Nitrate from minerals is either assimilated by biota or reduced to nitrogen gasses (N2 and N2O) under anaerobic conditions through denitrification
5.1. Soil Quality
 Soils forming from parent material that is rich in nitrogen (e.g., coal beds, carbonaceous slate) or has secondary inputs of nitrogen from hydrothermal sources, may become elevated in nitrogen concentration. Elevated nitrogen concentrations in soil forming on coal mine spoils in southeast British Colombia (up to 2000 mg N kg−1 [Fyles et al., 1985]) and Virginia (940 mg N kg−1 [Li and Daniels, 1994]) were attributed to nitrogen in the organic-rich parent material. Total nitrogen concentrations were elevated in subsoil horizons with low organic matter concentrations formed in a nitrogen-rich mica schist, ranging between 5600 and 9000 mg N kg−1 [Dahlgren, 1994]. Presence of high-nitrate soil in the western San Joaquin Valley, CA, was attributed to nitrogen-bearing clastic rock [Strathouse et al., 1980], although no direct correlation between soil and bedrock nitrogen concentrations was noted [Sullivan et al., 1979].
 Volcanism can enhance soil nitrogen concentrations through outgassing of mantle nitrogen or through fluid transport of nitrogen from organic matter sources. Nitrogen concentrations up to 2370 mg N kg−1 were reported in soils collected from geothermal areas in Antarctica [Greenfield, 1991]. Hydrothermal systems may affect soil nitrogen cycling as reported at Roosevelt Hot Springs in southwestern Utah [Klusman et al., 2000], where trace concentrations of nitrous oxide (1.22–103 ppmv N2O) formed presumably as a by-product of microbially mediated cycling of nitrogen gas. In one scenario, volcanogenic dinitrogen gas is fixed by soil microorganisms and nitrified to nitrate, which can undergo denitrification (Figure 2). Nitrous oxide is a trace intermediate product in both nitrification and denitrification.
 The nitrification step in weathering of ammonium-bearing rock (Figure 2) generates two protons per molecule of ammonium (equation (1)). The weathering of ammonium minerals thus has the potential to acidify soils, a phenomenon that was demonstrated in the laboratory using an ammonium-saturated vermiculite [Simon-Sylvestre et al., 1991]. In the Klamath Mountains of northern California, strong soil acidification occurred as a result of ammonium released from mica schist bedrock and its subsequent nitrification. Soil pH values were commonly less than 3.5 and in situ nitrate concentrations exceeded 7 mg N l−1. The acidity of the soils resulted in barren patches, devoid of the natural coniferous vegetation, indicating that geologic nitrogen may have substantial ecological effects on disturbed ecosystems [Dahlgren, 1994].
5.2. Water Quality
 Nitrogen released through weathering may contribute to nitrogen saturation of an ecosystem (more nitrogen available than required by biota), leading to elevated stream water nitrate concentrations. The extent to which bedrock acts as a source for nitrogen saturation is best evaluated on a case-by-case basis using a variety of appropriate geochemical tools. These tools may include mass balance calculation of nitrogen loss from rock through soil formation, in situ and laboratory assessments of nitrogen release from rock, soil solution chemistry, calculation of nitrogen flux and mass balance on a watershed basis, and isotopic composition of rock, soil, and water.
 Nitrogen in bedrock may be a large and reactive pool that can be mobilized through weathering. Mica schist bedrock in the Klamath Mountains of northern California contained 2700 mg N kg−1, corresponding to 7.1 Mg ha−1 of nitrogen contained within a 10 cm thickness of bedrock [Dahlgren, 1994]. Weathering experiments under simulated field conditions from soils and bedrock (shale, 1370 mg N kg−1; greenstone, 480 mg N kg−1) collected from the Sierra Nevada foothills, California, yielded steady state release rates on the order of 10−19–10−20 mol N cm−2 s−1 [Holloway et al., 2001]. When normalized for total elemental concentrations in the rock specimens, nitrogen release rates are similar to weathering rates for major elemental constituents in silicate rocks [Chou and Wollast, 1985; Swoboda-Colberg and Drever, 1992; White et al., 1996]. Scaling these weathering rates to an entire soil profile, nitrogen release fluxes were estimated from 4 to 37 kg ha−1 yr−1. Mass balance calculations for an entire soil profile indicate that nitrogen released during soil formation from these metasedimentary rocks ranged from 2350 to 2570 kg N ha−1, which could account for 30–50% of the total soil nitrogen pool in a typical California oak woodland [Holloway and Dahlgren, 1999].
