1. Introduction
Compost and other organic compounds are potentially beneficial amendments for potting soil mixtures. Organic compounds can contribute to improvements in plant growth and yield due to the nutrients and improved water-holding capacity they provide [
1]. Ecological management approaches that use organic amendments are preferred to reach multiple-benefit targets, such as enhancing agricultural production without harming the environment [
2]. Organic soil amendments are preferable as efforts are made to reduce the use of inorganic fertilizers [
3].
Sphagnum peat moss is one such amendment and has traditionally been used to provide these advantages in soil [
4]. However, more recent efforts have been made to replace peat moss due to the destruction of wetland ecosystems by the harvesting process [
4].
Vermicompost, worm castings, is an effective soil conditioner because it contributes plant-available nutrients and organic matter [
3,
5,
6]. Vermicompost derives from composted organic waste substrates in the presence of earthworms [
7] with a good physical structure, abundant labile resources, and high microbial activities [
8]. Farm animal manure is a beneficial material for addition to the soil by enhancing both physical and chemical characteristics [
9]; however, to be useful, organic matter, such as manure, must be decomposed to a humus-like state [
9]. Humus is the organic material that results from the decomposition of living organisms in soil. It is best to store manure so it can mature in a stable manner, thereby providing the physicochemical characteristics necessary for soil enrichment and crop growth [
9].
The introduction of earthworms can accelerate conversion of cow manure to a mature state for enhancement of plant growth [
10]. By way of function, earthworms have profound direct and indirect impacts on nutrient availability through increased decomposition of plant residues and turnover of soil organic matter [
11]. The conversion of manure by worms also addresses animal waste management regulations by stabilizing the waste chemically and reducing the microbial biomass [
10]. Vermicompost from cattle manure and paper waste has been shown to benefit growth of peppers (
Capsicum annum) var. King Arthur, at levels that are not significantly different than compost and inorganic fertilizers [
5].
Soil amendments with biochar have the potential of addressing both soil fertility and carbon sequestration. Wu et al. identified synergistic interactions between vermicompost and biochar in promoting crop yield on rice grown in a paddy soil [
2]. Additionally, it was found that, in the presence of vermicompost, the incorporation of biochar amendment significantly decreased the cumulative N
2O emissions.
The sugarcane industry generates large volumes of biological byproducts that are rich in nutrients that can be agriculturally beneficial and may otherwise be wasted [
12]. Two of these byproducts generated at the sugarcane factories are filter cake mud and bagasse. Particularly, composting has been used to convert filter cake mud into a humus-like state [
12]. A benefit of composting is that it generates heat, which kills microbial plant pathogens and weed seeds [
12]. Filter cake mud is a byproduct composed of sugar, fiber, coagulated colloids, albuminoids, inorganic salts, and particles of soil, which can be beneficial for plant growth [
12].
Highly weathered and/or used soils have reduced organic matter and fertility [
13]. In order to mitigate carbon losses, it is usually necessary to add organic inputs, of which worm castings and sugarcane filter mud compost are good examples. The blending of materials (worm castings or sugarcane filter mud compost) harboring microbes that promote plant growth with biochar was previously shown to have no deleterious effects on the enumerated microbial populations [
14]. It was theorized in that work that incorporating biochar, porous carbon clusters with mineral inclusions, into the blends stabilized the moisture and pH levels. It is possible that the release of low-volatile compounds from the biochar may lead to abundance of microbial activity in the soil. These factors might create an environment conducive to microbial survival. It is also possible that the biochar binds harmful bacterial waste products [
14]. The biochar may be functioning similarly to a soil aggregate of organic matter and minerals [
15].
A study on mycorrhizal responses to biochar in soil [
16] proposed several not mutually exclusive mechanisms, from altered nutrient availability and/or soil physicochemical parameters to alterations that are either beneficial or detrimental to other soil microbes, or alterations of signaling processes between plans and mycorrhizal fungi, to biochar serving as refuge from hyphal grazers.
Studies such as those reported in [
15] have enumerated microbial populations (but the microbes were not taxonomically characterized) to determine potential blending and storage effects on the numbers of microbes present. It is also necessary to qualitatively determine which microbes are present in the stored blends. Qualitative analysis was undertaken in this study to determine whether known pathogens are present, whether beneficial microbes are detectable after long-term storage, and whether, and in what way, blends with higher concentrations of biochar affect the microbial population. The potential for the presence of pathogens was due to the environmental samples containing unknown microbes, especially those from cow manure.
