Research Progress of the Improved Saline-Alkaline Soil by Biochar

. Biochar is a product for the recycling and utilization of solid waste from agriculture and forestry, and has more voids and larger specific surface area, and is widely used for improving saline-alkaline soil. Saline-alkaline soil is a key factor limiting crop growth. The research of changes in the environment of saline-alkaline soil caused by biochar application are of great significance for the sustainable utilization of saline-alkaline soil. This article summarizes the research progress on the improvement of saline-alkaline soil by biochar, and proposes future research priorities for the improvement of saline-alkaline soil by biochar. The research results provide data support and scientific basis for future research on the impact of biochar improvement on saline-alkaline soil on CH 4 uptake, and emissions of CO 2 and N 2 O.


Introduction
Biochar is a tiny particle porous of carbon-rich under anaerobic conditions (lack of oxygen) or at relatively low temperature [1]. The further explained concept and connotation of biochar from international biochar association (IBI) in 2013 is an obtained biomass by thermo-chemical conversion of solid products under anoxic conditions. Biochar is added as an additive to poor soil, and can improve the utilization rate of soil nutrients, and can prevent and control soil pollution, such as greenhouse gas emissions mainly containing carbon (C), hydrogen (H), oxygen (O), nitrogen (N) [2]. The carbon content of biochar ranges from 38% to 80% [3] Biochar is usually alkaline, with a pH between 5 and 12 [4]. Composition of biochar is highly aromaticity, including hydroxy phenol, hydroxyl carboxy, aliphatic double bond, and other special functional group [5].
Under change of the materials for producing biochar and pyrolysis temperature, there are significant differences in the properties of biochar [6]. The impact of biochar applied to widely distributed saline-alkaline soils on soil physicochemical properties needs to be further explored. Some severely salinized soils cannot be used for agricultural production, which are unable to support the growth of any plant, and have a negative impact to ecological system. China has about 3.4× 10 5 km 2 of saline-alkaline land, of which about 1.2× 10 5 km 2 can be used for agricultural production after improvement [7]. The improved saline soil by the biochar from agriculture and forestry waste reuse is one of the challenges promoting China's agricultural development in saline soil, and reducing the pollution for the environment of salinealkaline soil. The modified saline-alkaline land by biochar will be suitable for crop cultivation, which will be of great help to develope agriculture and improve local ecosystems.

Research progress of improvement measures in saline-alkaline soil
Scholars using biochar in the improvement of degraded soil at home and abroad had studied a lot in relevant fields. Biochar increased soil porosity [8], improved retention capacity of water and fertilizer in soil [9], promoted growth of crop, enhanced fertility of soil [10], and reduce carbon emissions [11]. However, previous research mainly focused on improving acidic soil by biochar. Research on improving saline-alkaline soil by biochar needs to be strengthened. The impact of biochar on saline-alkaline soil needs to be confirmed by longterm research data.
Saline-alkaline soil has high salinity and high pH high salt content and soil PH value. According to soil types and climate changes, distribution areas of salinealkaline soil are generally divided into five types (Table  1) [12]. Due to the complex composition of salinealkaline soil, it is difficult to implement management and remediation of saline-alkaline soil in the study area ( Figure 1).  Water management measures are based on transport of salt with water. A perfect drainage and irrigation system needs to be established [13]. The water level of groundwater is effectively controlled. Salt is discharged from the soil through drainage. The process of salt removal is divided into irrigation, washing salt, drainage, and desalination. The brackish water formed by washing salt is stored for irrigation [14]. The goal of reducing salinity of soil will be achieved. The combination of vertical shaft drainage and pipeline drainage buried in the subsoil can manage salt and water dynamics of soil and groundwater [15]. The physical measures for managing water and salt transport mainly include improvement of internal soil and mulching technology by crop straw. Improvement of the interior of saline-alkaline soil is lowcost mainly relying on natural precipitation [16]. Mulching technology by crop straw reduces accumulative evaporation of soil water, and effectively inhibits accumulation of salt [17]. Chemical measures for adding chemical modifiers change the physical and chemical properties of soil by ion exchange or neutralization reaction. Adding chemical modifiers, such as aluminum sulfate, or furfural residue, or gypsum can reduce the alkalinity of saline-alkaline soil, promote salt ions leaching downward [18]. Biological measures from planting salt-resisting and alkali-resisting plants can improve soil organic matter, increase available nutrient and improve soil structure by adding microorganisms and soil animals [19].
Previous research of many measures improving saline-alkaline land mainly focused on single measures to improve saline-alkaline soil. The single measure of improving saline-alkaline soil limits the research of impact of improvement measures on saline-alkaline soil. Therefore, the researchers apply comprehensive measures to improve different saline-alkaline soil ( Table  2), and find the most effective improvement methods of saline-alkaline soil.

Effect of biochar improving on salinealkaline soil
The research on saline-alkaline land governance is increasing gradually. The governance measures are also gradually improved. There is much attention paid to the modifiers made from reuse of solid waste at home and abroad, such as biochar [20], wood vinegar [21], gypsum [22]. These materials can reduce the degree of soil salinization, improve the content of soil nutrients, and have a positive effect on soil greenhouse gas emissions.

