Résumé
Biological soil crusts (biocrusts) are key components of dryland ecosystems, hosting diverse microbial communities that mediate ecosystem services. In grazed drylands, biocrusts help replenish nitrogen (N) incorporated into livestock; hence, it is important to understand how grazing management affects them. We used shotgun metagenomics to investigate the impacts of four grazing treatments - (1) non-grazed, (2) moderately stocked, (3) heavily stocked, and (4) moderately stocked with wet season rotational spelling (cattle removed every 3-4 years) - on the diversity and potential function of biocrust bacterial communities in two distinct land types (Ironbark and Box) within a northeast Australian semi-arid tropical savanna. Grazing significantly influenced taxonomic and functional gene profiles and reduced biocrust cover, although these effects differed between land types. In Ironbark, the effect of moderate stocking on community composition was intermediate between the non-grazed control and heavy stocking treatments, while in Box it more closely resembled the heavy stocking rate. Rotational spelling had negligible impacts on biocrust bacterial diversity and potential function but mitigated the negative impacts of grazing on biocrust cover, particularly in Box. At the genetic level, we identified diverse bacterial lineages potentially capable of N fixation and similar relative frequencies of N fixing genes between grazing treatments. Hence, to maximise N inputs, rangeland managers should reduce biocrust losses. Our findings indicate that the frequency and duration of rotational spelling should be explored as a promising approach to mitigate the negative impacts of grazing on biocrust cover to maximize the capacity of biocrusts to replenish N.