A microbiome is a community of microbes in an environmental niche [
17]. Metagenomics is the identification of the microbial community in environmental samples by direct genetic analysis [
18]. This identification of microbes within the population, also called metataxonomics, is made possible by extraction of total DNA from the sample followed by next-generation sequencing (NGS) [
17]. Microbes in the rhizosphere, the interface between plants and soil, provide nutritional support for plants and are a source of biochemical reactions that recycle nutrients [
19,
20]. NGS has become less expensive and more time efficient, enabling broad use of the technology on samples from multiple and varied environments [
21]. This work reports the determination of prokaryotic microbial populations in the metagenome of biochar blended with either worm castings or mud compost from sugarcane processing. These blends have been previously found to have beneficial chemical properties for use as soil amendments [
14].
3. Results and Discussion
Blends of biochar with either worm castings or filter mud from sugarcane processing were previously shown to have chemical properties that are beneficial for plant growth [
14]. Wu et al. [
2] determined the existence of synergistic interactions between vermicompost and biochar in promoting crop yield using only 1% biochar amendments. Using a soil column experiment, it was determined that biochar combined with vermicompost increased rice yields by up to 35%. In our previous study, microbes in the blends were enumerated, but were not taxonomically characterized [
14]. The blends are novel, so it was of use to determine whether the microbes also offered benefits for future plant growth, and to ensure that they did not potentially introduce foodborne pathogens to soil and plants. No bacteria were found in samples of biochar alone in the previous study [
14], so biochar samples were not taxonomically characterized. The results show that bacterial genera known to cause foodborne illness were not identified in any of the blends. These genera, listed by the Centers for Disease Control and Prevention (CDC), include
Escherichia,
Listeria,
Salmonella, and
Vibrio [
23], though other nonpathogenic members of the phylum Proteobacteria were identified in some samples. Orders, families, genera, and species that were found in all samples are discussed and indicated in blue in
Table 1,
Table 2,
Table 3,
Table 4,
Table 5,
Table 6,
Table 7 and
Table 8, those that were affected by higher ratios of biochar are discussed and indicated in red, and those that were found inconsistently in samples within an experiment are not discussed and are indicated in black. All data are reported as percent of total nucleic acids in the sample metagenome. In all cases, there were microbes with fractional percentages as well as unknown DNA. The percentages on the tables therefore do not equal 100%. In all cases, the predominant microbes are included.
3.1. Qualitative Analysis of Worm Castings Plus Biochar Blends
The following orders were identified in all samples of both experiments that included blending worm castings with biochar, in order of prevalence: Rhizobiales (10.34%), Actinomycetales (10.04%), Clostridiales (6.65%), and Rhodospirillales (5.19%) (
Table 2).
The order Bacillales was identified in all samples of the second, expanded experiment, and in blends of the first experiment that contained 5% or no biochar, with an average of 10.34% (
Table 1). The order Anaerolineales was found in both experiments in blends that contained less than 90% biochar, with an average of 4.3% (
Table 2). The order Actinomycetales was classified by Buchanan [
24], and it comprises Gram-positive bacteria that form branched chains, members of which are present in soil and offer the benefit of nitrogen fixation to plants. The order Clostridiales was classified by Prevot [
25], and it comprises Gram-positive, anaerobic bacteria that are present in the soil. Members are symbionts, which digest organic matter. Members of the included family Clostridiaceae were identified in all samples of both experiments (
Table 3), as were included members of the genus
Clostridium (
Table 4).
The order Rhizobiales was classified by Kuykendall [
26], and it comprises Gram-positive, nitrogen-fixing, symbiotic bacteria found in the soil. The order includes members of the family Hyphomicrobiaceae, which were identified in all samples of both experiments (
Table 3). The included genus
Rhodoplanes was identified in all samples except in those where the biochar ratio exceeded 50% (
Table 4).
The order Rhodospirillales was classified by Pfennig and Truper [
27] in the phylum Proteobacteria, which also includes several foodborne pathogens, none of which were identified in the worm castings plus biochar blends. Rhodospirillales are primarily purple nonsulfur bacteria that undergo photosynthesis. Members of the included family
Rhodospirillaceae were identified in all samples of the second experiment (
Table 3). The order Bacillales was classified by Prevot [
25]. The order includes Gram-positive bacteria in the phylum Firmicutes, which includes microbes found in, and beneficial to, the human gut [
28], none of which were identified in the worm castings plus biochar samples. Members of the included family
Bacillaceae were identified in all samples of the expanded second experiment and in samples of the first experiment that did not contain biochar (
Table 3). Included members of the genus
Bacillus were identified in all samples of the second experiment (
Table 4). No single species within this genus was found consistently in a range of samples (
Table 5).
The genus
Bacillus includes a broad range of characterizations, making generalizations at this level impossible [
29]. The genus
Paenisporosarcina, which is classified in the family
Planococcaceae, was identified in all samples from the second experiment which included 50% or less biochar (
Table 4). Similar to
Bacillus, it is an aerobic spore-forming bacterium, but is found in dairies at a much lower frequency [
29]. The order Anaerolineales was classified by Yamada [
30] and comprises Gram-positive bacteria. Members of the included family
Anaerolineaceae, anaerobic digesters of organic waste, were also identified in blends with less than 50% biochar (
Table 3). Two species of this family,
Bellilinea caldifistulae and
Longilinea arvoryzae, were classified after isolation from digester sludge and rice paddy soil, respectively [
31].
