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Soils are the most physically, chemically, and biologically diverse habitats on earth, so it is no surprise that they have the highest bacterial diversity, but that very few of them can be cultured or be identified as to type and to ecological roles and biogeochemical functions after more than 100 years of study.
An important new paper, A global atlas of the dominant bacteria found in soil, 2018, Delgado-Baquerizo et al., Science 359:320-325, examines bacterial diversity from the ribosomal RNA sequences (which make protein from DNA), and finds that despite tremendous diversity, with very few species similar in different soils, a mere 2% of bacteria make up 41% of global soil bacterial numbers.
Who they are and what they do is unknown, but there are characteristic types especially abundant in high pH, low pH, low productivity, drylands, and dry forests habitats.
Interestingly tropical soils are the most diverse, and least dominated by any one group, so they should show the largest range in bacterial populations from one soil type to another.
This is why soils should never be lumped together in simple averages without considering their staggering diversity!