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A major problem in all restoration and rehabilitation projects is to restore the function of the ecosystem. Nitrogen, next to water, is the most limiting factor for productivity of arid terrestrial ecosystems.

We used a successful restored area, completed 10 years earlier, in comparison with an undisturbed scrubland area and a remaining, disturbed area in the southern Sonoran Desert in Baja California, Mexico. We compared the abundance of the nifH gene, estimated by qPCR, potential N-2 fixation activity by acetylene reduction assay, and diversity of diazotrophs by denaturing gradient gel electrophoresis in the rhizosphere of the most representative plant species, the cardon cactus and the mesquite used for restoration.

The abundance of N-2-fixing bacteria in the rhizosphere of cardon growing with mesquite had significantly higher abundance of nifH gene than the rhizosphere of cardon that grew separately. Across all samples, the potential N-2 fixation was significantly higher in soil samples from the restored site than samples from the undisturbed and disturbed sites.

Successful long-term restoration improved the potential N-2 fixation to a level similar to undisturbed lands. Beneficial interactions between cardon and mesquite are a promising venue for desert reforestation by their contribution to improve N-2 fixing potential in degraded arid lands and increasing the population of diazotrophs.