Biological improvement of saline alkali soil reference system: A review

XueQin Wang , Xu Xing , FengJu Zhang , Kong Xin

1. Agricultural College of Ningxia University, Yinchuan, Ningxia 750021, China

2. Ningxia Science and Technology Development Strategy and Information Research Institute, Yinchuan, Ningxia 750021,China

3. Institute of Environmental Engineering, Ningxia University, Yinchuan, Ningxia 750021, China

4. Ningxia Science and Technology Development Strategy and Information Research Institute, Yinchuan, Ningxia 750021,China

ABSTRACT This work presents a reference system overview to improve the efficiency of biological improvement of saline-alkali soil developed during the last thirty years, ranging from connotation, general methods and species, soil desalination, soil structure, soil organic content, microbial flora, enzyme activity, yield and economic benefits. The reference system presented is divided into three main groups: suitable varieties, suitable cultivation measures, and a comprehensive evaluation system.There has been a lot of research on biological improvement of saline alkali soil, but these studies are very fragmented and lack a comprehensive standard system. Also, there is a lack of practical significance, particularly with regard to optimal species, densities and times of sowing for particular regions. On the other hand, the corresponding cultivation measure is very important. Therefore, a reference system plays an important role to the effect of biological improvement of saline alkali soil.

Keywords: biological improvement; saline alkali soil; reference system

1 Introduction

With the rapid development of a global economy,the associated problems can create concerns, such as resource shortage, environmental deterioration and ecological crisis (Tao, 2013). In recent years, arable land has declined globally, including China, which has been reduced to 2×105hm2from 2012 to 2016.Salt and alkali stress is one of the main abiotic stresses for plants, which has seriously affected agricultural productivity in large areas of the world(Moser and Vogel, 1995; Munns, 2002; Katerji et al.,2003; Joseph and Jini, 2010). Saline alkali soil is distributed over most of China, which includes 1/5 of the farmland (Wang et al., 2016). Saline alkali soil is challenging for agricultural production because increased soil salinity can disturb the plants' ionic homeostasis and create a hyperosmotic state. Secondary effects, such as oxidative damage and impaired photosynthesis, often occur from these initial disturbances (Zhu, 2001).

In recent years, most countries in the world have invested a lot of manpower, financial and material resources to improve saline alkali soil. Scientists have studied different kinds of approaches to solve the problem. Salinity control through reclamation of sa-linized land or improved irrigation techniques is often prohibitively expensive and provides only a shortterm solution (Ashraf, 1994; Shannon, 1997; Singh and Singh, 2000). The beneficial effects of flue gas desulfurization (FGD) gypsum in improving the physical properties of sodic and non-sodic soils have been well documented (Chen et al., 2001; Sakai et al.,2012). Flue gas desulfurization gypsum also provides extensive reclamation of saline-sodic soils (Chun et al., 2001, 2007). However, compared with these engineering measures, biological measures provide less investment and faster effect. Using halophytes can not only provide more economic benefits, but also improve saline soil quality (Wang et al., 2008).

Different scholars have different explanations about biological improvement of saline alkali soil.Wang et al. (2016) used living organisms to regulate the damaged ecological system function, such as cultivating salt-tolerant or alkali-tolerant plants, applying organic fertilizer, or adding microbial agents. Luo et al. (2001) used cultivated salt-tolerant plants under conventional irrigation, such that salinity is controlled under a soil layer that has a large number of roots by the interaction of irrigation water and root system, and the soil is fertilized by a reasonable rotation. Although there are numerous studies on the biological improvement of saline-alkali soil, there is a lack of practical significance, particularly with regard to optimal species, densities and times of sowing for particular regions. On the other hand, corresponding cultivation measures are very important, and a reference system plays an important role to the effect of biological improvement of saline alkali soil. Therefore, the objective of this paper is to describe the biological improvement of saline alkali soil reference system.

2 Methods on biological improvement of saline alkali soil

There are four biological methods to improve saline alkali soil. The first is to study plant physiology and improve plant salt tolerance; the second is to introduce halophytes and salt-tolerant plants which have economic value; the third is to cultivate salt plant varieties and salt resistant plant species by the hybridization technique and genetic engineering method(Arzani, 2008; Wang et al., 2008; Ashraf and Akram,2009; Agarwal and Ahmad, 2010; Yu et al., 2012;Agarwal, 2013; Arabbeigi et al., 2014; Arzani and Ashraf, 2016). The fourth is to improve plant salt tolerance by inoculating microorganisms (Rabie et al.,2005).

