Inoculation of paddy rice with Azospirillum brasilense and Pseudomonas fluorescens: Impact of plant genotypes on rhizosphere microbial communities and field crop production

Abstract

Rice is the third largest global food crop. Traditional practice to achieve maximum yields of rice is associated with the availability of mineral nitrogen and fertilization. This can lead to pollution of waterways. This can be particularly important in paddy rice production in north-eastern Argentina. Bio-fertilization or inoculation with plant growth-promoting bacteria (PGPB) is a sustainable alternative for agro-ecosystems. Inoculation of wheat, maize, and soybean is a widespread agricultural practice that has proved to be efficient in increasing production and promoting nutrition of these crops. This work measures the response of three rice cultivars to PGPB inoculation under field conditions with a commercial formulation containing strains of Pseudomonas fluorescens and Azospirillum brasilense. The experiment was performed in a farm plot located near Villa Clara, Entre Ríos. A factorial complete block design with four replicates was applied. Samples were taken at tillering and physiological maturity. Aerial biomass, grain yield, and its components were determined. Culturable microorganisms were analyzed in rhizosphere samples. Counts of most probable number of microaerophilic, nitrogen-fixing microorganisms and community-level physiological profiles of carbon-source utilization were evaluated at physiological maturity. Also, DNA extraction, nifH gene amplification, and terminal restriction fragment length polymorphism (T-RFLP) analysis were performed to analyze molecular diversity of diazotrophic communities associated with rice roots. Data showed differences between rice genotypes. Inoculation with PGPB did not have significant impact on culturable microbial communities and patterns of T-RFLP. Some fragments obtained by restriction with enzymes HaeIII and HhaI differentiated between inoculation treatments and rice genotypes. PGPB inoculation increased aerial biomass production, harvest index, and grain yield of the Supremo 13 cultivar by 4.7%, 16%, and 20.2%, respectively. Inoculation of the Yeruá cultivar increased aerial biomass by 1.9% and grain yield by 11%. On the other hand, control plants of the Cambá INTA cultivar produced 8.7% and 7.3% more aerial biomass and grain yield than inoculated plants, respectively. Inoculation reduced the percentage of chaffy grains of the three rice cultivars. The results indicate that the combined inoculation with P. fluorescens and A. brasilense has significant potential when applied to rice.