Nitrogen in rock: Occurrences and biogeochemical implications

Authors


Abstract

[1] There is a growing interest in the role of bedrock in global nitrogen cycling and potential for increased ecosystem sensitivity to human impacts in terrains with elevated background nitrogen concentrations. Nitrogen-bearing rocks are globally distributed and comprise a potentially large pool of nitrogen in nutrient cycling that is frequently neglected because of a lack of routine analytical methods for quantification. Nitrogen in rock originates as organically bound nitrogen associated with sediment, or in thermal waters representing a mixture of sedimentary, mantle, and meteoric sources of nitrogen. Rock nitrogen concentrations range from trace levels (<200 mg N kg−1) in granites to ecologically significant concentrations exceeding 1000 mg N kg−1 in some sedimentary and metasedimentary rocks. Nitrate deposits accumulated in arid and semi-arid regions are also a large potential pool. Nitrogen in rock has a potentially significant impact on localized nitrogen cycles. Elevated nitrogen concentrations in water and soil have been attributed to weathering of bedrock nitrogen. In some environments, nitrogen released from bedrock may contribute to nitrogen saturation of terrestrial ecosystems (more nitrogen available than required by biota). Nitrogen saturation results in leaching of nitrate to surface and groundwaters, and, where soils are formed from ammonium-rich bedrock, the oxidation of ammonium to nitrate may result in soil acidification, inhibiting revegetation in certain ecosystems. Collectively, studies presented in this article reveal that geologic nitrogen may be a large and reactive pool with potential for amplification of human impacts on nitrogen cycling in terrestrial and aquatic ecosystems.

1. Introduction

[2] The role and importance of geologic nitrogen (nitrogen contained in bedrock) is conspicuously absent from most reviews of global nitrogen cycling dynamics. The Earth's bulk rock nitrogen concentration has been estimated at 1.27 (±1) mg N kg−1 [Allègre et al., 2001], with roughly 20% of the global nitrogen pool tied up in rock [Schlesinger, 1997]. Elevated concentrations (>100 mg kg−1) of nitrogen have been found in metamorphic, igneous, and, particularly, in sedimentary and metasedimentary rocks. The nitrogen pool in sediment and sedimentary rocks was estimated to be 7.5 × 1020 g N with a δ15N value around +10‰ compared with the atmospheric values of 3.8 × 1021 g N and a δ15N value of 0‰ [Sweeney et al., 1978]. Because standard analytical methods for elemental analysis of rocks and minerals do not generally determine nitrogen concentrations, the presence and potential importance of geological nitrogen are often overlooked.

[3] Previous research on nitrogen in rock has focused on three general topics. The first area involves the transfer of nitrogen between different sources and lithologies (e.g., the incorporation of nitrogen from sedimentary sources into sedimentary rock, the hydrothermal transfer of nitrogen from sedimentary rock to metamorphic rock, the transfer of nitrogen from the mantle to the atmosphere through volcanism). The second area of research interest is nitrogen cycling in the Archean, with rock nitrogen concentrations and isotopic compositions used to elucidate differences between modern nitrogen cycling and nitrogen transformations before the presence of life on Earth. An emerging area of research is the contribution of nitrogen from rock as a naturally occurring source of nitrate in surface and groundwaters and as a source of excess nitrogen and acidity in soils. Geologic nitrogen may be a source of the missing nitrogen noted in several biogeochemical studies of ecosystem nitrogen budgets.

[4] Understanding bedrock nitrogen as a source of high background nitrate concentrations has potential human health and ecological implications. Geologic nitrogen has been shown to contribute to ecosystem nitrogen saturation (more nitrogen available than required by biota), which leads to nitrogen leaching and elevated concentrations of nitrate in surface and groundwaters [Dahlgren, 1994; Holloway et al., 1998]. Exposure to high nitrate concentrations (>10 mg N l−1) in drinking water has been linked to human health concerns including infant methemoglobonemia, gastric cancer [Bouchard et al., 1992], bladder and ovarian cancers [Weyer et al., 2001], and endocrine disorders [Seffner, 1995; Van Maanen et al., 1994]. Nitrate at much lower concentrations contributes to surface water eutrophication [Goldman, 1988] with subsequent compromise of fisheries and fouling of waterways with excess algal growth.