3 Variety study of biological improvement of saline alkali soil

In the Huang-Huai-Hai Plain of China, salt-tolerant crops (cotton, maize, sorghum, triangularis,Kostelezkya virginica), salt-tolerant herbage (alfalfa,Caudan grass, Amaranthus spp.) and salt-tolerant trees (cv. Hongyechun, Chinese Ash, willow, Tamarix chinensis, thread jujube, Lycium barbarum, purple leaf cherry plum, hibiscus, Prunus maritima) can improve moderate and severe saline alkali soils (Li et al., 2006). Also, Salicornia bigelovit, Suaeda salsa,Kender, Vitex trifolia L. var. simplicifolia Cham, Lycium barbarum, licorice, Tamarix chinensis and Puccinellia macranthera are economic salt-tolerant plants (Zhao et al., 2001), which is consistent with Li et al.. By introduction, scholars have found that sub can grow normally in a saline alkali soil of pH 9.5 after several years of planting and breeding (Journal of Hebei Agricultural Sciences, 2001). In the Yellow River Delta of China, wild soybean, Phragmites australis and Cynanchum chinense are well adapted to the saline alkali soil and effectively improves the soil quality (Wang et al., 2012; Zhang et al., 2014). Lygeum spartum L. is a native salt-tolerant forage species in the Algerian prairie, which not only stabilizes sand dunes but also repairs saline soil (Nedjimi,2009). In greenhouse research, when the concentration of NaCl solution is 300 mmol/L, the germination potential of cordgrass seeds decreased by only 50%,but the germination potential of switchgrass seeds decreased by 80%. When the concentration is 500 mmol/L, cordgrass seeds are still alive, but 30%of switchgrass seeds lost their vitality. Therefore,cordgrass has adapted to salt tolerance (Mooring et al., 1971; Kim et al., 2012). The genetic diversity of 33 species of switchgrass seedlings where analyzed by the sequence-related amplified polymorphism(SRAP) method, after treated with a NaCl solution of 0 mmol/L and 250 mmol/L concentration for 24 days.Results show that t-2086, t-2101, BN-11357-63 and BN-12323-69 were salt resistant species, while BN-18757-67, 70SG0021, summer, 70SG0016, T16971,Dacotah, Turkey and 70 SG 003 were salt sensitive species (Liu et al., 2014). In Kim and Mooring, some varieties of switchgrass are salt-tolerant while others were sensitive to salt.

The extracted gene encobetaine aldehyde dehydrogenase from Atriplex was transferred into corn inbred lines Zheng58 and Qi319, which provided a higher fresh weight, lower malondialdehyde (MDA)content, and higher chlorophyll content under salinity stress for the transgenic corn compared to wild corn(Hong et al., 2015). The putative gene xyloglucan endotransglucosylase/hydrolase (pexth) from a 1-year old Populus euphratica treated with a salt solution was transferred into tobacco. After sufficient growth,the tobacco plant became a salt-tolerant variety (Han et al., 2013).

4 The effect of biological improvement of saline alkali soil

Biological improvement of saline soil mainly uses forage, crop, shrub and wild salt-resistant plants(Dong et al., 2008), which can be measured by the following evaluation index.

4.1 Desalination rate

In the waste saline soil of Binhai, Yellow River Delta, Suaeda salsa, Atriplex centralasiatica,Puccinellia macranthera and Nitraria can effectively decrease the desalination rate (Zhang et al., 2001).Zhao noted that after cultivation of Suaeda salsa in saline soil, the Na content reduced by 5.53-8.53 kg/hm2within a depth of 0-60 cm a year later (Zhao et al.,2001). This result is consistent with the research of Zhang et al. (2001). After cultivation of forage grasses in an saline-alkali soil of the Yellow River Delta, desalination rate in decreasing order was: alfalfa (41.81%) ＞ Sesbania (35.93%) ＞ white sweet clover (30.82%) ＞ chicory (27.52%) ＞ sweet sorghum(24.21%); the desalination rate in the 0-20 cm soil was higher than the 20-40 cm and 40-60 cm layers(Hou, 2014). Salt-tolerant forage grasses or Lycium barbarum should be cultivated when the salt percentage is higher. However, when the salt percentage is lower, salt-tolerant wheat should be cultivated and multiple-cropped with white sweet clover or alfalfa.Salt-tolerant wheat intercropped with leguminous forage provide the best desalination rate mode, such that in the 0-40 cm soil, the desalination rate was 90.8%,while in the 40-100 cm soil, the rate was 85% (Ren et al., 2004).

