Abstract

Recent research implicates geologic nitrogen as a large and reactive pool of nitrogen that can have significant ecological effects. This study was conducted to determine the landscape distribution of geologic nitrogen and the fate of this nitrogen during rock to soil transformation in the lower Mokelumne River watershed located in the western Sierra Nevada of California. Mineralogy and chemistry were determined for six major lithologies in the study area to examine the spatial distribution of nitrogen in bedrock. In addition, soil samples were collected from two watersheds having high stream-water nitrate concentrations to determine the fate of nitrogen during weathering and soil formation. With the exception of diorite, all bedrock contained detectable concentrations of nitrogen: 153–1801 and 98–739 mg N kg−1 for metasedimentary and metavolcanic lithologies, respectively. A significant amount of nitrogen was released during soil formation: 2350 and 2570 kg N ha−1 for soils formed on greenstone and slate, respectively. This amount of nitrogen represents from 30% to 50% of the current soil nitrogen pool. In these ecosystems, nitrogen released from bedrock contributes to an excess of available nitrogen relative to biotic demands, leading to nitrate leaching and elevated concentrations of nitrate in stream water. Given the widespread occurrence of geologic nitrogen and the large global inventory of nitrogen contained within bedrock, the role of geologic nitrogen as a source of nitrogen for terrestrial biogeochemical cycling must be considered.

First Page Preview

First page PDF preview
You do not currently have access to this article.