Low-rank coal (LRC) is lower-quality coal that is rich in humic substances, improving the soil structure, water retention, and nutrient availability. It is therefore being used as a bio-soil amendment worldwide. More specifically, this review article summarizes the potential role of LRC in stimulating microbial activity, promoting nutrient cycling, and enhancing soil properties, which may ultimately lead to sustainable salinity management and long-term soil restoration. Specifically, we explore the role of coal-solubilizing bacteria (CSB), enzymatic biotransformation, and microbial-driven soil aggregation to delineate how LRC synergizes a sustainable soil microbiome. Moreover, the review discusses the challenges in applying LRC, such as variability in chemical composition, economic aspects, and applicator requirement strategies. Further work is needed to standardize processing methods, evaluate the long-term impacts on soil health, and better understand the interaction between LRC and other soil restoration practices. With a focus on the global need for high-quality, effective organic fertilizer, with no chemicals, LRC provides a solution to thoroughly, quickly, and accurately restore salinity in soil, using the humus oligosaccharide and the humic polymarker. LRC is a real solution for agricultural sustainability.

Keywords: low-rank coal, soil salinization, microbial activity, humic substances, soil amendment.

1 Introduction

Soil salinization is a global concern affecting about 20 % of irrigated lands, reducing crop yields. High salt concentrations have a negative effect on plant growth, disrupting the osmotic balance and impairing the absorption of nutrients [1]. Soluble salt accumulation in soil reduces crop development, changes microbial populations, and degrades soil structure.

Fig. 1. Approaches for Managing Saline Soils

Strategies for reducing soil salinity ( Figure 1 ) include leaching, which involves washing the soil with excess water to remove salts [2]. Chemical additions, such as gypsum, are also utilized to improve soil structure and remove hazardous sodium ions [3]. A more biological technique is phytoremediation, which involves extracting or stabilizing salts using salt-tolerant plants [4]. However, these procedures can be expensive, time-consuming, and have limited long-term efficacy, particularly in large-scale settings. Whereas low-rank coal offers a viable solution for soil amendment and salinity remediation with its high organic matter content, notably humic compounds [5]. Humic compounds can improve soil structure, increase water retention, and supply nutrients for plant growth [6]. Furthermore, LRC can boost microbial activity in the soil [5], which is critical for breaking down organic matter, releasing nutrients, and improving soil quality.

2 Soil Salinization: A Comprehensive Overview

Soil salinization is a serious environmental problem caused by the buildup of soluble salts in the soil, which can reduce agricultural output and ecosystem health. The underlying causes of soil salinity are diverse and linked and can be classified as natural (primary) or anthropogenic (secondary) processes. Primary salinity develops naturally as a result of the weathering of parent materials containing soluble salts, atmospheric salt deposition, and saline groundwater intrusion [7]. Arid and semi-arid environments are more vulnerable to primary salinization because of high evaporation rates, which concentrate salts in the topsoil. Primary salinity salinization is influenced by geological factors such as the existence of salt-rich parent materials and the migration of saline groundwater [8]. Secondary salinity is caused by human activity, specifically unsustainable irrigation methods. Irrigation with water containing dissolved salts can cause salt accumulation in the root zone, particularly in poorly drained soil. Deforestation and land clearing can increase salinity by lowering plant cover and increasing surface runoff, resulting in more salt deposition in the soil [3].

Elevated salinity levels have a negative impact on a variety of soil features, including dispersion and crusting, impaired water retention due to osmotic stress and reduced infiltration, and limiting nutrient availability by interrupting absorption processes and negatively influencing microbial activity. These changes contribute to decreasing agricultural output and soil health, providing considerable issues for long-term land management.

2.1 Soil Structure

Salinity has a significant impact on soil structure. At high salinity levels, the repulsion forces between like-charged particles cause clay particles to spread out evenly in the soil ( Figure 2 ). Dispersal breaks down the soil aggregates so that this «sodium-rich» material becomes lumpy and hard as gravel [9]. In addition, the development of saline soils predisposes them to surface crusting. This means that a hard coating forms over the surface of the soil, which hampers water infiltration and root growth [10].

Fig. 2. Saline soil structure

This crusting, driven by both physics and chemistry, is further compaction of the soil and lowers its porosity. The poor structure of the soil means that its hydraulic conductivity is low as well. This results in drainage and waterlogging problems, which further injure soil quality [3].

2.2 Water Retention

Soil salinity changes the soil's osmotic potential by a lot. It results in an osmotic gradient, brought about by high levels of salt in the soil solution, which stops plant roots from being able to absorb water. Plants are thus left water-stressed [1]. The evaporation rates increase and the water content of salt-affected soils decreases, while, on the other hand, soil structure deteriorates. The salts can also form a hydrophobic layer in the soil, which reduces the retention and availability of water [11]. Failure to take up water may cause wilt and lower growth rates.

