Low-Rank Coal as a Source of Humic Substances for Soil Amendment and Fertility Management

Abstract

Humic substances (HS), as important environmental components, are essential to soil health and agricultural sustainability. The usage of low-rank coal (LRC) for energy generation has declined considerably due to the growing popularity of renewable energy sources and gas. However, their potential as soil amendment aimed to maintain soil quality and productivity deserves more recognition. LRC, a highly heterogeneous material in nature, contains large quantities of HS and may effectively help to restore the physicochemical, biological, and ecological functionality of soil. Multiple emerging studies support the view that LRC and its derivatives can positively impact the soil microclimate, nutrient status, and organic matter turnover. Moreover, the phytotoxic effects of some pollutants can be reduced by subsequent LRC application. Broad geographical availability, relatively low cost, and good technical applicability of LRC offer the advantage of easy fulfilling soil amendment and conditioner requirements worldwide. This review analyzes and emphasizes the potential of LRC and its numerous forms/combinations for soil amelioration and crop production. A great benefit would be a systematic investment strategy implicating safe utilization and long-term application of LRC for sustainable agricultural production.

Assessing Factors Controlling Structural Changes of Humic Acids in Soils Amended with Organic Materials to Improve Soil Functionality

“…The benefits of HAs in soil are far-ranging and are well reported in the literature, including improvement of soil structure (water holding capacity, aeration, and aggregation) and promotion of plant growth through biostimulation (enzymatic activation, microbial growth stimulation, and root elongation) [12,14,15]. This highlights the suitability of HA-rich organic amendments for both environmental and agricultural applications [1,10,16].…”

“…Moreover, the peak at 1240-1200 cm −1 was less intense, and the shoulder at 1080-1040 cm −1 developed into a stronger peak relative to the other samples. Collectively, these changes indicated reduced condensation and a lower degree of humification in samples T7-T8 [7,10], which can also be inferred from their significantly low C/N ratios (Table 3). The structure of functional groups in both samples were generally similar, another indication that the effects of BIO amendment may have diminished eight years after it was added once to soil, unlike soils from the other sites that had been amended every 2-5 years prior to sampling.…”

“…The correlation between the C/N ratio and fraction of aromatic C (determined by NMR) of the HA samples was moderately positive (R = 0.6) (Figure 4). This suggests that the degradation and condensation of SOM are somewhat intrinsically linked with the formation of aromatic compounds in HAs [7,10]. However, the concentration of aromatic compounds determined by FTIR was poorly correlated with the C/N ratio (R = −0.004), and thus there was a weak relationship between the NMR and FTIR (R = 0.018) aromatic fractions.…”

“…During the transformation of SOM, C is preferentially concentrated in HAs relative to fulvic acids, the secondary components of HSs [8]. Thus, the elemental ratios of C in HAs, especially C/N, are used as indicators of humification and aromatisation [7,9,10]. At the latter stages of humification, the structural complexity of HAs increases, leading to the formation of complex substituted aromatic and carboxylic functional groups, which behave as di-or tribasic acids.…”

“…At the latter stages of humification, the structural complexity of HAs increases, leading to the formation of complex substituted aromatic and carboxylic functional groups, which behave as di-or tribasic acids. This enhances their surface charge and reactivity, making them important for nutrient retention and contaminant immobilisation in soil [10][11][12][13]. The benefits of HAs in soil are far-ranging and are well reported in the literature, including improvement of soil structure (water holding capacity, aeration, and aggregation) and promotion of plant growth through biostimulation (enzymatic activation, microbial growth stimulation, and root elongation) [12,14,15].…”