An intricate cycle of geological and chemical interactions accounts for the highly acidic soils of mysterious barren areas in the Klamath Mountains of Northern California, according to a researcher at the University of California, Davis. Nitrogen, released by the weathering of rock, increases soil acidity, leaches out critical soil nutrients and mobilizes aluminum into its toxic form, preventing revegetation of areas apparently left barren by lightning fires, reports biogeochemist Randy A. Dahlgren in this week's issue of the scientific journal Nature. The findings suggest important implications for dealing with ecosystems affected by acid rain, excessive use of nitrogen fertilizers, and timber harvesting, according to Dahlgren, an assistant professor in the UC Davis land, air and water resources department. "Although these barren areas created by acidification from geologic nitrogen are probably quite rare, they present an important analogy to acid rain, which has affected the soils of some parts of Europe and the Northeastern United States," said Dahlgren. "In the case of acid rain, humans have created a nitrogen-saturated ecosystem. If those areas are clear-cut in timber harvest and the nutrient-rich soil surface eroded, we would likely see the same type of barrenness. As long as the organic material of the forest floor is maintained, this can be prevented." When standard soil and rock analyses failed to yield a cause for the acidity of the Klamath barren areas, each covering as much as 40 acres, in the midst of the forested mountains, Dahlgren used a centrifuge, to force the water from the soils, and examined the soil constituents in solution. This process, he notes, better detects only the ongoing, rather than historical, chemical and biological processes. "We found a huge amount of nitrate, which is negatively charged, in the soil solution from the barren areas," said Dahlgren. "The negative charge of the nitrate was balanced by hydrogen, so what you get is nitric acid, causing the soils to be extremely acidic." Yet, there was no explanation for the source of the nitrate, until Dahlgren and colleagues examined the rock underlying the barren areas. In analyzing the rock, the researchers found that a major component was mica, which contained high levels of ammonium, normally found only in trace amounts in rock. When exposed to weathering and resultant oxidation, the ammonium in the rock is converted to nitric acid. In addition to causing further breakdown of the rock, the nitric acid yields negatively charged nitrate, stimulating the leaching out of positively charged magnesium and calcium -- nutrients crucial to plant growth. In a forested area, this leaching would be prevented by the normal uptake of nitrogen by plants and trees, according to Dahlgren. Interestingly, the only vegetation on the virtually barren sites is a tiny plant commonly called pussy paws, which is known for its tolerance of acidic soils and low levels of both calcium and magnesium, he says. The nitrate causes further problems by converting aluminum, normally bound up in minerals and organic matter of the soil, into a soluble, toxic form. "So there is the potential on these barren sites for aluminum toxicity to occur, a process that is exacerbated by the lack of calcium in the soil," Dahlgren said. The Klamath Mountains, whose rock formations likely originated in oceanic sediment, present the ideal, although uncommon, setting for this acidification process, according to Dahlgren. Geologic ages ago, the organic matter in the ocean floor was mixed with the sediment. When heat and pressure combined to turn the sediment into metamorphic rock, nitrogen was virtually squeezed from the organic matter into the rock in the form of ammonium. Having identified the cause of the soil acidification, Dahlgren hopes to help foresters revegetate the barren areas by applying lime and compost to restore the critical nutrients and break the cycle of acidification. Although it may take a century to completely regrow a mature forest community, recreation of the organic forest floor will provide the crucial first step, he says. Dahlgren's work was funded by a grant from the University of California Division of Agriculture and Natural Resources