The Diversity Of Exopolysaccharide-Producing Bacteria And Their Effects On Aggregation In Soils

Soil aggregates, also called soil structure, are the regulating center of soil fertility and a major index of soil assessment. The formation of soil aggregates is a complicated process. Microorganism can impact the formation of soil aggregates in two manners. On the one hand, microbes can affect aggregation by themselves, referring mycelium, electric charge and so on. On the other hand, the cohesion of metabolites of microbes, including exopolysaccharides, lipid and protein, can promote the formation of soil aggregates. This article mainly studied the diversity of exopolysaccharide-producing bacteria, and the effect of exopolysaccharide-producing bacteria on aggregation in soils, especially the role of exopolysaccharides.Soils were collected in Yangting Agro-ecological Experimental Station of Purple Soil. The diversity of bacterial communities in soils from forest and grassland was investigated. The results of PCR-DGGE showed that bacterial communities in soils from forest and grassland had little difference, including Firmicutes, Proteobacteria, Actinobacteria, Acidobacteria, Gemmatimonadetes and unclassed bacteria. The diversity of exopolysaccharide-producing bacterial communities in soils from forest and grassland was also investigated. Eighteen exopolysaccharide-producing bacteria strains were isolated from forest soil, and identified as Bacillus、Ensifer、Arthrobacter、Sinorhizobium、Pseudomonas and Cupriavidus. Fourteen exopolysaccharide-producing bacteria strains were isolated from grassland soil, and identified as Bacillus、Sphingobacterium and Enterobacter. Pseudomonas chlororaphis A20、 Bacillus cereus A27、 Bacillus sp B17-1and Bacillus aerophilus B23were selected as the tested strains.The structure of exopolysaccharides from the tested strains was investigated. The results of IR showed that characteristic absorption peaks of the exopolysaccharides could be found from the IR spectra. O-H,-CH, C＝N, C=O and C-N were found in the IR spectra of exopolysaccharides from all the tested strains; C=C was found in the IR spectra of exopolysaccharides from B17-1and B23yet. The monosaccharide composition of exopolysaccharides from the tested strains was also analysised by GC. The results showed that the monosaccharide composition of exopolysaccharides of the tested strains were different. D-xylose and D-mannose constituted the monosaccharide composition of exopolysaccharide from A20. L-arabinose, D-xylose and D-mannose constituted the monosaccharide composition of exopolysaccharide from B17-1. L-rhamnose, D-glucose, D-xylose and D-mannose constituted the monosaccharide composition of exopolysaccharides from A27and B23.The effect of exopolysaccharide-producing bacteria on aggregation in soil was investigated. Inoculation with the tested strains A20、 A27、 B17-1and B23could significantly promote the formation of water-stable soil aggregates (P<0.05).The percent of soil aggregates (>0.25mm) was increased by128.6%、169.0%、79.1%and77.5%, comparing to the controls. The results of PCR-DGGE showed that bacterial communities in soil were influenced more obviously by B17-1and B23, comparing to the controls.The effect of exopolysaccharides on aggregation in soil was also investigated. Inoculation with1.4mg/g exopolysaccharides from strains A27、B17-1and B23respectively could significantly promote the formation of water-stable soil aggregates, the percent of soil aggregates (>0.25mm) was increased by43.8%,24.5%and61.7%, comparing to the controls (P<0.05). Inoculation with2.8mg/g exopolysaccharides from strains A20、B17-1and B23respectively could significantly promote the formation of water-stable soil aggregates, the percent of soil aggregates (>0.25mm) was increased by19.1%、38.4%and45.2%, comparing to the controls (P<0.05). The results of PCR-DGGE showed that bacterial communities in soil was influenced more obviously by exopolysaccharide from B17-1and B23, comparing to the controls.The monosaccharide composition of exopolysaccharides from A27and B23were more complicated, and had promoted the formation of soil aggregates more obviously.