Geologic Nitrogen as a Source of Soil Acidity

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The origin of highly acidic (pH<4.5) barren soils in the Klamath Mountains of northern California was examined. Soil parent material was mica schist that contained an average of 2,700mg N kg^<-1>, which corresponds to 7.1Mg N ha^<-1> contained in a 10-cm thickness of bedrock. In situ soil solutions were dominated by H^+, labile-monomeric Al^<3+> and NO_3^-, indicating that the barren area soils were nitrogen saturated-more mineral nitrogen available than required by biota. Leaching of excess NO_3^- has resulted in removal of nutrient cations and soil acidification. Nitrogen release rates from organic matter free soil ranged from 0.0163 to 0.0321mg N kg^<-1> d^<-1>. Nitrogen release rate from fresh ground rock was 0.0465mg N kg^<-1> d^<-1>. This study demonstrates that geologic nitrogen may represent a large and reactive nitrogen pool that can contribute significantly to soil acidification.
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