Potential of Reinoculation Using R. tropici Applied under Cover to Increase Bean Yield

Abstract

The common bean is known to require nitrogen, with mineral fertilizer being the main form of supply to the plants. However, more efficient inoculants containing strains of Rhizobium tropici that are potentially more efficient in fixing atmospheric N2 could replace the use of nitrogen fertilizers as long as some of the existing challenges in the application methodology are overcome (for example, ensuring the existence of active nodules in the most demanding crop phases: flowering and grain filling). This study aimed to verify the responses of common bean plants subjected to reinoculation using Rhizobium tropici in topdressing applied at different stages in a greenhouse and in a field between March and April 2020 and in the spring–summer harvest of 2020/2021, respectively. A randomized block design was used, along with four replications: T1 = uninoculated control; T2 = inoculation via seed (VS); T3 = VS + reinoculation at the V4 stage; T4 = VS + reinoculation at the R5 stage; T5 = VS + reinoculation at the R6 stage; T6 = VS + reinoculation at the V4 and R5 stages; T7 = VS + reinoculation at the V4 and R6 stages; T8 = VS + reinoculation at the R5 and R6 stages; T9 = VS + reinoculation at the V4, R5, and R6 stages; and T10 = mineral nitrogen fertilization. Nodulation and morphological variables were evaluated at the R7 stage, and agronomic variables were evaluated at the R9 stage at harvest. The use of reinoculation, in addition to inoculation, positively influenced the bean nodulation process. The morphological characteristics of the plants showed good development with the supply of N (carried out via inoculation and reinoculation), equivalent to the development that could be achieved via the addition of a mineral nitrogen fertilizer. Reinoculation from the R5 to R7 stages proved ineffective in supplying N to the common bean crop. Inoculation in the seed associated with reinoculation in coverage at the V4 stage provided higher yields (3.271 kg ha−1), reinforcing the potential for mineral nitrogen fertilizers to be replaced. The use of the seed inoculation technique with subsequent reinoculation under cover can completely replace the use of nitrogenous mineral fertilizers in fertilizing bean plants, making the cultivation of this fabacea more sustainable in terms of the associated costs and environmental problems.