4.2 Soil physical properties

The physical properties of saline alkali soil are improved due to interspersed roots of salt-tolerant plants. After cultivation of salt-tolerant plants in the saline-alkali soil of the Yellow River Delta, bulk density in the 0-20 cm layer decreased significantly.In the 0-60 cm layer the decreased rate was: alfalfa(16.66%) ＞ white sweet clover (15.23%) ＞ Sesbania(14.11%) ＞ sweet sorghum (11.02%) ＞ chicory(6.83%) (Hou, 2014). From 1989-1992, the bulk density of soil decreased from 1.65 to 1.45, and the seepage velocity increased from 15 mm/d to 200 mm/d for cultivated salt-tolerant wheat in an alkaline soil. In 1992, the bulk density of soil decreased from 1.45 to 1.30 for cultivated alfalfa, and soil physical properties obviously improved after three years (Ren et al.,2004). Cultivated salt-tolerant herbage can also promote the formation of soil aggregate structure (Zhang,2005). Yellow sweet clover can effectively improve soil permeability, where the number of sticky grains in the size range of ＜ 0.001 mm and 0.001-0.005 mm decreased, the number of sand grains at 0.05-0.25 mm increased and porosity increased (Tang et al., 2004).After five years of Elaeagnus angustifolia cultivation in saline soil of the Yellow River coastal delta, the porosity increased by 11.8% in the 20 cm soil, 11.9%in the 40 cm soil, and 4.0% in the 60 cm soil. After four years of Nitrari cultivation, the porosity increased by 62.9%. After four years of Puccinellia macranthera cultivation, vegetation coverage increased by 100%, and porosity increased by 103.4%(Liu and Xie, 2007). After cultivation of perennial herbs such as oat, sweet clover, alfalfa, Elymus junceus, and E. novae-angliae in the saline soil of the Altai region, physical properties improved, the content of water-stable aggregates ＞ 0.25 mm increased,bulk density of the topsoil decreased, and permeability increased. Sweet clover and alfalfa improved the physical properties of soil most effectively because of deep and well developed root systems (Kursakova,2006) which is consistent with Hou (2014) .

4.3 Soil organic matter

In the Fukang saline-alkali soil of Xinjiang, China,the grass field rotation system provided 11,500 kg/hm2of organic matter, N 319 kg/hm2, P2O5101 kg/hm2,and K2O78 kg/hm2every year, and increased available nutrients to the soil (Hou, 2014). Organic matter N, P and K all improved after cultivation of Suaeda salsa, Atriplex centralasiatica, Puccinellia macranthera, Nitraria, and Elaeagnus angustifolia (Liu and Xie, 2007). After cultivation of Dixie, Smyrna,Seabrook, Georgia and native grasses in the coastal saline soil of Petchaburi Province, Thailand, the levels of organic matter for phosphorus and potassium increased (Pongwichian et al., 2014).

4.4 Microbial flora and soil enzyme activity

Microbes provide material circulation in the soil,which is closely related to soil fertility. Symbiosis of roots and fungi can improve salt adaptability of plants(Qin et al., 2016). After two years of castor cultivation in the coastal saline soil, soil microbial activity and diversity increased significantly. Functional bacteria also increased, such as halophilic, phosphate-solubilizing, potassium-solubilizing, cellulose decomposing, ammonification and nitrogen-fixing bacteria(Wu et al., 2012). The ratio of fungal C to bacteria C was about 0.7 in uncultivated saline desertification soil of Xinjiang Province. After two years of cotton cultivation in Xinjiang Province, this ratio was about 1.4. The microbial biomass C increased to 300 μg/g from the 4th year to the 16th year, and the ratio was 100%. The ratio of Gram-negative bacterial PLFA and Gram-positive bacterial PLFA increased with time (Liu et al., 2016). After cultivation of Suaeda salsa, Phragmites australis and Tamarix chinensis in saline soil of the Yellow River Delta, the activity of catalase, protease, urease, dehydrogenase and acid phosphatase were closely related to plant community succession in a salt marsh ecosystem (Di et al., 2014).For Suaeda salsa, the number of microbes increased significantly. Compared with the control, bacteria, actinomycetes, and fungi increased 2.5x, 4.3x and 71x,respectively. As soil salinity decreased, the lower salt tolerant microbes gradually became dominant. Based on phylogenetic analysis, Bacillus subtilis was the dominant species after soil improvement (Lin et al.,2006). Treated with fresh Leymus chinensis and animal manure, fresh alfalfa and animal manure, fresh alfalfa covering could improve the activity of urease significantly in a soda meadow alkali soil (Wu et al.,2009).

