Redundancy maintains microbial ecosystem functional potential despite taxonomic shifts between serpentine and nonserpentine soils.

Environmental filtering and buffering are complementary processes responsible for stabilizing ecosystem services across landscapes. The extent to which either of these processes structures microbial composition and functions in extreme soil systems can be highly context-dependent, requiring site-specific examination to elucidate general principles of community function under stress. Serpentine soils have high amounts of heavy metals and magnesium and low levels of plant nutrients and exist in close proximity to nutrient-rich and plant-productive nonserpentine soils, making them an ideal system for evaluating selection and redundancy under stress. Combining biogeographical field surveys and shotgun metagenomic sequencing of microbiomes from 23 pairs of serpentine and nonserpentine soils across California, we investigated selection and functional redundancy between disparate soil types. We hypothesized that the strong selective pressure present in serpentine soils would result in distinct bacterial and functional profiles. Bacterial and fungal community taxonomic compositions were indeed distinct between serpentine and nonserpentine soils, while archaeal communities were similar between soil types. In contrast, functional repertoires of all microbial community groups were largely similar between soil types, with many of the same taxa carrying out functions across soil types. Still specialized functions in serpentine soils represented adaptations to stress in contrast to the carbon-rich environment of nonserpentine soils. For example, specialized functions - such as siderophore biosynthesis proteins, which are involved in the biosynthesis of iron-chelating compounds - were distinctive features of serpentine soils and specialized functions notably had similar functional redundancy but distinct taxa carrying out the functions across soil types. These results highlight taxa that perform the functions that have been selected for survival in an extreme soil ecosystem and the functions that are most at risk in the face of environmental disturbances.