Rhizobium Tropici-Produced Biopolymer Impact on Soil and Plants

Increasing soil stability is important for addressing increased stream and river turbidity, eutrophication, decreased biodiversity, and desertification of farmland. A potential solution is the use of extracellular polymeric substances (EPS) produced by the bacteria Rhizobium Tropici, which offers a low-cost and environmentally friendly solution to soil erosion. The form of RT-EPS used in this research is Ethanol Precipitable Material (EPM). Characterized by its ability to retain water and nutrients and enhance subterranean root structure, EPM offers applications in sustainable agriculture and environmental preservation in the face of intense weather patterns. This study aims to investigate how EPM influences the mechanical properties of sand and promotes growth in Bermuda Grass and Bush Bean Plants.

To study EPM’s adhesive properties, an experiment was conducted to measure the angle of repose for sand samples mixed with varying concentrations of EPM. The angle of repose reflects the stability of a granular material, and it was used to indirectly measure the adhesive interactions between the sand particles mixed in with EPM. Different concentrations of EPM (0 mg/kg, 12.5 mg/kg, 50 mg/kg, and 125 mg/kg) were mixed with sand, and these mixtures were then molded into cones using a funnel. The measurements were then recorded at different time points for a prolonged period of time. A significant difference in the angle of repose was noted between 125 mg/kg and control samples, indicating that greater amounts of EPM imparted the sand with improved stability.

Bermuda Grass samples germinated and matured over a 2 month period. On average, the shoot count and root mass increased with higher EPM concentrations of 25 mg/kg and 125 mg/kg. This promotion of shoot and root development led to the hypothesis that the EPM may also be promoting nutrient availability and uptake for the plants. This was investigated with the Bush Bean Plants, as the main focus was on the number of bean pods produced and the mineral content in those pods. Samples grown with 25 mg/kg EPM in sand produced the greatest number of bean pods and had the highest root mass and root-shoot mass ratio. XRF analysis was then performed to quantify the presence of important minerals in the pods, and a significant increase in the amount of potassium and chlorine with higher amounts of EPM was noted. At the higher concentration of 125 mg/kg, though XRF analysis delivered results that followed the trend of increased mineral content, the bean pod count was lower than the 25 mg/kg samples and equivalent to the control.

In conclusion, it was observed that sand samples treated with EPM maintained a higher angle of repose over longer periods of time, demonstrating its ability to enhance adhesive properties between the granules. This suggests its potential to effectively reduce soil erosion in vulnerable areas. In addition, the results showcase EPM’s ability to increase shoot count and root mass in Bermuda Grass, highlighting its ability to influence both above and below-ground development. This is further supported by XRF analysis in Bush Bean pods, as EPM treated samples display an increase in important minerals for growth and metabolism. Angle of repose will continue to be measured to observe the long-term impact on adhesive properties. The scope of the XRF studies may also be broadened to include the observations of toxic metals: different studies showed that just as the EPM assists in delivering minerals to the plants in controlled amounts it can also sequester toxic and non-essential metals.

The authors would like to thank the US Army Corps of Engineers (ERDC) and the Louis Morin Charitable Trust for their support (W912HZ-20-2-0054) in this research.