[5] This review synthesizes information from several disparate fields concerning geologic nitrogen: measurement of nitrogen concentrations and isotope composition in rock, occurrences and processes influencing nitrogen content of rock, cycling of nitrogen in ancient environments, and the role of bedrock nitrogen chemistry in biogeochemical cycling in modern ecosystems. While much work has been done on nitrogen in bedrock, little has been done to synthesize the current knowledge into a cohesive understanding of nitrogen cycling in multiple contexts. Given the emerging recognition that bedrock may be a nonpoint source for elevated background nitrogen concentrations in certain ecosystems, it is important to document the current state of knowledge of nitrogen in rock, including its forms, quantification, and fate during weathering.

2. Forms and Characterization

2.1. Bulk Nitrogen Characterization

[6] Nitrogen is generally not measured in rocks because of the widely held assumption that the nitrogen concentrations are insignificant and because there is not a widely adopted standard method for analyzing the form and concentration of nitrogen in rock and soil mineral fractions. As a result, biogeochemical cycling of nitrogen is rarely addressed from a geologic context. Nitrogen is incorporated in rock as recalcitrant organic matter (e.g., kerogen), nitride minerals in meteorites and possibly in mantle rocks, salts of ammonium or nitrate, or as fixed NH4+ in silicate minerals (Table 1). The contrasting forms of nitrogen (organic versus inorganic forms) and the stability with which ammonium is held in silicate minerals, make analytical quantification challenging. Spectral methods, pyrolysis, Kjeldahl extracts, and sealed-tube combustion are the most commonly used techniques in quantifying nitrogen in bulk rock samples and mineral separates. Isolated studies have measured nitrogen concentrations of ammonium minerals by 14N nuclear microanalysis (precision ± 50 mg N kg−1 [Mosbah et al., 1993]) and electron microprobe techniques (detection limit 10–100 mg N kg−1 [Bastoul et al., 1993]). Homogeneous crystalline organic compounds, such as sulfanilimide and acetanilide, are commonly used for standardization, with ceramic materials such as TiN used in the context of microprobe analysis [Bastoul et al., 1993; Wilson et al., 1992]. Universal, homogenous reference materials need to be established that are similar in composition to silicate minerals and kerogen-bearing rocks to verify nitrogen recovery rates by various techniques, to enhance the comparability of different analytical methods, and to facilitate comparison of data acquired by differing methods.

Table 1. Nitrogen Minerals and Occurrences
MineralFormulaOccurrencesReference
  1. a

    References: 1, Story-Maskelyne [1870]; 2, Mason [1966]; 3, Keil [1968]; 4, Ericksen [1981]; 5, Ericksen et al. [1988]; 6, Mansfield and Boardman [1932]; 7, Hill [1999]; 8, Martini [1980]; 9, Wallace and Pring [1990]; 10, Toussaint [1956]; 11, Williams [1961]; 12, Effenberger [1986]; 13, Keys [1981]; 14, Ericksen et al. [1968]; 15, Odum et al. [1982]; 16, Shannon [1927]; 17, Dunning and Cooper [1993]; 18, Allen and Day [1927]; 19, Goldsmith [1877]; 20, Jan et al. [1985]; 21, Valentino et al. [1999]; 22, Parwel et al. [1957]; 23, Martin et al. [1999]; 24, Bridge and Clark [1983]; 25, Cavé et al. [2001]; 26, Hori et al. [1986]; 27, Altaner et al. [1988]; 28, Erd et al. [1964]; 29, Krohn [1989]; 30, Kimbara and Nishimura [1982]; 31, Gulbrandsen [1974]; 32, Loughnan et al. [1983]; 33, Higashi [1982]; 34, Wilson et al. [1992]; 35, Drits et al. [1997].

