Soil salt and microbiome diversification over the past 3700 million years

A geological history of pedogenic salts and their microbiomes can now be reconstructed from a review of thousands of described paleosols ranging in age back 3700 Ma. The current diversity of evaporite minerals within paleosol gypsic (By) horizons may have begun with kieserite (MgSO4.H2O at 3700 Ma), then barite (BaSO4 at 3458 Ma), and gypsum (CaSO4.2H(2)O at 3217 Ma). Pedogenic carbonate of calcic (Bk) horizons appeared later, first nahcolite (NaHCO3 at 3016 Ma), then dolomite ((Mg,Ca)(CO3)(2) at 2403 Ma), and low-magnesium calcite (CaCO3 at 1460 Ma). The earliest occurrence of each salt is shallow in paleosol profiles (12-25 cm), but deeper (50-100 cm) salt horizons (By and Bk) horizons appear later in the Proterozoic and Paleozoic. These changes can be normalized for estimated differences in mean annual precipitation in the same paleosol from both depth to salts and from chemical composition, which demonstrated that depth to salts and soil productivity measured as respired carbon dioxide showed unchanged relationship with mean annual precipitation in deep time. Stepwise increases in soil respiration through time inferred from depth of soil salts reflects evolving soil microbiomes and atmospheric composition cued to major advances in the evolution of life on land, such as the evolution of anaerobic, then aerobic photosynthetic microbes, then land plants, and trees.