Does revegetation cause soil microbiota recovery? Evidence from revisiting a revegetation chronosequence 6 years after initial sampling

The global biodiversity and land degradation crises have brought about an urgent need and great demand for restoration actions. However, restoration outcomes are often less than ideal, indicating a need for improved restoration practices. Soil microbiota are extremely diverse and functionally important and should be further considered in restoration. However, despite their importance, there remains a gap in understanding of how soil microbiota respond following native plant revegetation. Several studies have used cross-sectional study designs of restoration chronosequences to infer that revegetation causes the recovery of soil microbiota, but it is near-impossible to determine cause and effect relationships with cross-sectional study designs. Here we used high-throughput amplicon sequencing of the bacterial 16s rRNA gene from soil samples collected at two timepoints, 6 years apart, at a revegetation chronosequence in South Australia. Our results show some indications of recovery but not the additional recovery in bacterial community composition toward the reference sites as expected after this 6-year period—a result that appears at odds to the expected patterns of revegetation causing recovery of soil microbiota. Spatially dependent factors (e.g. soil chemistry), biotic and abiotic barriers, seasonal differences in sampling, and variability among the ecological reference sites could each help explain this apparent lack of additional microbial recovery. More detailed longitudinal and/or experimental manipulation work is required to further examine the cause-effect relationships. Our study contributes important new information and highlights knowledge gaps in how soil microbiota respond to revegetation, and we urge caution when attempting to infer causation from cross-sectional chronosequence studies.