OsborniteTiNchondrite1
SinoiteSi2N2Ochondrites2,3
NitratineNaNO3caliche: Atacama Desert, Chile; Mojave Desert, USA; western USA; cave deposit: USA4–6
NitreKNO3caliche: Atacama Desert, Chile; western USA; cave deposit: USA4,6
NitrocalciteCa(NO3)2·4H2Ocave deposit: AZ, USA7
SveiteKAl7(NO3)4Cl2(OH)16·8H2Ocave deposit: Venezuela8
GerhardtiteCu2(NO3)(OH)3Cu ore deposit: Australia; Congo; AZ, USA9–11
LikasiteCu3(OH)5(NO3)·2H2OCu ore deposit: Congo12
DarapskiteNa3(SO4)(NO3)·H2Ohydrothermal: Antarctica; caliche: Atacama Desert, Chile; Mojave Desert, USA13,4
HumberstoniteNa7K3Mg2(SO4)6(NO3)2·6H2Ocaliche: Atacama Desert, Chile, Mojave Desert, USA5,14
Ammoniojarosite(NH4)Fe3+(SO4)2(OH)6ore deposits: WY & UT, USA; hydrothermal: The Geysers, CA15–17
Boussingaultite(NH4)2Mg(SO4)2·6H2Ohydrothermal: The Geysers, CA18,19
Lecontite(NH4,K)Na(SO4)·2H2Oevaporite, Pakistan20
Letovicite(NH4)3H(SO4)2hydrothermal: The Geysers, CA, USA; Campi Flegrei, Italy17,21
Mascagnite(NH4)2SO4hydrothermal: The Geysers, CA, USA17
Sal-ammoniac(NH4)Clhydrothermal: Etna and Vesuvius, Italy22
Tschermigite(NH4)Al(SO4)2·12H2Ohydrothermal: The Geysers, CA. USA; Campi Flegrei, Italy; Taupo, New Zealand17,21,23
Mundrabillaite(NH4)2Ca(HPO4)2·H2Ocave deposit: Australia24
Ammonian fluorapophyllite(NH4,K)Ca4Si8O20(F,OH)·8H2Ohydrothermal: Calvinia, South Africa and Guanajuato, Mexico25
Ammonioleucite(NH4)AlSi2O6metamorphosed volcanic rock: Japan; hydrothermal: The Geysers, CA26,27
BuddingtoniteNH4AlSi3O8hydrothermal: Sulphur Bank Mine, CA; Cedar Mt, NV, USA; Japan; Phosphoria Fm.: ID, USA; oil shale: Australia28–32
Tobelite(NH4,K)Al2(Si3Al)O10(OH)2hydrothermally altered clay: Japan; UT, USA; oil shale: North Sea33–35

[7] The spectral properties of ammonium-substituted minerals have been used to quantify ammonium concentration in minerals and whole rock (detection limit 8 mg N kg−1 [Deines et al., 1993]; 40 mg N kg−1 [Duit et al., 1986]). However, interference from inorganic and organic carbon resulted in the overestimation of nitrogen concentrations [Kydd and Levinson, 1986]. More recently, FTIR spectroscopy was used to isolate individual grains of mica [Boyd, 1997; Boyd and Philippot, 1998] and tobelite (ammonium end-member of illite) [Higashi, 2000] to determine the presence of ammonium, and is being developed as a quantitative tool for determination of mineral ammonium concentrations [Boyd, 1997].

[8] Initial difficulties in quantifying nitrogen in rock using spectral methods led to the use of pyrolysis for rock nitrogen quantification. Nitrogen was extracted from rock samples through pyrolysis at 960°C by elemental analyzer following removal of organic nitrogen by preheating the powdered sample at 550°C [Kydd and Levinson, 1986]. This method was extended to operationally defined inorganic nitrogen as the fraction of nitrogen remaining after the sample is roasted at a somewhat lower temperature of 450°C for 8 hours following pretreatment to remove labile organic matter, carbonates, and iron oxide/hydroxide coatings (detection limit 100 mg N kg−1 [Schroeder and Ingall, 1994]). Pyrolysis at temperatures exceeding 1000°C is generally more effective in completely volatilizing nitrogen from geological samples (detection limit 40 mg N kg−1; [Holloway and Dahlgren, 1999]). The use of acid, hydrogen peroxide, and other oxidizing wet chemistry techniques as pretreatments may release some fixed ammonium prior to analysis, resulting in a decreased apparent value for total nitrogen.