 While assessing nitrogen loss from rock through weathering provides a perspective of nitrogen saturation from a long-term perspective (thousands to tens of thousands of years), the calculation of watershed-scale nitrogen flux provides a short-term assessment of immediate impact of human activity superimposed on an ecosystem with geologic nitrogen. Median stream water nitrate concentrations in the Mokelumne River watershed, California, from catchments with geologic sources of nitrogen ranged from 0.3 to 1.4 mg l−1 compared to median values less than 0.03 mg l−1 for watersheds with no detectable source of geologic nitrogen [Holloway et al., 1998]. Stream water nitrate fluxes were greater than 10 kg N ha−1 yr−1 (maximum fluxes ≈20 kg N ha−1 yr−1) in low-order watersheds containing geologic nitrogen compared to the values <2 kg N ha−1 yr−1 for watersheds with no appreciable geologic nitrogen [Holloway et al., 1998]. An estimated mass balance for the entire Mokelumne River watershed (≈980 km2) indicates that greater than 90% of the nitrate flux originates from the 10% portion of the watershed containing geologic nitrogen. Thus the watershed area that contains geologic nitrogen contributes a disproportionately large amount of the total nitrate to the downstream reservoirs. Given the current emphasis on developing total maximum daily load (TMDL), criteria for nonpoint source pollutants, it is critical to recognize the possible contributions of nitrate from natural sources.
 In the Mediterranean climate of California, seasonal dynamics of nitrate concentrations in soil solution and stream water from these watersheds show a pulse of nitrogen release from soil at the beginning of the wet season and minimum concentrations during the spring [Holloway and Dahlgren, 2001]. Nitrogen released from bedrock during the summer-autumn period of limited rainfall accumulates in the soil micropores. In situ pore water extracted by centrifugation from saprolite layers contained up to 20 mg NO3-N l−1 prior to the onset of the rainy season. These maximum nitrate concentrations are often associated with low pH values believed to result from nitrification of ammonium released from the bedrock.
 The variation in δ15N for rock (Tables 2–6), in combination with the extent to which nitrogen is transformed during weathering by soil microorganisms, suggests nitrogen isotope signatures for rock are useful tools for assessing the extent of influence on water quality in limited settings. The δ15NO3− values may be used to distinguish fecal sources from naturally occurring nitrogen in many cases. Fecal matter from septic tank leachates or cattle feedlots is enriched in δ15N, generally >8‰ [Fogg et al., 1998; Kreitler and Browning, 1983; Schroeder et al., 1996] relative to soil organic matter (<7‰). The δ15N for nitrate (0.6–3 mg l−1 NO3-N) in low-order streams draining areas with nitrogen-bearing bedrock in the Mokelumne River watershed, California, were between 2.8 and 5.7‰, eliminating fecal matter as a primary source of nitrogen [Holloway, 1999].
 Laboratory leaching experiments with KCl (used to extract labile NH4+ and NO3−) produced up to 177 mg N kg−1 from soils associated with Tertiary age sedimentary rock and much lower extractable nitrogen concentrations (<6 mg N kg−1) in the Sierra Pelona basin in southern California [Williams et al., 1998]. The δ15N of rock and undisturbed soil leachates in this region were low (<2‰) in comparison to soil leachates from plots influenced by cattle feedlots and septic system leachates. A mass balance approach indicated that 10% of groundwater nitrogen in the Sierra Pelona basin originated from natural (e.g., bedrock) nitrogen sources while over 49% of the nitrate was attributed to sewage and other fecal matter sources [Williams et al., 1998].
