Habitat filtering but not microbiota origin controls microbiome transplant outcomes in soil

Human activities cause global losses of soil microbiome diversity and functionality. Microbiota transplants offer a potential solution, but the factors influencing transplant success remain unclear. We investigated how microbiota origin affects microbiome mergers, hypothesizing that native strains through niche preference are better adapted to their habitat and will outcompete non-native ones. To test this, we contrasted transplants between soil microcosm-cultured topsoil or lakewater communities with a community of 21 soil bacteria (SynCom). In both cases, SynCom transplant increased resident productivity but permanently shifted compositions, although its abundance dropped from an initial 50-80% to <1% within two months. Both merged and non-merged communities resembled natural soil microbiota in comparisons with over 81,000 soil, sediment and lake compositional data. Our results show that habitat filtering and niche competition, more than microbiota origin, determine transplant outcomes. Despite the limited proliferation of SynCom transplants, their capacity to instill lasting community trajectory changes opens new paths for microbiome engineering. TEASEREven transiently present microbiota transplants can alter resident microbiome composition through processes governed by habitat filtering.