Investigation of the Biopolymer Rhizobium tropici for Use in Soil Improvement

The effectiveness of the biopolymer Rhizobium tropici for use a soil additive to improve vegetation drought resistance is described. Rhizobium tropici (ATCC 49672) is a gram-negative, aerobic, rod-shaped bacterium that attaches to plant roots and is responsible for nitrogen fixation. The bacterium uses plant sugars to produce an extracellular polymeric substance (EPS), or biopolymer that can form a hydrogel. The EPS was produced using 111 Rhizobium X Medium (ATCC). The structure of the biopolymer was observed using SEM cryo-microscopy imaging. Dependencies of the spatial properties of the gel network were observed. Hydrogel structures with coiled structure providing reinforcement and a bimodal distribution of mesh sizes were typical. The bulk soil properties of plastic limit and compactibility were measured for biopolymer modified and non-biopolymer amended soils, and the presence of biopolymer in these systems resulted in changes to these properties. The change in the bulk soil property compactibility resulting from biopolymer amendment showed significant changes at biopolymer loading rates as low as 0.0025% by dry weight. The hydroscopic property of the biopolymer hydrogel was observed through thermogravimetric analysis to show a fraction of strongly adsorbed water. Modification of the bulk soil properties plastic limit and compactibility using biopolymer indicate that this biopolymer may be well suited to improve slope stability, surface erosion resistance, and vegetation drought resistance.