[9] Standard Kjeldahl extraction, which reduces all nitrogen to ammonium via acidic distillation, does not completely recover nitrogen relative to combustion methods from various materials, including marine plankton and kerogen (87–94% recovery [Minagawa et al., 1984]), silicate rock (50–75% [Stevenson, 1962]), coal and oil shale (inconsistent recovery; 71–111% [Rigby and Batts, 1986]), and sediments (87% recovery [Terashima, 1993]). Infrared spectroscopy data and values obtained by pyrolysis indicate that Kjeldahl digestions extracted as little as 37% of ammonium from metamorphic rock [Duit et al., 1986]. Kjeldahl extraction techniques using different combinations of acids were compared and found to be inconsistent, with a greater recovery of ammonium in muscovite using hydrofluoric acid [Bradley et al., 1990]. An interlaboratory comparison between Lehigh University and the Academy of Sciences of Germany found δ15N derived from HF-Kjeldahl extracts by Brauer and Haendel were similar to values using sealed-tube combustion [Bebout et al., 1999].

[10] Nitrogen loss and fractionation through chemical pretreatments were reduced by combusting rock in a sealed tube with copper and cupric oxide [Bebout, 1997; Bebout and Fogel, 1990, 1992; Boyd, 1997; Boyd and Philippot, 1998]. The sealed-tube combustion technique is used to measure isotopic composition (analytic precision ∼0.1‰ [Bebout et al., 1999]) of nitrogen in rock, with nitrogen concentration measured using manometry prior to introducing the gas to the mass spectrometer. An advantage of this method is the sensitivity (±5% uncertainty for a detection limit ≤0.01 mg N kg−1 [Pinti et al., 2001]). Recent studies have experimentally determined that nitrogen is fully extracted from mica separates at 1200°C using this technique [Pinti and Hashizume, 2001; Sadofsky and Bebout, 2000], although most nitrogen (>70%) was released at 1000°–1100°C. Slightly lower temperatures, between 900° and 1000°C, were determined to be optimal for nitrogen release by pyrolysis from granite [Boyd et al., 1993] and Precambrian cherts [Sano and Pillinger, 1990].

2.2. Organic Matter Characterization

[11] Nitrogen in rock occurs in part as recalcitrant organic nitrogen, largely kerogen, which remains in the rock through lithification and low-grade metamorphism. Studies of nitrogen in kerogen tend to neglect the overall concentrations in rock, but organic geochemistry techniques may be applied to understand the role of weathering in mobilizing nitrogen from kerogen. Kerogen was isolated from dolomite using 30% hydrochloric acid [Beaumont and Robert, 1999; Spangenberg and Macko, 1998], and from Precambrian cherts using hydrochloric and hydrofluoric acids [Beaumont and Robert, 1999]. The nitrogen was then extracted by combustion with copper and cupric oxide in quartz glass chambers for analysis by mass spectrometry. Replicate samples gave poor reproducibility for kerogen nitrogen and δ15N, suggesting that aggressive treatment with hydrofluoric acid may give inconsistent recovery of kerogen from silicate minerals, possibly owing to volatilization [Beaumont and Robert, 1999]. Specific organic matter fractions, including alkylporphyrins in oil shale kerogen [Chicarelli et al., 1993] and amino acids in estuary sediments [Macko et al., 1994], have been isolated using HPLC. This level of organic fractionation provides further interpretive data for determining organic matter sources and organic nitrogen transformations during diagenesis.

2.3. Mineral Characterization

[12] Ammonium silicate minerals form by substitution of ammonium for potassium, resulting in a slight rearrangement of the mineral crystal structure due, in part, to slight differences in ionic radius and the covalently bonded, tetrahedral nature of the ammonium ion [Juster et al., 1987]. There is a resulting shift in diagnostic peaks from X-ray diffraction of tobelite [Higashi, 1982, 2000], ammonium-micas [Kawano and Tomita, 1988, 1990], ammonioalunite [Altaner et al., 1988], and buddingtonite [Erd et al., 1964]. Infrared spectrometry based on absorption of the 1430 cm−1 band was demonstrated to produce a vibrational effect proportional to ammonium content in the interlayer site of mica [Shigorova, 1982], with near-infrared spectral bands near 4720, 4950, and 6410 cm−1 diagnostic for ammonium in a mineral matrix [Krohn and Altaner, 1987]. Infrared spectral detection may be applied to mineral identification in hand-samples [Altaner et al., 1988; Sterne et al., 1982] or used as a remote sensing tool [Krohn, 1986, 1989]. These techniques are effective with 20–50% substitution of ammonium for potassium in the mineral matrix [Juster et al., 1987] and are not applicable to samples where there is only minor ammonium substitution within the mineral matrix.

