**3.2 Effects of different treatments on soil chemical indexes**

Accumulation of soil carbon and nitrogen is regarded as an indicator of soil fertility and productivity [29]. Algae spraying in different proportions had different effects on soil organic carbon. Among them, the ratio of single algae species Scytonema sinense and Phormidum lucidum, and mixed algae species Microcolus vaginatus: Scytonema sinense 5:1 did not change soil organic carbon content, while the ratio of single algae species Microcolus vaginatus, Microcolus vaginatus: Scytonema sinense: Phormidum lucidum 1:1:1 showed a trend of increasing soil organic carbon content. The soil total carbon content was significantly increased by 72.6% (P < 0.05) by spraying mixed algae with the ratio of Microcolus vaginatus: Scytonema sinense: Phormidum lucidum 3:2:1 (**Figure 3**).

Previous studies have shown that biological soil crusts can fix atmospheric carbon and nitrogen [30, 31]. Fixed carbon and nitrogen are released into the surrounding environment and used by other organisms such as vascular plants, fungi and bacteria [32]. The study of the algal crust under the canopy of Artemisia sphaerica vegetation and the organic matter in the underlying soil in Mu Us Sandy Land found that most of the algal crust developed stably between canopies, and most of the algal crust under the canopy was at the early or middle stage of development, and the content of organic matter in the intercanopies and underlying layer increased significantly [33]. Nitrogen fixing cyanobacteria can fix atmospheric nitrogen and increase soil organic matter content. The nitrogen-fixing activity of algal crusts in the artificial vegetation sand-fixing area and the natural vegetation area were compared, the results indicated that the nitrogen-fixing activity of algal crusts in the artificial vegetation area increased significantly with the extension of vegetation restoration time [34]. The results showed that the development of algal crusts promoted soil nitrogen content and soil development after the implementation of artificial vegetation restoration measures.

The difference in total nitrogen content is an indication of different nitrogen inputs [13]. Soil total nitrogen content and soil organic carbon content had a similar response trend. Neither single algal culture medium could significantly increase soil total nitrogen content, but the soil total nitrogen content in the ratio of Microcolus

#### **Figure 3.**

*Effects of different treatments on soil chemical indexes. CK T1: Microcolus vaginatus: Scytonema sinense: Phormidum lucidum 1:1:1; T2: Microcolus vaginatus: Scytonema sinense: Phormidum lucidum 3:2:1; T3: Microcolus vaginatus: Scytonema sinense 5:1; T4: Microcolus vaginatus; T5: Scytonema sinense; T6: Phormidum lucidum.*

vaginatus: Scytonema sinense: Phormidum lucidum 3:2:1 was significantly increased (P < 0.05), with the increase ratios of 56.2% and 70.3%, respectively.

All the algal culture media had no effect on soil total P content. Carbon input is an important ecological function of biological soil crust in arid regions, and the ability of carbon fixation is affected by the development degree of biological soil crust. In well-developed biological soil crusts, the carbon fixation rate is about two times higher than that in poorly developed soil crusts, which is mainly attributed to the increase of chlorophyll a in well-developed biological soil crusts [35].

Most of the nitrogen fixed by biological soil crusts can be immediately released into the soil [36]. At low soil moisture content, biological soil crusts had no effect on soil organic matter and soil total nitrogen content, while at high soil moisture content, Biological soil crusts can significantly increase soil organic carbon and soil total nitrogen content at 0–5 cm depth [37]. This may be because the available water under the surface of the soil is too high, and the water condition is high enough to activate carbon fixation components [38] and drive photosynthesis to produce ATP and carbohydrates for nitrogen fixation [39].

#### **3.3 Effects of different treatments on soil biological indexes**

Soil microbial biomass fluctuated between 0.29 and 2.02 g C m−2 in soil biological crust, and its value was significantly correlated with the development degree of soil crust [40]. Increasing organic matter and polysaccharides offers a plentiful carbon source for microorganisms and invertase, resulting in an increase of microbial biomass C [41, 42]. Increase of microalgal biomass is helpful to improve organic C and available P. The release of carbonic acid by the algal cells can accelerate the weathering of minerals, hence make improvement of inorganic ions [43].

