Synergistic Complementary Effects of Elemental Sulfur and Micro-Algae on Saline- Alkali Soils

MA commonly refers to the collective name of single-celled micro-algae that contain chlorophyll-a, which are capable of photosynthesis. MA belongs to a kind of protists. A variety of soil bio-fertilizers have been developed by using different MA species as raw materials. The bio-fertilizers were commonly used in paddy fields previously, but currently, in dryland, the bio-fertilizer application is increasing [16]. a) MA as bio-fertilizers can reduce soil sulfur enrichment. Cyanobacteria (one of the algae species) first appeared in the Archean oceans on the Earth. With increasing oxygenation and more abundant sulfate (SO₄²-) levels, algae (green algae together with cyanobacteria) became the main primary producers in the Earth’s oceans. Sulfur is one of the important components of algae cells. The sulfur is consumed by algae in the form of SO4 2-. Therefore, sulfur has become a macronutrient necessary for the growth of micro-algae [17]. The application of MA bio-fertilizers can inevitably consume part of SO4 2- and reduce sulfur enrichment and sulfate content in the soil. b) The cyanobacteria integrated with salt-tolerant plants can remove much more salts from soils [18]. The major contribution of cyanobacteria is to re-establish micro-ecology [18] and absorb Na+ in the soil [19]. c) MA bio-fertilizers can increase soil availability of nutrients [16,20]. In drylands, MA species can grow in symbiosis with crop roots [21], significantly affecting microbial communities [19,22], improve plant rhizosphere microbes [23-25], decompose soil compounds to generate available nutrients [21], enhance crop N uptake, and fix N [16]. d) MA can increase Soil Organic Carbon (SOC) through photosynthesis [16], thereby increasing SOM. For example, the application of MA biofertilizers in desert soil in the United States has been reported to increase SOM from 10 to 30 g kg^-1 within three years [26]. e) MA bio-fertilizers can significantly reduce N₂O emissions from the soils [27].

However, little information is available on the interaction between S⁰ and MA. To our knowledge, only one study has so far evaluated the impacts of sulfur and algae fertilization on the productivity of soybean (Glycine max (L.) Merrill) and mung bean (Vigna radiata (L.) Wilczek) in Egypt. The results showed that compared with the single application of S⁰, MA, or the control, under the condition of combined application of S⁰ and MA, the yields of soybean and mung bean were significantly increased, and the protein, carbohydrate, and oil contents also increased accordingly [12]. However, the effects of S⁰-MA interaction on soils have not been reported yet.

As a fertilizer, S⁰ can reduce soil pH and ESP, improve N nutrient availability in the soil, and reduce soil NO₃^--N leaching, but can lead to soil sulfur enrichment (i.e., increase in soil SO₄^2- content) and an increase in soil total salt content (Table 1). Whereas MA as bio-fertilizers can significantly reduce soil SO₄^2- content, soil total salt content, soil N₂O emission, can improve SOM content and fix N in the soil (Table 1). Therefore, the S⁰ and MA have strong synergistic and complementary effects on saline-alkali soil, by which we proposed a novel idea that the interaction between S⁰ and MA could be used to improve the saline-alkali soils in the HID or other globally arid regions.

Table 1:Synergistic complementary effects of elemental sulfur and micro-algae as bio-fertilizers on saline-alkali soils.

† ESP, Exchange sodium percentage; NO₃^--N, Nitrate nitrogen; SO₄^2-, Sulfate ion; N₂O, Nitrous oxide.

This study was financially supported by the Inner Mongolia Natural Science Foundation (NO. 2020MS04001), the Research Funding Project of Hetao College Talent Introduction (NO. HYRC2019006), and the Hetao College Science and Technology Research Project (NO. HYZX201952).

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