B. caldifistulae was found in samples of the second experiment in blends containing 50% or less biochar (
Table 5).
L. arvoryzae was found in samples of both experiments in blends containing less than 75% or less biochar (
Table 5). Both species are syntropic, allowing growth in environments that would otherwise be deleterious to the growth of these bacteria [
32]. They reverse the homoacetogenic pathway, thereby preventing accumulation of inhibitory concentrations of acetate and allowing continued conversion of agricultural waste [
32].
The species
Runella limosa and
Bifidobacterium bombi were also identified in samples of worm castings plus biochar, as was the genus
Rhodoplanes.
Runella limosa is classified in the order Cytophagales and family Cytophagaceae. It was isolated from activated sludge [
33] and found in all samples of the second experiment and samples of the first experiment that contained 10% or more biochar (
Table 5).
Bifidobacterium bombi is classified in the order Bifidobacteriales and the family
Bifidobacteriaceae and is found in the digestive tract of bumblebees [
34]. The genus is characterized in the gastrointestinal system of humans and several other animals.
B. bombi is not one of the two
Bifidobacterium species that have been detected in both animals and humans [
34].
Rhodoplanes is classified in the order Rhizobiales and family
Hyphomicrobiaceae [
35]. It produces hopanoids, which are characteristic bacterial biomarkers in the biomass of soils and sediments [
36].
3.2. Qualitative Analysis of Mud Compost Plus Biochar Blends
The following orders were identified in all samples blending mud compost with biochar, in order of prevalence (
Table 5): Actinomycetales (11.87%), Clostridiales (7.95%), Rhizobiales (7.47%). The order Chromatiales (4.32%) was identified in all samples that contained 50% or less biochar. The orders Rhodospirillales (4.32%) and Thermoanaerobacterales (3.37%) were found in samples that contained less than 75% or less biochar (
Table 6).
The order Chromatiales was classified by Imhoff [
37], and comprises photosynthetic, purple, sulfur bacteria found in water environments. The included family
Chromatiaceae was also identified in samples containing 50% or less biochar (
Table 7).
Within the order Clostridiales,
Clostridiaceae was the only family identified in all mud compost plus biochar blends (
Table 7), and
Clostridium was the only genus found in all samples (
Table 8). Within the order Rhizobiales, both the family
Hyphomicrobiaceae and the genus
Rhodoplanes were identified in samples that contained 75% or less biochar (
Table 6 and
Table 7). Within the order Rhodospirillales, the family
Rhodospirillaceae was identified in samples containing 75% or less biochar (
Table 7). The order Thermoanaerobacterales was classified by Wiegel [
38], and it comprises bacteria that share the survival at high temperatures and includes families that produce biohydrogen more efficiently than the families
Clostridiaceae and
Enterobacteriaceae [
39].
The following species were also identified in samples of mud compost plus biochar:
Azospirillum,
Caldithrix, and
Gemmatimonas.
Azospirillum is among the best-studied rhizobacterial genera, capable of colonizing and enhancing growth of multiple plant species [
40] and was identified in samples containing 50% or less biochar (
Table 8).
Caldithrix is a newly recognized genus comprising two species, having been originally isolated from a hydrothermal vent [
41].
Caldithrix paleochoryensis was isolated from geothermally heated sediment [
42], and was the only species identified in all samples of mud compost + biochar (
Table 9).
Gemmatimonas is a newly recognized genus found in soil, sludge, and water [
43]. It, as well as the included species
G. aurantiaca, was identified in samples of mud compost + biochar with 50% or less biochar (
Table 8 and
Table 9).
4. Conclusions
Characterization of soil microbes is a necessary step toward the goals of tracking population changes, their impact on plants, and breeding for genotypes that are conducive to beneficial interactions. In the study presented here, microbial populations were qualitatively identified in blends of worm castings or mud compost with biochar sourced from sugarcane bagasse, with the intention to apply them as soil amendments. As anticipated, microbes most likely to survive long-term storage, including spore formers, were identified in the samples. Identified orders and their potentially beneficial characteristics include Actinomycetales and Rhizobiales, which are involved in nitrogen fixation, Clostridiales, which digest and recycle organic matter, Anaerolineales, which digests organic waste and sequesters compounds that could be harmful to plant growth, and Thermoanaerobacterales, which produces biohydrogen. Bacterial genera known to cause foodborne illness were not identified in any of the blends.
Based on the microbes identified in this study, the application of worm castings or mud compost blended with 50% biochar would provide beneficial microbes as well as the previously identified physicochemical properties to enhance soil quality and plant growth, even after long-term storage.