All medium significantly increased the content of soil microbial biomass carbon (P < 0.05), but there was no significant difference among different medium (P > 0.05).

Except for Scytonema sinense, other algal culture media significantly increased the soil single species Scytonema sinense and Phormidum lucidum significantly increased soil microbial biomass phosphorus (P < 0.05), and other species had an increasing trend, but the statistical difference was not significant (**Figure 4**).

#### **Figure 4.**

*Effects of different treatments on soil biological indexes. CK T1: Microcolus vaginatus: Scytonema sinense: Phormidum lucidum 1:1:1; T2: Microcolus vaginatus: Scytonema sinense: Phormidum lucidum 3:2:1; T3: Microcolus vaginatus: Scytonema sinense 5:1; T4: Microcolus vaginatus; T5: Scytonema sinense; T6: Phormidum lucidum.*

*Cultivation of Artificial Algal Crust and Its Effect on Soil Improvement in Sandy Area DOI: http://dx.doi.org/10.5772/intechopen.98716*

#### **3.4 Existing problems and suggestions**

As a new method to prevent and control desertification, artificial cultivation of algae crust has made substantial progress and breakthrough in the practice of preventing and controlling desertification. However, the following two aspects need to be further discussed: sand burial is one of the most common disturbance factors of desert ecosystem in sand area, especially in arid desert ecosystem with frequent eolian sand activities. Sand burial affected the growth and survival of BSC by changing the light, temperature and soil physical and chemical properties of BSC habitat. Especially in the early stage of the construction of artificial algae crust, the survival ability of algae is relatively weak, and the existence of sand crust seriously threatens the further development and formation of algae crust. Artificial crust is more fragile, and its resistance to adversity is weak. Therefore, after the construction of artificial algae crust is completed, how to prevent and control the damage of sand burial to it. It is a problem to be faced in constructing artificial algae crust. The key links of technology still need further optimization and innovation, still need to be improved and perfected through a lot of scientific research and production practice. In addition, the early artificial hydration measures after inoculation are conducive to the normal synthesis and metabolism of extracellular polysaccharide, and the accumulation of extracellular polysaccharide is helpful to restore the drought-tolerant ability of algae and adapt to the external drought environment, thus increasing the biomass of algae, improving the drought-tolerant ability of algae and promoting the formation of artificial algae crust. Therefore, early water acquisition is the key factor for the successful formation of artificial algae crust. However, water is the most important ecological limiting factor in arid areas. If water is replenished continuously in the early stage, it cannot be realized in largescale desertification prevention and control practices. Therefore, how to cultivate the algae with stronger drought resistance and more suitable for the formation of artificial algae crust is the key technology to be broken through in the construction of artificial algae crust.

#### **4. Conclusions**

This study shows that different proportions of algae crusts can improve the physical, chemical and biological properties of desert soil to different degrees. The results showed that the soil bulk density in the 0–5 cm surface layer had a decreasing trend after spraying different proportion of algae culture solution, and the soil bulk density in the mixed algae Microcolus vaginatus: Scytonema sinense: Phormidum lucidum 3:2:1 was significantly decreased (P < 0.05). The soil organic carbon content was increased with the ratio of single species Microcolus vaginatus, Microcolus vaginatus: Scytonema sinense: Phormidum lucidum: 1:1:1. The soil total carbon content was significantly increased by 72.6% (P < 0.05) when the ratio of mixed species Microcolus vaginatus: Scytonema sinense: Phormidum lucidum: 3:2:1. The soil total nitrogen content was significantly increased with the ratio of Microcolus vaginatus: Scytonema sinense: Phormidum lucidum 3:2:1 (P < 0.05), increasing by 56.2% and 70.3%, respectively; All medium significantly increased the soil microbial biomass carbon content (P < 0.05). And artificial cultivation of algae crusts can greatly shorten the formation time of algae crusts. The results in this study highlight the significant role of BSCs for soil improvement in semiarid and arid areas. Therefore, when carrying out ecological construction in arid desert, it is necessary to fully consider the difference of effects of different ratio of algal solution, so as to achieve the optimal soil nutrient improvement effect of biological crusts.
