Effect of Bacillus mucilaginosus D4B1 on the structure and soil-conservation-related properties of montmorillonite

Abstract

Bacillus mucilaginosus is one of the common soil bacteria and is widely used as a microbial fertilizer to release potassium from K-rich soil minerals. But few researchers have discussed possible effects that the bacteria may have on montmorillonite. This study investigated the effect of B. mucilaginosus strain on the capability of montmorillonite to retain water/cations. After 20days of interaction with B. mucilaginosus D4B1, the expansion ratios of montmorillonite samples decreased from 83.2% to 76.7%, and the cation exchange capacities (CEC) decreased from 80.8 mmol/100g to 67.8 mmol/100g, suggesting the durative interaction with the bacteria may reduce the capabilities of incorporating water molecules and nutrient into the interlayer space of montmorillonite. Morphological changes of discrete layers and etch pits were observed with scanning electron microscope (SEM). In the synchrotron radiation X-ray diffraction (SR-XRD) profiles, the full-width at half maximum (FWHM) of 001 reflection increased to 0.24nm as compared to the abiotic sample (0.19nm), indicating a decreased structural order in bacteria-treated montmorillonite. Also, a new band at 1540cm−1 assigned to asymmetric stretching vibrations of carboxylate was detected by micro fourier-transform infrared spectroscopy (FTIR) in bacteria-treated samples, suggesting the alteration in the mineral structure of montmorillonite was possibly triggered by the organic acids produced by bacteria. Moreover, in bacteria involved dispersion, a significant decrease of the pH value was coupled with a consecutive increase of total protein concentration, indicating the continuous activities of the bacteria during 20days of interaction. Above all, the decreased water-holding or cations-exchange capacity of montmorillonite was related to the partial alteration in the mineral structure of montmorillonite by B. mucilaginosus D4B1. Such a durable interaction on structural alteration of soil clays would have detrimental impact on long-term sustainability of soil resources.