2.3 Nutrient Availability

Instead of enabling the plant to flourish, as needed, live salty soil changes the availability of some elements in the soil. There is an imbalance in nutrition, particularly the uptake of macronutrients like nitrogen (N), phosphorus (P), and potassium (K ⁺ ). Potassium deficiencies can result from sodium ions competing with it for absorption by plants [1]. Moreover, concentrations of Na ⁺ inhibit the intake of calcium (Ca ²⁺ ) and magnesium (Mg ²⁺ ), compounding nutritional deficiencies [11]. The productivity of the soil is decreased by salinity through the microbial populations, which have such a crucial role in cycling nutrients and breaking down organic matter. Microbial biomass and enzymatic activity are both reduced at high salinities, resulting in decreased nutrient availability and fertility of soils [12]. High salinity is particularly deleterious to the activity of soil enzymes like urease and phosphatase, which are essential for nitrogen and phosphorus cycling [3].

3 Low-Rank Coal: Composition and Properties

LRC has potential as a soil amendment material in areas with heavy salinity problems. This organic substance, originating from the finest grains of coal, sets a new low on its processing technology level. The following text is a look at LRC composition, how it interacts with soil, and what effects it has on soil pH, cation exchange capacity (CEC), organic matter content, and other indicators.

3.1 Composition of Low-Rank Coal

Low-rank coal, which includes lignite and brown coal, has a lower carbon content but higher moisture and ash content than higher-rank coal [13]. The organic composition of LRC includes:

LRC is abundant in humic substances (HS) — complex organic compounds formed through the decomposition of plant and animal matter. HS falls into three main categories:

— Humic Acids (HA): Soluble at alkaline pH, HA has a significant effect on soil fertility and structure.

— Fulvic Acids (FA): Soluble at either alkaline or acidic pH, FA is vitally important for the availability of nutrients to plants and soil microorganisms.

— Humin: Insoluble at either alkaline or acidic pH, humin contributes to the total organic matter content of soil [6].

Table 1

Structural Composition of Low-Rank Coal

Table 1 characterizes the chemical and structural features of LRC. Additionally, LRC contains significant inherent moisture trapped in its porous matrix. Which lowers the heating value, as energy is spent evaporating water during combustion. This moisture originates from condensation reactions during coalification. Also LRC has fewer hydrogen atoms relative to carbon, which directly reduces calorific value [22].

3.2 Mechanisms of Interaction with Soil

LRC interacts with soil in various ways, which is useful for soil structure, microbial activity, and nutrient availability. This is what makes it such a valuable remediation agent for soil. Most visibly, of all those advantages, is an improvement in soil structure. LRC can help to improve soil aggregation and porosity—and so, water retention capacity, drainage, and ventilation conditions. This is especially beneficial in deteriorated soils, where compaction and poor structure constrain the development of both plants and fungi [6]. Apart from this structural advantage that LRC brings, it stimulates microbial activity in soils. LRC can act as a substrate for various soil microbial entities. Injured plants, in turn, release humus that becomes food for soil microbial life. This includes CSB such as Bacillus mycoides and Acinetobacter baumannii . These microorganisms may convert LRC into humified organic matter (HOM), which will increase soil health by fostering nutrient cycling and biodiversity among soil microorganisms [5, 14]. This brings us to yet another function of LRC to excrete nutrients. From LRC-derived humic compounds, nutrients are chelated with convenient efficacy for plants. This process will increase the soil's capacity to retain nutrients, especially in saline-sodic areas where resources are often scarce [6].

3.3 Effects on Soil Properties

Moreover, the effects of LRC are not just simply structural biological matter, it can also change and enhance important soil functions long-term. One crucial property is the effect on soil pH, particularly in acidic soils. One advantage of organic acids in LRC is that they can help neutralize soil acidity, creating better conditions for plant growth. However, in saline-sodic soils, pH does not change much. This view has been confirmed by researchers using LRC [14]. Another consistent advantage of LRC is its ability to augment soil CEC. LRC has a high CEC, as it contains functional groups such as carboxyl (-COOH) and phenolic groups (Ph-OH). Applying LRC increases soil CEC, making it easier for the soil to retain important nutrients like ammonium or potassium. This is especially beneficial in saline-sodic soils where nutrient leaching is an issue [5, jk14]. Of all the contributions LRC makes to generating organic matter in the soil, this may well be its most transformative effect. When LRC is added, soil organic matter content increases dramatically. This is critical for improving soil fertility, microbial activity, and overall soil health. Organic matter produced from LRC helps to form stable soil aggregates that enhance soil structure and reduce erosion [2, 6].

