LRC accrue benefits for soil structure by enhancing its water retention ability, aggregate stability/porosity, aeration, and bulk density. The water holding capacity of brown coal HS due to its partial hydrophilicity and porous character is well-understood [49]. Piccolo et al. showed that coal-derived HS can improve the structure and water retention of degraded arable soils and argued that the higher the HS content, the better the water retention of soil [50]. Cihlar et al. [51] suggested that modification of brown coal HS by formaldehyde cross-linking may provide an effective strategy for achieving high water uptake kinetics. Oxidation may enhance the HA content of coal sources to be used as soil conditioners. Two independent experimental studies showed that nitric acid (HNO₃) oxidation of brown coal leads to the increase of HA content with richer functional groups and ensures the retention rate of nutrients, which consequently improves soil aggregate stability and associated structure [52,53].

Brown coal-derived humic acid can reduce the disaggregating effects of cyclic wetting and drying on soil structural stability [54]. Soil porosity, an essential component of the soil skeleton structure and site productivity, can be maintained after coal mining by conducting site reclamation [55]. Being a rich source of carboxylic acid and phenolic hydroxyl functional groups, HS can provide reactive sites, increasing the CEC and the pH buffering of soils [56]. The high CEC of brown coal results in greater retention of NH₄⁺ and consequently lowers the NO₃⁻ leaching loss [57].

Most of lignin oxygen-containing functional groups result in a low pH level when ionized in solution [58]. For this reason, LRC can be rather effective in neutral-to-alkaline soils. However, in combination with lime, brown coal is well suited for application to soils with a low pH. Imbufe et al. [59] have found LRC humates to be effective for increasing pH and electrical conductivity in acidic soils. Further acid ameliorating effects by LRC have been studied in different conditions [20]. According to another recent report, the LRC at a dose of 5 kg m⁻² contributed to the decrease of electrical conductivity and sodium adsorption ratio of saline-sodic soils, whereas pH levels and bulk density displayed no significant changes [25].

The high content of TOC and its relatively slow mineralization suggest LRC be attractive for increasing plant nutrient supply in the soil the same way as known organo-mineral fertilizers [52]. The application of LRC by B. Dębska with colleagues [21] resulted in an increase in TOC content (by ~300%) and elevated soil organic carbon with higher aromaticity (38.6% compared to 35.4% in controls), which implies higher C sequestration potential and recalcitrance. Along with LRC, the LRC-derived humic acid products can outperform conventional organic wastes such as farmyard manure (FYM) in ameliorating soil quality and fertility [60]. Enhancement of SOC content and sequestration following LRC-derived HS is well-documented by R. Spaccini et al. [61].

The good adsorptive properties of LRC have aroused intense interest for its potential as a versatile environmental adsorbent. The utilization of the coal-based HS in soil remediation [45,62,63,64] and water treatment systems (municipal wastewater and acid mine drainage) [65,66,67] are recently well-documented. Detoxification studies by research groups led by Qi and by Skłodowski [58,68] have employed LRC as an attractive low-cost adsorbent for the removal of different pollutants from the aquatic and terrestrial environments. The complex and heterogeneous coal matrix is created by amorphous polymers containing double- or triple-substituted aromatic rings which makes LRC highly suitable for immobilizing di- and trivalent metals in soil, consequently reducing their uptake by plants. Brown coal-derived HA has been used already multiple times for the environmentally beneficial adsorption of metal ions (Al³⁺, Pb²⁺, Fe³⁺, Ca²⁺, Mn²⁺, Mg²⁺, Cu²⁺, Ni²⁺, Co²⁺, Cd²⁺), that strongly reduced their mobility, bioavailability, and phytotoxicity [69,70,71,72,73].

The system of interactions between HA and dissolved metal ions creates a complex supramolecular network given by their heterogeneous, polyelectrolyte, and polydispersive character [74]. In comparison with HA/HS isolated from various soils, HA/HS from brown coal exhibit a remarkably high sorption capacity and a low desorption profile [75]. A. Pusz [76] showed that brown coal can be especially effectively employed on soils strongly contaminated with heavy metals, and suggested using it at the dose of around 90 t ha⁻¹ (roughly equivalent to a dose of 150 g pot⁻¹ in their studies).

Coal-derived humic substances are considered to be effective for the extraction and concentration of many organic pollutants as well. The recovery degree of phenols using magnetic Fe₃O₄ nanoparticles modified with HA from natural sources (brown coal, peat, chernozem, and sapropel) exceeded 94% [77]. The sorption rate of polar organic pollutants can be strongly influenced by the degree of the HS aromaticity [78]. Brown coal amendment to soil contaminated with the pesticide pentachlorophenol resulted in a distinct improvement of its biodegradation, enhancing the growth of the inoculated bacterial strain Comamonas testosteroni [79].

The application of exogenous organic matter is often critical to improving soil fertility and nutrient management. Only such a treatment can substantially stimulate microbial activity, root respiration, enzyme turnover, and many other biological processes in soil. Studies assessing the impact of LRC on soil microbial community structure and activity are scarce. However, existing reports consistently show that the LRC amendment increases soil microbial activity, manifesting in elevated soil respiration, higher enzyme activity, and larger CEC [36,84,85]. The high specific surface area and porosity of LRC promotes ventilation and moisture retention, providing a favorable habitat for the growth and activity of microbial communities [31]. Activity levels of various hydrolytic and ligninolytic enzymes (including esterases, peroxidases, phenol oxidases as well as supporting enzymes, e.g., H₂O₂-generating oxidases; all predominantly of fungal origin) are strongly positively correlated with the enrichment of soil with LRC [86].

Due to its chemical and physical properties LRC act as a “storehouse” for nutrients that attracts soil microbial communities. Microorganisms with different physiological properties and metabolism transform LRC and generate HS through the so-called “ABCDE-system” (A = alkali, oxidative; B = biocatalysts; C = chelators; D = detergents; E = esterases) [87]. Microbially produced chelators and alkaline substances attack the macromolecular coal matrix and dissolve HS [87].

Metagenomic analyses revealed that both endophytic and epiphytic microorganisms are abundant in the LRC environment, as coal is generally originated from plant materials and therefore exhibits inherent plant interaction abilities [88,89,90]. LRC supplementation usually promotes the relative abundance of Actinobacteria. Due to their filamentous nature, these bacteria favor and can readily colonize the leonardite-rich environment [36,91]. Many members of Actinobacteria and Firmicutes are able to solubilize and depolymerize coal matrix [92,93,94]. However, further data regarding microbial functional responses to LRC exposure are inconsistent. Victorian brown coal had a short-term effect on the soil microbial community after 60 days of application, i.e., it temporarily increased the peroxidase and phenol oxidase activities, suppressed the heterotrophic respiration, and induced shifts among microbial populations [24]. Bekele et al. [34] observed that leonardite amendment had no effect on microbial biomass carbon (MBC) of the receiving subsoil, while application together with labile organic mix resulted in intermediate MBC values. It is important to note that the current understanding of microbial colonization and its activity is mainly drawn from short-term studies; thus, more testing should be done yet, especially focusing on long-term studies.