3. Nitrogen Cycling and Geologic Processes

3.1. Origin of Nitrogen on Earth

[13] Volatilization of ammonia and dinitrogen gas from the earth's crust and mantle through hydrothermal activity and volcanism is thought to be the source of atmospheric nitrogen (Figure 1) [Hutchinson, 1944; Rayleigh, 1939; Rubey, 1951]. This implies that mantle outgassing is the ultimate source of nitrogen in the biosphere, with nitrogen incorporated into the biosphere by microbial N2 fixation from the atmosphere. Nitrogen contained in organic matter appears to be a primary source of nitrogen in different lithologies, with some contribution of mantle nitrogen to igneous rocks. Nitride minerals associated with meteorites (Table 1) may reflect the dominant form of nitrogen in the early Earth (circa 4.5 Gyr), which has been subsequently remobilized through nitrogen cycling in the biosphere, hydrothermal transport, and crustal recycling during subduction.

Figure 1.

Nitrogen cycling in a geologic context. Major global pools of nitrogen are shown in boxes with processes represented as arrows. Larger arrows show processes that transfer nitrogen between global pools. This diagram represents a summary of processes that transfer nitrogen between these pools, but is not inclusive of all processes.

[14] Mantle nitrogen chemistry has been inferred using diamond-bearing kimberlites and ultramafic rocks, assuming these lithologies originate from the aesthenosphere (mantle). Average mantle nitrogen concentration was estimated to be as much as 40 mg N kg−1 based on nitrogen concentrations for diamonds collected globally [Cartigny et al., 2001]. Data from coupled δ13C–δ15N isotope analyses indicated a purely mantle origin for both carbon and nitrogen, as opposed to subducted organic matter, for most of the 150 diamonds analyzed from different localities [Cartigny et al., 1998]. In contrast, diamonds from the Mbuji Mayi kimberlite district, Democratic Republic of Congo (formerly Zaïre), were markedly depleted in δ15N (average −5‰) relative to the upper mantle (average +18–20‰), perhaps indicating a sedimentary nitrogen origin [Javoy et al., 1984]. Thirty to sixty percent of the mantle nitrogen in ultramafic xenoliths from San Carlos, Arizona was interpreted to originate from recycled crustal nitrogen using noble gas and nitrogen isotope systematics [Mohapatra and Murty, 2000].

3.2. Hydrothermal Fluids and Volcanism

[15] Nitrogen commonly occurs as a gas (e.g., N2, NH3) in thermal waters associated with volcanism. Presence of dinitrogen gas in glasses from mid-ocean ridge and island arc basalts was attributed to varying proportions of mantle, sedimentary, and atmospheric based on variations in δ15N2/36Ar, with an estimated nitrogen flux of 1.3 × 1019 mol over 4.55 Gyr [Sano et al., 2001]. Similarly, ammonia gas associated with volcanism was attributed to magma degassing at depth [Valentino et al., 1999], concentration of meteoric waters [Minissale et al., 1997], and leaching of underlying sedimentary rock by hydrothermal fluids [Allen and Day, 1935; Lowenstern et al., 1999; Minissale et al., 1997; Valentino et al., 1999]. Elevated ammonium concentrations have been reported for thermal waters associated with the Washburn Hot Springs complex, Yellowstone National Park (210–884 mg NH4-N l−1 [Ball et al., 1998; Gooch and Whitfield, 1888; Thompson and DeMonge, 1996; Thompson et al., 1975]), the northern California Coast Range (10–l76 mg NH4-N l−1 [Roberson and Whitehead, 1961]), Ketetahi Hot Springs, New Zealand (25–282 mg NH4-N l−1 [Moore and Brock, 1981]), the Mt. Amiata area of Central Italy (13–225 mg NH4-N l−1 [Minissale et al., 1997]), and thermal pools at Pisciarelli in the Campi Flegrei volcanic system, Italy (170–796 mg NH4-N l−1 [Valentino et al., 1999]). Elevated ammonium concentrations associated with seafloor hydrothermal systems in the Guaymas Basin in the Gulf of California (144–238 mg NH4-N l−1 [Von Damm et al., 1985]) and the Gorda Ridge off northern California (78 mg NH4-N l−1 [Campbell et al., 1994]) were attributed to the decomposition of buried organic matter. A number of ammonium salts, probably originating from ammonium-rich thermal fluids, have been identified in association with volcanic centers (Table 1).

