Geological nitrogen and terrestrial nitrogen cycling in the Mokelumne River watershed, California

Elevated concentrations of nitrate in stream water are reported throughout the world and are commonly attributed to anthropogenic activities such as atmospheric deposition, livestock grazing and feedlots, agricultural runoff, timber harvest practices, and effluent. Nitrate also originates from natural pathways such as biological fixation of N₂ gas and minor amounts from inorganic pathways including lightening. Geologic nitrogen is an additional source of nitrogen in terrestrial ecosystems that is often overlooked in studies examining nitrogen cycling. As we show, the presence of geologic nitrogen may exacerbate human inputs of nitrogen into terrestrial ecosystems that are already saturated with respect to nitrogen.; Metasedimentary and volcanogenic metasedimentary bedrock in the Mokelumne River watershed was found to have high concentrations of nitrogen, up to 1800 mg N kg-1. Evidence from field and laboratory analyses indicate that this nitrogen is released through weathering and is flushed to surface water during the rainy season. Stream water chemistry data collected over a period of four years from watersheds with and without geologic nitrogen suggest that bedrock does act as a non-point source of nitrogen in this region. As 75 percent of the rocks now exposed at the earth's surface are sedimentary in origin and as these rocks contain approximately 20 percent of the global nitrogen inventory, geologic nitrogen may be a large and hitherto unappreciated source of nitrate to surface waters.