In this paper, the production of humic substances (HS) through the bacterial solubilization of low rank coal (LRC) was evaluated. The evaluation was carried out by 19 bacterial strains isolated in microenvironments with high contents of coal wastes. The biotransformed LRC and the HS produced were quantified in vitro in a liquid growth medium. The humic acids (HA) obtained from the most active bacterial strain were characterized via elemental composition (C, H, N, O), IR analyses, and the E4/E6 ratio; they were then compared with the HA extracted chemically using NaOH. There was LRC biotransformation ranged from 25 to 37%, and HS production ranged from 127 to 3100 mg.L−1. More activity was detected in the isolated strains of Bacillus mycoides, Microbacterium sp, Acinetobacter sp, and Enterobacter aerogenes. The HA produced by B. mycoides had an IR spectrum and an E4/E6 ratio similar to those of the HA extracted with NAOH, but their elemental composition and their degree of aromatic condensation was different. Results suggest that these bacteria can be used to exploit the LRC resulting from coal mining activities and thus produce HS in order to improve the content of humified organic matter in soils.

Biofertilizers are an alternative to face the sustainability problem that chemical fertilizers represent in agriculture. Among them, plant growth-promoting rhizobacteria (PGPR) is a microbial group with high potential, but lack of reproducible results from their application is a bottleneck for its use in agricultural production. Here we highlight a factor that could partially explain this inconsistency: the total auxin level in the soil–plant system. Auxin production is recognized as a main mechanism for plant growth promotion by PGPR; however, the final effect of auxins depends on a fine balance of its content, and this will be a result of all the sources of auxin compounds in the system. In addition to the auxins produced by inoculated bacteria, the plant itself produces its own hormones as part of complex physiological processes, varying in amount and sensitivity. Also, soil organic matter displays like auxin activity, causing plant responses just like those produced by added auxins. Therefore, the inoculation of an auxin-producing PGPR on plants might cause a wide variety of responses, ranging from effective growth promotion to growth restriction, depending on the total auxin content in root tissue. We think this must be considered for the practical use of bacterial biofertilizers, in order to have better and more consistent results of inoculation. Graphical Abstract

Background: This study assessed, under greenhouse conditions, the use of low-rank coal (LRC) generated in the "El Cerrejón" (La Guajira Colombia) mine as a source of humified organic matter, which is released by the activity of coal solubilizing bacteria (CSB), in order to improve the properties of edaphic materials (EM) used in post-mining land reclamation processes. Methods:In this trial, using pots with 10 kg of EM, the effect of LRC applications was tested, in contrast with applications of LRC inoculated with CSB at two different doses and EM without LRC. The responses of the cation exchange capacity (CEC), soil respiration, and fluorescein diacetate hydrolysis (FDA) were evaluated in each treatment. The contents of P and N and the dry weight were measured in maize seedlings. Soil microaggregates were observed with scanning electronic microscopy. Results:After 12 months of treatment with LRC, an increase of 75 % was registered in the CEC, along with an increase of up to 59 % in the EM respiration (CO 2 production) and up to 50 % in the FDA activity. The LRC caused a significant increase in the dry weight and the total contents of N and P in the maize seedlings; the observations with the scanning electronic microscopy showed evidence of the formation of bigger aggregates, as compared to the control, and the presence of organic material on mineral particles in the treatments with LRC inoculated with CSB. Conclusions:These results provided evidence that supports the use of LRC, alone or inoculated with CSB, as a humic amendment to improve the EM in soil reclamation processes (Technosols construction) after mining activities.

The humic acid (HA) obtained with the activity of three bacterial strains that transformed a lignite-type low rank coal (LRC) and released humified organic matter (HOM) was characterized. The HA obtained with the activity of Bacillus mycoides, Microbacterium sp. and Acinetobacter baumannii were compared with HA obtained with the traditional alkaline extraction method using NaOH 0.5 M (HA-NaOH). These characterizations included the elemental composition (C, H, N, O), Fourier transform infrared (FTIR) analysis, cross polarization-magic angle spinning-nuclear magnetic resonance (13 C-CPMAS-NMR), tetramethylammonium hydroxide (TMAH)-thermochemolysis followed by gas chromatography-mass spectrometry (GC-MS) and size exclusion chromatography. The two evaluated HA types showed differences in the elemental composition, aromaticity degree, content of aliphatic groups, molecular weight, polydispersity, profile of the spectra 13 C-CPMAS-NMR and products of TMAH-thermochemolysis. The results indicate that the process of biotransformation of LRC with bacterial activity results in the structural transformations of HA, which leads to the formation of HA with a lower degree of aromaticity, more of a hydrophilic tendency, lower oxygen content, enriched with nitrogenated functional groups and polar aliphatic chains, as compared with HA-NaOH. The HA generated with the three bacterial strains exhibited a high structural similarity to each other; however, some differences were evident in the type of metabolites generated by the TMAH-thermochemolysis of the HA obtained with B. mycoides, as well as a higher polydispersity for the HA generated with A. baumannii.