Efficacy of plant growth promoting rhizobacteria to minimize the usage of synthetic fertilizers in amaize mesocosm

Nitrogenous runoff and food crises are two of the major environmental and humanitarian issues worldwide. Plant growth promoting rhizobacteria (hereafter PGPR) are considered as benign microbes that associate with plants to promote plant growth and yield. PGPR are a key element of the rhizosphere, wherein plant roots and soil interact chemically and biologically. The idea to harness PGPR capability to reduce synthetic fertilizer application in conventional agriculture is exciting. There are limited studies to fully address the biofertilizer potential of PGPR regarding nitrogen (N) mobilization. In here, we aim to evaluate the impact of a commercial PGPR bioinoculant Bacillus subtilis (BU1814) on B73 maize plants, under varying fertilizer concentrations (essentially determining the most effective ratio of bioinoculum to fertilizer), so that fertilizer usage could be reduced to minimize runoffs. We used B. subtilis BU1814 concentration at 5.6 x 10^9 CFU/pot, and the fertilizer concentrations varied from 100% to 6.25% (1.63 g L^-1 - 0.10 g L^-1) for all the experiments. Plants were treated with an initial dose of PGPR followed by a booster dose on day 14 post emergence. In addition, plants with/without BU1814 were irrigated with fertilizer concentrations ranging from 100% to 6.25%. Plants were harvested and parameters such as total biomass, and root to shoot (R/S) ratio were taken. Plants were also evaluated for Photosystem-II efficiency using a chlorophyll fluorescence meter. Plants treated with lower concentrations of fertilizer along with BU1814 treatments showed increased R/S ratio compared to the non-bioinoculum treated control with 100% fertilizer amendments. In contrast, the aboveground biomass of plants was not significantly influenced under low fertilizer and BU1814 treatments. In addition, qPCR targeting the gyrB gene for BU1814 showed higher abundance of B. subtilis in BU1814 inoculated rhizospheres compared to the 100% fertilizer control, indicating successful establishment of BU1814. Our data showed that combining bioinoculum with synthetic fertilizer could help minimize the application of synthetic fertilizers for crop plants. In sum, the implication of the study amplifies the role of bioinoculums beyond protecting plants to reduce runoffs in conventional agriculture practice.