Connection of metabolites with microbial diversity in different fallow periods and Rhizopshere niches, RI and BS
In our study, specific functional groups were more represented in different niches. The affiliation of the sequences in the KEGG database and further analysis in STAMP indicated the metabolism-related with ’ABC transporter,’ ’Amino acid and sugar metabolites metabolism, ’Lipopolysaccharides biosynthesis, ’pyruvate metabolism’ in theJhum field soil.
ABC transporter re involved in the secretion of phytochemicals present in the root exudates in most plants (50-53). Any mutation in the ABC transporter alters the root exudation of phytochemicals, which in turn changes the microbial diversity near the plant root rhizosphere (54). In the bacterial system, it helps in nutrient uptake as well as in the secretion of toxins and antimicrobial agents. In a comparative study, we have found that in bulk soil where there is no influence of crop roots, there is a higher presence of ABC transporters in 5 years than in 20 years fallow periods (Fig. 7 a,b,c,d). This may be attributed to the high stress put by the bacterial community to make the soil stable, unlike the soil in a long fallow period, which is already in a stable state. A similar pattern was observed in the case of LARS (Fig. 7 b); however, the ABC transporter in SARS and RI is opposite and is higher in the case of the long fallow period (Fig 7 c,d). Interestingly, it has been found that the phylum Proteobacteria is abundant in thejhum fields irrespective of the crop, rhizosphere niches, or the fallow period (Fig. 2 a,b,c,d). Proteobacteria , along with other soil phyla such as Bacteroides and Acidobacteria, uses the ABC transporter as an active transport system (55).
Our study has found greater amino and nucleotide sugar metabolism gene abundance in fields with a long fallow period of LARS (Fig. 7b) and BS (Fig. 7a) as compared to the short fallow periods of SARS (Fig. 7c) and RI (Fig 7 d). The amino and nucleotide sugar indicates the soil microbial community composition. It has been reported that any change in the ratio of the amino and nucleotide sugars can be used as a qualitative indicator of time-integrated compositional changes in the soil microbial community (56, 57).
Lipopolysaccharides are complex glycolipids attached to the outer membrane of most Gram-negative bacterial cells (58). These compounds aid soil stability by ”gluing” soil particles together (59). Bacteria use these compounds for attachment to soil particles, promoting the formation of soil aggregates (60, 61). In the jhum field ecosystem, we have found that the long fallow periods in the BS, LARS, and SARS have more lipopolysaccharide metabolism than short fallow periods (Fig. 7 a,b,c). This indicates that the long fallow periods have more soil aggregating propensity than the shorter fallow periodsjhum fields. In the case of the pyruvate metabolism, while RI follows a similar trend to lipopolysaccharides metabolism (Fig. 7d), however, it is predominant in the BS, LARS, and SARS of short fallow periods than the long fallow period(Fig 7a,b,c). Pyruvate is a vital intersection in the network of metabolic pathways that unites several vital metabolic processes in both aerobic and anaerobic bacteria (62). Our finding suggests more pyruvate metabolism in the short fallow periods.
Another interesting observation is the high sequence abundance for benzoate in long fallow periods of SARS and RI as compared to the short fallow periods, while the pattern is opposite for LARS and BS. Benzoate is a xenobiotic compound (63). Soil microorganisms collectively decompose such compounds to the mineral salts, which are then utilized by plants (64). Some of the bacterial family-likeSyntrophobacteraceae, Helicobacteraceae, which belongs toProteobacteria phylum, were reported for the benzoate degradation (65), and it was found in the jhum field soil mainly in short fallow periods of LARS and BS. Clostridiales were more predominant in the long fallow periods of SARS and RI, which is also reported for benzoate degradation (65).
In general, our result suggests that the metabolite profile is regulated by the microbial communities present in the different rhizosphere niches of jhum field soil.