[16] Granitic rocks can have nitrogen concentrations up to 250 mg N kg−1 (Table 2) with ammonium partitioned into the feldspar orthoclase to a greater extent than muscovite or biotite [Boyd et al., 1993]. Moderate nitrogen concentrations in granitic rocks from Japan [Tainosho and Itihara, 1991a, 1991b], southwest England [Boyd et al., 1993], South Dakota [Solomon and Rossman, 1988], and central Spain [Hall et al., 1996] were attributed to anatexis, the partial melting of preexisting rock through the emplacement of a magma body (Figure 1). Concentrations of nitrogen in granite may be enhanced by hydrothermal alteration [Bradley et al., 1990; Cooper and Bradley, 1990; Hall, 1993; Itihara and Honma, 1979; Tainosho and Itihara, 1991b], with regional metamorphism resulting from hydrothermal fluid migration generated by the magma body through sedimentary nitrogen sources [Bebout et al., 1999]. The δ15N values from granites ranged from 1 to 10‰ [Bebout et al., 1999; Boyd et al., 1993], and may reflect a combination of nitrogen source (e.g., sedimentary organic matter) and thermal history of the rock.

Table 2. Nitrogen Concentration and δ15N Composition of Igneous Rocks
OccurrenceN, mg kg−1δ15N, ‰MethodaReference
  • a

    IR, infrared spectrometry; K, Kjeldahl extract; P, pyrolysis.

  • a

    Methods: C combustion in sealed quartz glass tube with Cu and CuO.

Granite, central Spain1–243 K[Hall et al., 1996]
  Granite, SW England, – whole rock8–1875.9–10.2P[Boyd et al., 1993]
  Orthoclase509–10508.9–10.9  
  Muscovite123–39011.4–12.7  
  Biotite17–1341.7–14.9  
Ryoke Belt, Japan, unaltered granite5–53 K[Itihara and Honma, 1979]
  Altered granite15–149   
Bath District, UK, altered granite17–2251.0–4.8C[Bebout et al., 1999]
Antarctica, granite, granodiorite, diorite36–126 K[Greenfield, 1991]
  Basalt, scoria56–102   
Bishop Tuff, CA, USA85 ± 20 K[Sheridan and Moore, 1981]
Diamonds    
Mbuji Mayi kimberlite, Zaïre98–2109−11.2–+6.0C[Javoy et al., 1984]
Orapa kimberlite, Botswana11–1351 IR[Deines et al., 1993]
Africa, Australia, South America0–3350 C[Cartigny et al., 2001]
Ultramafic xenoliths, AZ, USA0.1–0.32.31–5.14P[Mohapatra and Murty, 2000]

[17] Low to moderate concentrations of nitrogen has also been detected in a variety of extrusive igneous rocks. Volcanic gasses and atmospheric deposition have been suggested as the possible origin of nitrogen in tuffs from California [Sheridan and Moore, 1981] and other extrusive igneous rocks, including basalt, rhyolite, and obsidian [Mayne, 1957]. Accumulation of nitrate salts is a plausible nitrogen source in arid and semi-arid climates, however, the interchange between sedimentary nitrogen pools and molten rock is a probable origin for nitrogen in these rocks.

