Soil forms from rock. The process — mineral weathering — breaks down parent material through chemical and biological attack, releasing nutrients and creating the substrate that supports terrestrial life. The standard pedogenesis models treat this as a continuous process that accelerates or decelerates with climate but remains fundamentally the same.
Von Hippel and colleagues (Science Advances, 2025) extracted ancient DNA from Lake Lama sediments in Siberia spanning 23,000 years. During the Late Glacial period, from 23,000 to 11,700 years ago, lichens dominated mineral weathering. They attacked basaltic bedrock through acid secretion and mechanical fracturing, driving carbon-dominated nutrient cycling. Then forests arrived. Mycorrhizal fungi replaced lichens as the primary weathering agents, dissolving minerals through organic acid exudation linked to plant nutrient demand and driving nitrogen-dominated cycling. Podzolization — the acidification and iron mobilization that defines boreal soils — began only with this second regime.
The two mechanisms are not variations of the same process. Lichen weathering is carbon-intensive and independent of vascular plants. Mycorrhizal weathering is nitrogen-intensive and coupled to root networks. The chemistry is different, the organisms are different, the nutrient pathways are different. But the output — soil — continued forming throughout. The product appeared continuous. The process was not.
The structural insight is about what continuity of output conceals. Soil accumulated across the transition. A geologist sampling the profile would see a column of material deepening with time, thinner in the glacial layers, thicker in the Holocene, suggesting the same process running at different speeds. The ancient DNA reveals that the process itself was replaced. The soil didn't form faster after the transition. It formed differently — by different organisms, through different chemistry, along different nutrient pathways. The continuity of the product hid a categorical discontinuity in the mechanism.