulic conductivity of BSCs through the production of EPSs. Higher hydraulic conductivity may lead to higher dissipation of water in sandy soils, but EPSs likely promoted a better water distribution within the first soil layer due to the higher content of silt and clay in the investigated soil (Rossi et al. 2012). However, in the type of soil investigated here, a previous study showed that the EPSs of IBSCs played crucial role in trapping and retaining humidity, thus increased the water availability in the first layer of soils and reduced the infiltration of water (Colica et al. 2014), so as to promote the growth of IBSCs. The SA might play the similar role in IBSCs. The higher water content in the SA treated groups may be due to the combined effects of SA and EPSs in the mature stage of IBSCs. Moreover, the cost of SA is relatively low. For example, the cost of the industrial grade SA is about US$0.008 per gram. In practice, if an amount of 2 g m−2 SA is used for BSCs induction, the cost of SA is US$0.016 per square meter. This is much cheaper than the straw checkerboard barriers (US$0.47 per meter) in the IBSC technology. However, it is better to combine these two methods in the IBSC technology, so as to achieve an ideal restoration of desertified land.

Acknowledgements This work has been jointly supported by the Special Fund for Forest Scientific Research in the Public Welfare (20140420402), National Key R&D program (2016YFD0200309-4), and Fund for Sand Stabilization and Circular Economy (2013-N-121) from Qinghai Science and Technology Department. The authors wish to gratefully express their thanks to Yuan Xiao in Testing and Analysis Center of Institute of Hydrobiology, Chinese Academy of Sciences, for her assistance in the ultrastructure observation.

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