The cumulative effect of these changes is to alter soil, rather than just restore it. By reinforcing soil structure, stimulating microflora growth, and improving nutrient conditions, LRC offers a comprehensive means, even in the most challenging conditions, to restore soil health.

4 Microbiological Activity in Saline Soil

Beneficial microbes interact with plant roots and also affect soil parameters, which brings the effect of guarding against salty incursions along with a general enrichment in the fertility of the soil. This is therefore especially important to stress the role of halotolerant bacteria and mycorrhizal fungi in bringing down salinity by different means with a common purpose but equally necessary to achieve it.

4.1 Halotolerant Bacteria

We know that halotolerant bacteria are a class of organisms that are adapted to live and thrive in high salinity environments. One of their most important activities is the osmoregulation as it allows their cells to maintain a state of homeostasis in spite of external stress by maintaining suitable solutes such as trehalose and glycerol, which protect cellular structures from dehydration or ionic imbalances [10]. Besides living themselves, halotolerant bacteria also actively promote nutrient solubilization, which makes phosphorus, vital for plant growth, more bioavailable to the plant. This characteristic renders them particularly useful in saline environments where nutrient availability is often restricted by high ionic concentration [16]. Halotolerant bacteria also enhance soil chemistry. These bacteria synthesize plant growth-promoting chemicals such as Indolacetic acid (IAA) and aminocyclopropane carboxylic acid (ACC) deaminase, which stimulate root elongation, reverse salinity's inhibitory effects on plant metabolism, and promote plant growth in saline conditions [1, 17]. At the same time, they form biofilms around plant roots that increase soil aggregation, water retention, and microbial stability. The result is a protective microbe-friendly environment for plants to survive salinity [18].

4.2 Mycorrhizae

Mycorrhizal fungi form beneficial partnerships with plant roots and are much more efficient in improving a plant’s tolerance to salinity. They extend the root surface area by forming extensive networks of their own hyphae so that plants may absorb water or nutrients in a more efficient way even when placed into saline soils, which are poor in nutrients. They are especially good at improving the uptake of phosphorus and potassium, which are essential nutrients for growth and stress resistance [1]. In addition to nutrient absorption, mycorrhizae enhance soil structure. The fungal hyphae lead to soil aggregation (and hence air and water do better). This raises root temperature as well as protects the growing tips of plants because they no longer come directly into contact with salty water [11]. Moreover, these fungi reactivate plant defense mechanisms after salinity strikes. They increase the activity of antioxidant enzymes, make osmotic adjustments for plants so that plants can still survive under salty conditions by preventing oxidative damage and bringing about a state of water equilibrium inside cells that suits them [18]. Lastly, by improving the carbon cycle and fertility of the soil, mycorrhizal associations also speed up the decomposition of plant residues and organic matter. This adds much-needed nutrients to the soil, which further assists in salinity management over a longer term [11].

5 The Role of Low-Rank Coal in Enhancing Microbial Activity

LRC has recently attracted great interest as a soil amendment for promoting microbial activities, nutrient bioavailability, and soil structures. As it is a rich source of humic substances and organic matter, LRC not only directly interacts with soil microorganisms and biochemical processes in the terrestrial ecosystem, but also interacts with salinity stress on soil microorganisms and biochemical processes, eventually enhancing soil fertility and ecosystem stability in the long term.

5.1 Enhancing Microbial Growth and Nutrient Availability

Among the most elemental ways that LRC improves soil function is that it promotes microbial growth. Its rich humic substances act as an organic carbon source, providing necessary nutrients to promote microbial growth. Therefore, LRC has remarkably increased soil respiration and enzymic activity, which serve as important indices for the activity of the soil microorganisms [14, 19]. On the other hand, LRC supports microbial biomass. Certainly, some loss of nutrients will result from microbial absorption, but by combining with clay and by increasing the CEC, LRC offers an advantage in the salinity of soils. It can prevent a shortage of nitrogen that leaves fields unable to grow crops and can cramp plant growth as well [5, 20].

5.2 Shaping Soil Microbial Communities and Biotransformation Processes

With the inorganic minerals in LRC serving as substrates, it can not only strengthen the microbial biomass but also guide the structure and diversity of the microbial community. Moreover, LRC-enriched soils are home to specific bacteria such as CSB Bacillus mycoides , Acinetobacter baumannii , and Microbacterium sp . The decomposition of the organic matter derived from coals produces HOM, which serves to enhance soil microbial habitats [15]. Further, interaction of LRC with microbial populations is a key factor in biotransformation, especially since microbial effort produces the key enzymes lignin peroxidase (LiP) and manganese peroxidase (MnP). They also facilitate the breakdown of complex organic matter, contributing to better nutrient cycling and soil organic matter turnover [5]. Such a cycle will improve soil nutrients and be more dynamic and robust, hence a thriving microbe community.

