Effect of Nitrogen Topdressing Associated with Growth-Promoting Rhizobacteria on Yield, Nutrition, and Chlorophyll Index of Rice

Nitrogen (N) is a key nutrient for upland rice (Oryza sativa L.), and plant growth-promoting rhizobacteria (PGPR) have been investigated as a sustainable strategy to improve plant nutrition and crop performance. This study evaluated the effects of N topdressing and PGPR inoculation on leaf chlorophyll index (LCI), leaf nutrient concentrations, and yield components in upland rice. A field experiment was conducted in a randomized block design (4 × 6 factorial) with four N rates (0, 40, 80, and 120 kg ha−1) and five PGPR strains (Azospirillum brasilense, Nitrospirillum amazonense, Bacillus subtilis, Priestia aryabhattai, and Methylobacterium symbioticum), plus a non-inoculated control. No significant interaction between N rates and PGPR inoculation was observed. Nitrogen increased leaf phosphorus (P), potassium (K), and magnesium (Mg) concentrations and panicle number; however, it also increased unfilled grains, reduced grain weight, and did not affect grain yield. Azospirillum brasilense increased LCI by 25.7%. Bacillus subtilis and A. brasilense increased leaf N, K, Mg, copper (Cu) and manganese (Mn) concentrations. Azospirillum brasilense, B. subtilis, N. amazonense, and P. aryabhattai reduced unfilled grains, increased grain weight and grain yield by up to 10.7%, whereas M. symbioticum did not differ from the control in grain yield. Under the conditions of this study, nitrogen was not limiting for grain yield, and all strains, except M. symbioticum, were associated with increases in grain yield and changes in plant nutritional status.