 Arid to semi-arid regions may be more susceptible to nitrate loading from nitrogen-bearing rock as a result of incomplete leaching of soils and a relatively discontinuous rate of soil microbial transformation of nitrogen species owing to irregular wetting and drying cycles. As a result, nitrogen released from bedrock can accumulate to high concentrations. High nitrate concentrations (>10 mg l−1 NO3-N) in groundwater in the Libyan Sahel were originally attributed to interaction with nitrate-rich shale, although soil organic matter was also implicated as a nitrogen source [Edmunds and Gaye, 1997]. Similarly, high nitrate concentrations in groundwater from eastern Utah were attributed to accumulations of nitrate salts associated with siltstone and sandstone in this region of the Colorado Plateau [Stewart and Peterson, 1914].
 The limited soil microbial activity in arid regions allows the use of δ15N as an interpretive tool in establishing bedrock sources of elevated nitrate concentrations. Böhlke et al.  determined that low concentrations of nitrate in groundwater from the Victorville, California area were more isotopically similar to accumulations of nitrate salts in the region (δ15N – NO3− = 0 – +3‰) compared to potential septic tank leachates. High nitrate concentrations (>10 mg l−1 NO3–N) in groundwater with moderate to low δ15N values (<11‰) at Ft. Irwin, California were attributed to natural sources rather than sewage [Densmore and Böhlke, 2000]. The somewhat enriched natural background isotopic signature of groundwater (6–11‰) and soils (41–159 mg kg−1; 7.7–10.8‰) are isotopically similar to the biotite schist bedrock in the area (67 mg N kg−1, 7.7‰ for a single sample) [Densmore and Böhlke, 2000]. In contrast, parts of the basin at Ft. Irwin impacted by wastewater disposal had elevated nitrate concentrations (up to 3374 mg kg−1) and δ15N in excess of 11‰.
 While elevated nitrogen concentrations (361–457 mg N kg−1) were measured in samples of Pierre Shale in eastern Colorado [McMahon et al., 1999], the contribution of this rock to natural background nitrate levels was obscured in the Platte River alluvial aquifer by extensive agricultural input of nitrogen. However, the δ15N–NH4+ (+3.2 ± 0.4‰) of the shale-hosted groundwater underlying the alluvium was isotopically similar to the δ15N (+2.8 ± 0.1‰) of the shale itself, suggesting that the deeper shale aquifer ammonium was derived directly from the Pierre Shale [McMahon et al., 1999]. Denitrification was favored by abundant electron donors, including organic carbon and sulfide, in the shale, but was limited by a low flux of nitrate-contaminated water from the alluvial aquifer through the low-conductivity shale aquifer [McMahon et al., 1999].
 Approximately 200 kg ha−1 of exchangeable ammonium was estimated to reside in a 1 m thickness of the Paleocene Fort Union Shale in North Dakota and eastern Montana [Power et al., 1974]. The ammonium was rapidly nitrified to nitrate under favorable moisture, temperature, and oxygen concentration. Little nitrification of exchangeable ammonium was detected under field conditions owing to lack of moisture, oxygen, and nitrifying organisms. Power et al. warned of potential enhanced nitrate build-up in surface and groundwaters through disturbance of shale by strip mining and irrigation, which disturb soil structure that would favor the removal of nitrate through denitrification.
 Fine-grained geologic materials, including loess and glacial till, may provide a reservoir for nitrogen through increased surface area that allows for sorption of ammonium ions from the atmosphere. Pleistocene loess was proposed as a source of nitrate in central Nebraska where nitrate (25–87 mg NO3–N l−1) was encountered in groundwater at a 7 m depth and continued till a depth exceeding 30 m [Boyce et al., 1976]. The rapid onset of irrigation in this agricultural area mobilized nitrate in groundwater making the loess interpretation inconclusive. High groundwater nitrate concentrations (up to 335 mg NO3–N l−1) in a glacial till in southwest Alberta were thought to result from the nitrification of exchangeable ammonium on clay surfaces [Hendry et al., 1984].
 Carbon and nitrogen isotopic shifts in suspended sediment were used to demonstrate a spatial shift in nitrogen from biogenic (soil and plant) to lithogenic (soil and rock fragments) sources in a low-order stream in Taiwan [Kao and Liu, 2000]. Two samples of argillite-slate and sandstone bedrock had 800 mg N kg−1 and a δ15N of 3.9 ± 0.1‰, compared to relatively depleted nitrogen (−7.2 to −7.6‰) in vegetation samples. While there were insufficient data for rock samples to be conclusive, suspended sediment enriched in δ15N relative to vegetation was interpreted as controlled by human-induced erosion of bedrock and mineral soils [Kao and Liu, 2000].