3.3. Sedimentation and Diagenesis

[18] Nitrogen in sedimentary rock originates as organic matter accumulated during sedimentation, which is subsequently incorporated as ammonium into clay minerals and as organic matter during diagenesis. During early diagenesis, new labile organic matter is produced in sediments through the breakdown of more refractory compounds in marine biomass [Macko et al., 1994]. Multiple studies indicate a decrease in total nitrogen concentration in marine sediments and an increase in pore fluid ammonium concentrations with depth, indicating that ammonium is mobilized in anoxic pore fluids through microbial nitrogen cycling during diagenesis. A decrease in total nitrogen concentration from 1100 to 250 mg N kg−1 was accompanied by an increase in δ15N for organic nitrogen from 6.1‰ at sediment surface to 8‰ at depth in shallow cores (32 cm depth) collected off the coast of Morocco [Freundenthal et al., 2001]. The δ15N values were attributed to Raleigh fractionation kinetics related to microbial metabolism (e.g., mineralization, nitrification, and denitrification; see Figure 2), a hypothesis supported by pore fluid nitrate and ammonium concentration profiles through the sediment column [Freundenthal et al., 2001]. Total nitrogen concentration decreased from 400 to 600 mg N kg−1 at seafloor surface, to 100 mg N kg−1 at approximately 300 m depth in sediment cores collected in the North Philippine Sea [Waples and Sloan, 1980]. Sediment cores collected from the North Atlantic as part of the Deep Sea Drilling Project yielded pore fluids with ammonium concentrations increasing from nondetectable at the seafloor surface to 310 mg NH4+-N l−1 at 500 m depth with 3 to 5‰ δ15NH4+ [Borowski and Paull, 2000]. Similarly, pore fluids extracted from sediment cores collected off the coast of southwest Africa increase from 0 to 700 mg NH4+-N l−1 at 200 m depth [Wefer et al., 1998].

Figure 2.

The relationship between nitrogen cycling and mineral weathering is represented as three forms of nitrogen in rock: (1) sedimentary organic matter, (2) ammonium minerals, and (3) nitrate minerals, entering soil microbial pathways for nitrogen metabolism.

[19] Sufficient ammonium in solution during diagenesis can result in tobelite, an ammonium end-member of illite [Drits et al., 1997]. The transfer of ammonium from organic matter to illite/smectite interlayers during diagenesis and low-grade metamorphism is a complex interplay between mechanisms including sorption and desorption, ammonium fixation as smectite layers collapse to form illite, and dissolution of clay minerals [Schroeder and McLain, 1998]. Elevated nitrogen concentrations (Table 3) are associated with different depositional environments, including river basins (Cumnock Shale, North Carolina [Krohn et al., 1988]), deep marine basins (Pierre Shale, Colorado [McMahon et al., 1999; Williams and Ferrell, 1991]), and offshore detrital accumulations of terrestrial and marine sediment (Phosphoria Formation, Idaho [Gulbrandsen, 1974]).

Table 3. Nitrogen Concentration and δ15N Composition of Sedimentary Rocks
OccurrenceN, mg kg−1δ15N, ‰MethodaReference
  • a

    K, Kjeldahl extract; P, pyrolysis.

  • a

    Methods: C combustion in sealed quartz glass tube with Cu and CuO.

Cretaceous sedimentary sequences, North Atlantic  C[Rau et al., 1987]
  Marlstone, black clay stone (>1.0 mg Corg kg−1)400–7000−2.68–+2.27  
  Limestone, green clay stone (<1.0 mg Corg kg−1)20–3001.45–5.72  
Organic-rich sediment, various German localities8000–14,0003.5–6.3K[Stiehl and Lehmann, 1980]
Coal, Germany, Poland, USA1000–80004.2–10.7  
Serpiano oil shale, Italy, kerogen −0.90C[Chicarelli et al., 1993]
  Aliphatic hydrocarbons −3.04–+3.38  
Australia, oil shale2100–18,900−0.5–+12.7C[Rigby and Batts, 1986]
  Coal3900–20,300−0.7–+0.29  
Coal, northern Germany9000–17,000−2.5–+3.5K[Parwel et al., 1957]
Coal, Appalachian region, VA, USA15,976 K[Li and Daniels, 1994]
Pierre Shale, unaltered, CO, USA361–4372.67–2.91C[McMahon et al., 1999]
Pierre Shale, hydrothermally altered, CO, USA300–760  [Williams and Ferrell, 1991]
Nonesuch Shale, MI and WI, USA260–12402.72–8.48C[Imbus et al., 1992]
Mudstones and siltstones, Bath District, UK803–8373.1–3.9C[Bebout et al., 1999]
W. San Joaquin Valley, CA, sandstone33–1974 K[Sullivan et al., 1979]
  Claystone/mudstone/shale113–4832   
Antarctica, sandstone17–101 K[Greenfield, 1991]
  Siltstone1424   
Miocene sale, Gulf of Mexico540–790 P[Schroeder and McLain, 1998]
Shale, Cumnock Formation, NC, USA150–3200 P[Krohn et al., 1988]
Paleocene Fort Union Shale, ND and MT, USA300–500 K[Power et al., 1974]

