Geologic nitrogen as a source of soil acidity

Abstract

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 2,700 mg N kg-1, which corresponds to 7.1 Mg N ha-1 contained in a 10-cm thickness of bedrock. In situ soil solutions were dominated by H+, labile-monomeric Al3+ and NO3-, indicating that the barren area soils were nitrogen saturated-more mineral nitrogen available than required by biota. Leaching of excess NO3- has resulted in removal of nutrient cations and soil acidification. Nitrogen release rates from organic matter free soil ranged from 0.0163 to 0.0321 mg N kg -1 d-1. Nitrogen release rate from fresh ground rock was 0.0465 mg 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.