Whether nitrogen is divided into consumers, producers or decomposers is not determined by whether nitrogen can be fixed, but by its energy acquisition method.

Plants gain energy by synthesizing organic matter through photosynthesis, so they are autotrophs, that is, producers.

Animals get energy from other creatures through the process of predation (feeding), so they are heterotrophs and called consumers.

Rhizobia obtain organic matter from plants in leguminous plants and from soil outside leguminous plants, so they are heterotrophs, but their acquisition mode is not predation or feeding. This creature is generally called a decomposer.

As for the problem of nitrogen fixation, the nitrogen fixation process of rhizobia can't get energy, but consumes energy, so it's different from using chemosynthetic bacteria as producers. This is one of the reasons why some rhizobia can't fix nitrogen in soil and have insufficient energy.

However, it should be noted that rhizobia refers to a large class of bacteria, and some bacteria can also fix nitrogen when they live freely. However, they all belong to the decomposer.