Coevolution of Soil Genesis and Life.

Planetary scientists have increasingly recognized the great variety of mineral species on Earth as compared to other planets and the moon and attributed much of this great diversity to the coevolution mineralogy and life. Similarly, the great variety of soils is largely the result of life which can be traced back to at least the Great Oxidation Event about 2.4 billion years ago when pedogenic iron oxide minerals formed after free oxygen accumulated in Earth’s atmosphere as the result of cyanobacterial photosynthesis. Plants on land in the Silurian, and even more so in the Devonian, brought forth a major change to soil genesis. Like today, vegetative cover would have slowed erosion enabling the formation of soil horizons.  Nutrients extracted from soil minerals would have been uplifted to topsoil by plant-root-fungal networks where they were recycled in O and A horizons. Pedogenic carbonates in arid and semiarid regions would have started forming as a consequence of ecologically-induced biomineralization in the Silurian. Accompanying the plant colonization of land, burrowing invertebrates, reptiles, small dinosaurs, and mammals would have bioturbated soils and re-distributed nutrients. By the Holocene, humans were having a progressively greater impact on soil genesis as the result of agriculture. Currently, industrial agriculture involves global-scale additions of nitrogen and phosphorous, compaction from heavy machinery, and movement of crops and animals across intra- and inter-continental distances. Humans are now the largest geomorphic agent moving more soil and sediments than glaciers, wind, or rivers.  Cities and transportation structures, such as interstate highways and railroads, are anthropogenic landforms easily seen from space. Without life on Earth, soil genesis would be much different.