4.5 Production benefits

By salt-washing, the average yield of maize (SC-7OC) and wheat (new winter 17) was 6,000 hm2and 3,750 hm2in a saline-alkali soil. After three years of cultivation, the average yield of maize (SC-7OC) and wheat (new winter 17) was 11,260.5 hm2and 6,000 hm2.Thus, the biological improvement of saline-alkali soil has higher production potential than salt-washing(Zhang, 2005). In 1991, after cultivation of salt-tolerant wheat in a saline and alkaline wasteland of Shihezi, Xinjiang Province, the salinity was 1.019%-1.650%, the yield was 3,900 hm2, and the seedling area was 92%. The second stubble cultivated cotton got high yield. In 1999, after cultivation of salt-tolerant wheat in the same wasteland, the salinity was higher than 2%, but seedling area was only 65% due to poor irrigation conditions. However, continuation of salt-tolerant wheat cultivation in 2000, seedling area was higher than 85% (Wang et al., 2008). After three years of planted grass and raised livestock on high saline alkali soil, production benefits were 2.89 times higher than cultivated cotton on low saline-alkali soil (Ren et al., 2004). In India, the line spacing of eucalyptus was 1 m×1 m on the saline soil, the biomass was the largest (Dagar et al., 2016).

5 Conclusions

Numerous studies on the biological improvement of saline alkali soils have been carried out in terms of theories, species, cultivation measures and evaluation methods. At present, scientists generally use soil quality evaluation indices, such as physical, chemical and biological indices to evaluate the effect of biological improvement. The desalination rate, soil bulk density,soil organic matter, microbial flora and soil enzyme activity are used to evaluate the effect of biological improvement of saline alkali soil, but the comprehensive evaluation method is more reasonable. The characteristics of soil quality evolvement can be analyzed by comparative evaluation and dynamic evaluation. The comparative evaluation is a parallel comparison between different management modes, and the dynamic evaluation is a comparative evaluation of a long-term series evolution of soil quality (Larson and Pierce, 1994). The soil quality is an instant value,but the dynamic change of soil can reflect whether the soil is evolved or degenerated. The soil quality evaluation value can be obtained by multiplying the elements of soil quality (Doran et al., 1994). The method of MVIK (multiple variable indicator kriging) can synthesize the unlimited single soil quality index into a total soil quality index (Smith et al., 1993).

There have been numerous research achievements in the physiology and improvement of salt-tolerant plants, cultivating salt-resistant varieties, and transgenic plants with salt resistance. However, in China, this research is in the primary stage. Generally,studies mainly focus on the introduction of halophytes and salt-tolerant plants, such as Suaeda salsa,Puccinellia macranthera, P. distans, Atriplex centralasiatica, sweet clover, alfalfa, sesbania, and chicory. The main crops are Salicornia bigelovit,kender, and sweet sorghum. The main shrubs and wild salt-resistant plants are Lycium barbarum, Vitex trifolia, Tamarix Chinensis, and Nitrari.

Recently, numerous scholars have found that Pennisetum purpureum, Echinochloa, Lycoell Bamboo,and Arundo donax L. are good salt-tolerant plants. In addition, the biological improvement of saline alkali soil generally requires a long period of time. Therefore, in order to apply these species widely, it is necessary to study more appropriate cultivation measures.

Acknowledgments:

This project is supported by the National Key R&D Program of China (No. 2016YFC0501307) and the Key R&D Program of Ningxia Hui Autonomous Region (No. 2018BBF23008).