Distinct mediating patterns between metal filtering and species coexistence of rare and abundant subcommunities in heavily polluted river sediments

•Deterministic processes prevail in the subcommunity assembly of river sediments.•Rare subcommunity assembly was governed more by interspecific interactions.•Introduction and colonization processes disentangle the abundant assembly.•Metals dominate the two processes, but nutrient availability prevail in the latter.•A balance between metal filtering and species coexistence within subcommunities.