Hi @pau ,
You are observing very high alpha diversity because you are using the kmer-diversity action, which measures diversity of kmers instead of ASVs (see here for an explanation)
We would expect kmer diversity to be very high relative to ASV diversity, as the ASVs are decomposed into many kmers prior to calculating diversity metrics with this action.
For "normal" ASV diversity metrics, you should use the core-metrics action that @colinbrislawn mentioned above.
good luck!