Effects of biochar on the physical and chemical properties of saline-alkaline soil
Biochar can not only improve saline soils to make them more suitable for cultivation, but also provide new solutions to solid waste disposal. Different types of biochar ( Figure 2). Biochar applied to saline-alkaline soil can improve soil texture, and affect physical and chemical properties such as soil moisture, pH and salt content. Adding biochar to the saline and alkaline soils could significantly improve the saturated water conductivity, increase the total effective porosity of the soil and the effective porosity of radius >100 m [23]. Biochar is mostly alkaline. the effect of biochar on soil pH in saline soils is controversial. Some studies have shown that the application of biomass charcoal can slightly reduce the pH value of saline soil, and the pH value of soil decreases with the gradual increase of biomass carbon [24]. However, Wu et al. (2020) found that enteromorpha biochar applied to coastal saline soil increased the soil pH by 1.49%, resulting in a negative effect [25]. The research of Sun et al. (2020) showed that the treatment of sand soil combined with biochar can significantly improve soil infiltration performance, inhibited the return of soil salt to the surface, and improved soil moisture and salt form [26]. The application of biochar in salt-stressed soil could alleviate degradation of crop growth caused by salt stress, improve the Na/K ratio, and reduce the soil EC [27].

Effects of biochar on the nutrient of salinealkaline soil
After 1.5% biochar was applied to the coastal saline soil, the content of soil organic matter was significantly increased, the proportion of organic carbon and available phosphorus in the total carbon and total phosphorus in the soil was increased significantly, and soil mineral elements were significantly increased [27]. At a higher biochar application rate of 50 t/ha, the organic carbon content and cation exchange capacity of the saline soil increased [28]. Straw biochar can significantly increase the content of available phosphorus and potassium in the soil with high salinity [29]. In general, adding biochar can improve the fertility status of salt-affected soils and the nutritional status of plants growing in these soils [30], mainly due to abundant amount of mineral elements in biochar.

Effects of biochar on microorganisms of saline-alkaline soil
Microorganisms are important participants in soil biochemistry. Microorganisms play an important role in the formation of soil fertility. Distribution and activity of microorganisms in soil are closely related to the formation of soil structure and the transformation of nutrients. However, the activity of microorganism will be inhibited, which will affect the growth of crops in salinealkaline soil. The large specific surface area and surface porosity of biochar can absorb dissolved organic carbon in the soil. The abundant pores of biochar are also conducive to the storage of organic carbon, which can indirectly provide nutrients to microorganisms and thus increase the abundance of microorganisms.
The study of microbial activities and changes provides data support for remediation of saline-alkaline soil by the application of biochar. Adding biochar to salinized soil of rice can increase the activity of soil dehydrogenase, sucrase, urease, catalase, alkaline phosphatase and protease, and significantly promote the microbial activity of the system [28]. Biochar was applied to alkaline soil with a pH of 8, and significantly increased microbial biomass carbon content of soil [31].
The combination of laboratory simulation and corn pot planting experiment proved that corn stalk biochar can increase the microbial biomass and microbial biomass carbon of saline-alkaline soil, and promoted the growth of corn seedlings [32].

Effects of biochar on greenhouse gas emissions of saline-alkaline soils
The greenhouse effect caused by greenhouse gases leads to global warming, sea level rise, desertification and other environmental problems. The emitted greenhouse gases from farmland soil, such as CO 2 , N 2 O and CH 4 , account for 10%~12% of the global anthropogenic greenhouse gas emissions, and are one of the important emission sources of greenhouse gases. Therefore, it is of great significance to study the effect of biochar on greenhouse gas emissions from saline-alkaline soil. Some studies show that the addition of biochar to salinized soil significantly reduces CO 2 emission flux and seasonal total accumulative emissions, and reduces Global Warming Potential (GWP) [33]. The addition of biochar of 30 t/ha can significantly reduce the total seasonal accumulative emissions of CO 2 and N 2 O, and has the best effect of carbon sequestration reduction [11]. The application of corn stalk biochar can effectively reduce the total greenhouse gas emission of corn farmland ecosystem [34]. The application of biochar in alkaline soil increases the content of soil organic matter and effectively inhibits the emission of soil CO 2 [35]. Adding biochar to paddy soil reduces the accumulative emissions of CH 4 and increases the accumulative emissions of N 2 O, but overall reduces the net emissions and intensity of greenhouse gases [36]. Foreign studies have also shown that biochar can reduce the accumulative emission of CO 2 and N 2 O, and has the potential to alleviate the flux of CO 2 and N 2 O in the actual ecosystem [37]. The application of biochar into the soil may influence the abundance of soil microorganisms, and further affect soil greenhouse gas emissions.

Conclusions
Biochar can improve soil texture and physicochemical properties, and increases soil nutrient, promotes crop growth, and reduces greenhouse gas emissions of soil. However, there are some issues that need to be addressed and proved by further research in the application of in saline-alkaline soil.
(1) Biochar is mostly alkaline, improvement of saline-alkaline soil by biochar is still controversial. Future research should focus on large-scale and longterm in-situ field observations.
(2) To solve the possible negative effects of biochar on saline-alkaline soil, the modification of biochar and the combination between biochar and other materials will be one of the emphases of research in the future, such as wood vinegar.
(3) Biochar may contain heavy metals or polycyclic aromatic hydrocarbons (PAHs). The study on the environmental toxicity of biochar will be the key direction in research of future.