Plant-growth promoting rhizobacteria increase soil water retention by changing soil physical and hydraulic properties
Date
2019
Authors
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Publisher
University of Delaware
Abstract
The growing global population and food consumption is challenging agriculture for higher productivity. Water is a key factor limiting crop yield in (semi-) arid regions, in this case, increasing water use efficiency is of great importance. The plant growth promoting rhizobacteria (PGPR) could potentially increase agricultural productivity in (semi-) arid regions as its beneficial effects on enhancing plant drought stress tolerance, which has been increasingly documented in the literature. However, most of previous researches have focused on PGPR-root interactions, less is known about PGPR’s effects on physiochemical and hydrological properties in rhizospheric soil that may also contribute to plant drought stress tolerance. This study aimed to investigate changes in soil physical and hydraulic properties induced by Bacillus subtilis FB17, a generalist PGPR that has been commercialized (named as UD1022) for its ability to benefit plant growth and disease protection. In this study, soil water retention curves (SWRC) and water evaporation in soils with various textures (i.e., pure sand, sandy soil, and clay) as influenced by UD1022 were measured using HYPROP. In addition, X-ray and neutron radiography/tomography, an in-situ, non-destructive imaging technique were used to image water movement in UD1022-treated and control soil samples during evaporation. Results from both HYPROP and radiography imaging experiments showed that all UD1022-treated soils held more water and had reduced conductivity and cumulative evaporation compared to their corresponding controls. Analyses the HYPROP results combined with neutron radiography and SEM imaging revealed two potential mechanisms responsible for the changes in hydraulic properties and soil evaporation upon UD1022 treatment: (1) EPS alter the structure of soil matrix and connectivity of pore spaces and (2) EPS modify the physicochemical properties of water (surface tension and viscosity). These physicochemical and structural changes can lead to reduced evaporated and increased water retention. ☐ To further understand how EPS mediate changes in water retention and evaporation, neutron and X-ray tomography were used to obtain 3D structures of UD1022 treated pure sand sample and its control. The estimated water content in the UD1022-treated sand column from neutron tomography images was higher and the water was more heterogeneously distributed compared to its control. With similar water content of both treatments based on weight measurement, the higher estimated water content in UD1022 treated sand column could come from artifacts in phase identification in image processing. In neutron tomography images, the histograms showed higher portions of pixels with greater grey scale value in the UD1022 treated sample, indicating that water was more concentrated as “clusters” that were more easily identified as water phase leading to the higher estimated water content in UD1022 treated sample than its control. There are two possible reasons that may be responsible for the changes in water distribution upon UD1022 treatment: 1) the presence of B. subtilis FB17 reduces the surface tension of liquid phase thus increases water retention in soil pore space sand 2) bacteria distribution is heterogeneous in soil pore space. The findings from this study suggest the potential effectiveness of PGPR in modulate changes in soil physiochemical and hydraulic changes that favor increased water retention, which may translate to enhanced plant stress relief during a drought event by increasing the time available for plants to make metabolic adjustment and improve plant drought tolerance.