5.3. Management Implications
 The exact mechanisms for nitrogen release and export from geologic nitrogen are not well constrained and will vary with climate, ecosystem structure, and pattern of land use. The influence of bedrock chemistry on soil and water quality must be addressed on a case-by-case basis, without overlooking obvious human impacts on cycling of nitrogen weathered from rock or introduced sources of nitrogen. A global emission inventory for ammonia emissions indicates that the largest sources include domestic animals (21.6 Tg N yr−1), synthetic fertilizers (9.0 Tg N yr−1), oceans (8.2 Tg N yr−1), and biomass burning (5.9 Tg N yr−1) [Bouwman et al., 1997]. On a more localized scale, septic tank leachates, cattle feedlot emissions, fertilizer, and industrial emissions can provide an overwhelming source.
 Soil disturbance by construction, timber harvest, grazing, or other agricultural practices may enhance the extent to which nitrogen from a geologic source is accessed and accommodated by soil biota. Ecosystems that develop on nitrogen-bearing bedrock may be more susceptible to nitrate export resulting from these disturbances [Power et al., 1974; Holloway et al., 1998]. Controlled burning coupled with vegetation management may be a feasible tool for attenuating nitrate leaching in certain grasslands and forests that have formed on nitrogen-bedrock terrains. Wildfires volatilize soil nitrogen [Schlessinger, 1997] and may play a significant role in attenuating nitrate leaching in Mediterranean, arid and, semi-arid ecosystems. For example, in California oak woodlands, oak trees attenuate the deep leaching of nitrate by cycling nitrogen back to the soil surface as litterfall (e.g., leaves, twigs/branches, and acorns). From 50 to 90 kg ha−1 yr−1 of nitrogen is returned to the soil surface as litterfall beneath blue oak (Quercus douglasii) [Dahlgren et al., 1997]. Oak trees have an extensive rooting system that may reach depths greater than 30 m in some locations. As a result, they are highly effective in retaining nutrients within the active biosphere. By increasing the fire frequency or intensity, it may be possible to volatilize an appreciable amount of the nitrogen returned to the soil surface as litterfall. The current management practices of thinning oak trees for pasture improvement and fire suppression have probably exacerbated nitrogen saturation in California oak woodland ecosystems.
 On a more limited scale, it may be possible to incorporate organic materials having a high carbon:nitrogen ratio into the soil to immobilize excess nitrogen in microbial biomass and to enhance denitrification. For example, sawdust has a C:N ratio of about 400:1, resulting in microbes taking up labile nitrogen (i.e., NH4 or NO3) from the soil to build their body tissue (microbial biomass C:N is about 8–10). The addition of a labile carbon source may also be employed to enhance microbial activity and consumption of oxygen. Under depleted oxygen levels, facultative and anaerobic organisms will continue to process the labile carbon source while using nitrate as the terminal electron acceptor, the denitrification process. On small streams, it may be possible to construct small retention dams to increase the hydrologic residence time within lower-order streams. Higher retention times have been shown to remove ammonium and nitrate by biological uptake (e.g., algae, riparian vegetation) and nitrate by denitrification within the hyporheic zone and reservoir sediments. In densely vegetated riparian zones, high rates of denitrification can cause significant loss of nitrogen from water flowing through the rooting zone [Groffman et al., 1992]. Riparian vegetation may cause the loss of N both directly, through uptake and incorporation in plant biomass, and indirectly, through the stimulation of microbial processes in the vicinity of the roots [Schade et al., 2001]. As plants allocate photosynthate to the production of root biomass, a certain proportion of this organic matter is lost to the soil in a process known as rhizodeposition. This input of organic matter fuels microbial processes, generating high rates of microbial activity in the vicinity of roots resulting in an increase in the denitrification potential. Thus increasing the density of woody vegetation in the riparian zone may greatly increase nitrate removal from small streams.