5.3 Improving Soil Structure and Salinity Management

The structural reforms that are achieved through the application of LRC are directly related to microbial activity. Microbes also help with soil aggregation while decomposing organic matter, which in turn improves aeration and water retention. This is especially important in degraded and compacted soils where structural limitation reduces microbial colonization and biological function [6] Additionally, LRC humic substances help in soil aggregate stabilization, which helps in the erosion and nutrient losses. LRC can also enhance soil structure by improving soil porosity and thus water infiltration. It creates a more hospitable environment for microbial life, allowing beneficial bacteria and fungi to thrive and help soil restoration processes [5]. One factor is that by creating less harsh soil conditions, LRC encourages microbial resilience and promotes beneficial microbial communities, leading to improved plant-microbe interactions. This effect is important in degraded soils where salinity-induced stress limits microbial and plant productivity. [20].

6 Challenges and Future Perspectives in the Use of Low-Rank Coal for Soil Restoration

The application of LRC in soil restoration has bright future prospects. However, in order to maximize its effectiveness and sustainability over the long term, various issues remain to be addressed. These challenges revolve around the chemical variability of LRC, microorganism interactions, and economic feasibility, as well as environmental concerns.

6.1 Limitations of Using LRC for Soil Restoration

LRC application's chemical composition is highly variable, so one of the challenges is to overcome this. Therefore, the focus switched to lignite and other low-rank coals with different compositions shown as suitable candidates as soil amendments. Sources of LRC are heavy metals and polycyclic aromatic hydrocarbons (PAHs) that could have a negative effect on soil and plant health [6]. Standardized processing methods are required to make LRC food safe and effective for agricultural utilization. Release of nutrients is yet another concern. LRC improves CEC and soil structure, but nutrients in this organic matter are released as microbes that are restricted by soil conditions multiply in the soil. This variability can complicate predictions of plant nutrient uptake and optimization, necessitating further study on controlled release mechanisms [5, 14]. Further, the interactions between microorganisms and LRC are complex and not yet fully understood. Although CSB convert LRC into HOM, the long-term effects of these interactions on soil microbial diversity and health require more in-depth investigation [5, 14].

6.2 Economic Feasibility and Sustainability Concerns

There were some quality data to suggest seeing below that full seeing is not nearly so zonally redeemable all that it represents excellent topsoil with a high nitrogen content. To those who watch these figures on an annual basis, above 30 % organic matter is far more common. For developing new variants of the LRC process (in response to different circumstances in its uptake). Conduct further experiments in land management and application, in order to define best practices for these respective areas of work (e.g., marketing your product). This LRC was greatly in demand in Iran and Afghanistan. However, planting was not always successful because the initial investment and the ongoing management may be cost-prohibitive for farmers and land managers [6]. Also, no one knows how sustainable LRC supply is. With increasing demand for organic amendments, high-quality sources of LRC may be limited, particularly if extraction is not sustainably managed [6]. Indeed, these must be explored as sustainable sourcing strategies and alternative processing methods need to be looked into to ensure long-term viability.

6.3 Research Gaps and Directions for Future Work

Determining the application rates to be used under strictly controlled conditions is also one of our pressing needs at present. Soil modifies the effects of LRC on soil chemistry, and its influence on microbiota as well as environmental stress can be considerable. Also, by upgrading application methods and by the simultaneous use of LRC with microbial inoculants, these benefits might be achieved [14, 21].

7 Conclusion

In the face of salinization and poor soil fertility, the application of LRC to restore soil quality feels like an encouraging biological answer. By harnessing humic substances and the activity of various microorganisms in soil, LRC enhances its nutrient retention, structure, and microbial diversity. As a result, it offers a sustainable model for long-term soil health management. LRC not only effectively improves soil conditions when it interacts with the microorganisms involved to condition the soil structure, it also makes a mechanism for supplying nutrients. The ability of enzymatic processes to turn soil organic matter into more readily plant-available forms further confirms how valuable microbially-based soil remediation can be. Also, LRC improves soil aggregation and porosity which reduces salt-induced stress, making the soil a more stable environment for plants and animals. While advantageous, there remain nonetheless several stumbling blocks. These include differences in chemical composition, cost restraints, and a dearth of long-term field research. To achieve the full benefits of LRC, standardized methods for preparing it and suitable application rates have to be found. The task at hand is to integrate these with other kinds of soil renewal—such as cover cropping and farmyard waste in general—in order that its effectiveness may be maximized. One area for further study is to examine the long-term ecological effects of LRC, particularly its role in the carbon cycle, and in engineering a natural system of land use. The succession of different microbial communities would also be worth watching.

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