[20] Nitrogen concentrations in sedimentary rocks (Table 3) are generally greater than that in igneous rocks (Table 2). Large accumulations of organic matter, such as petroleum or coal, result in much higher nitrogen concentrations in rocks, commonly exceeding 10,000 mg N kg−1 [Li and Daniels, 1994; Rigby and Batts, 1986; Stiehl and Lehmann, 1980]. Nitrogen concentration and δ15N values of sedimentary rock may reflect the origin of accumulated organic matter. Cretaceous (65–145 Ma) sedimentary sequences collected as cores from the North Atlantic through the Deep Sea Drilling Program showed an association between elevated organic carbon concentrations, elevated total nitrogen concentrations and depleted δ15N values, interpreted to originate from terrestrial organic matter [Rau et al., 1987]. The lower δ15N values (−2.68–+2.27‰) for organic-rich units [Rau et al., 1987] overlap with values for aliphatic hydrocarbons in oil shale (−3.04–+3.38‰ [Chicarelli et al., 1993]) and for some coal deposits (−0.7–+0.3‰ δ15N [Rigby and Batts, 1986]; −2.5–+3.5‰ [Parwel et al., 1957]). The higher δ15N values (1.45–5.72‰) from the low-organic Cretaceous sequences were interpreted to reflect a dominantly marine origin for organic nitrogen [Rau et al., 1987], although these values may also result from the incorporation of ammonium during diagenesis rather than organic matter. The δ15N range reported by Rau et al. overlaps with δ15N values reported for Mancos Shale, a marine sedimentary unit (2.67–2.91‰ [McMahon et al., 1999]), and the Nonesuch Shale, a volcaniclastic sedimentary sequence with a significant terrigeneous input of sediment (2.7–8.5‰ [Imbus et al., 1992]). It is likely that the δ15N values in the Nonesuch shale reflect some degree of hydrothermal influence [Imbus et al., 1992].

3.4. Metamorphism

[21] The onset of metamorphosis, where primary minerals are transformed by fluids under heat and pressure to secondary minerals, begins between 200° and 350°C, which overlaps the upper temperature range for diagenesis. The thermal history of the rock during diagenesis and low-grade metamorphism will effect nitrogen concentration and δ15N values. For example, a comparison of coal from different localities found δ15N values from +3.5 to +6.3‰, with increasing values from low-grade bituminous coals to anthracite, a hard coal that has undergone more extensive thermal degradation [Stiehl and Lehmann, 1980]. Ammonium associated with shale host rock has been interpreted as resulting from enrichment by solution expelled from coal seams during diagenesis at temperatures exceeding 200°C [Daniels and Altaner, 1990] or during low-grade metamorphism. Fixed nitrogen in a unit of Pierre Shale in Colorado increased during diagenesis from 300 to 760 mg N kg−1 with increasing temperature up to 300°C, beyond which ammonium was apparently volatilized [Williams and Ferrell, 1991].

[22] The dependence of rock nitrogen concentration and δ15N on thermal history is apparent in metasedimentary rock sequences, with progressive δ15N enrichment and lower nitrogen concentrations with increasing metamorphic grade [Bebout, 1997; Bebout and Fogel, 1992; Haendel et al., 1986; Mingram and Bräuer, 2001] (Table 4). The uniformity of δ15N between mica separates and metasedimentary rock within the same unit of the Catalina Schist, California, was interpreted to indicate that nitrogen was preferentially partitioned into micaceous mineral phases (fuchsite, white mica, biotite) [Bebout, 1997]. When both mineral phases are present, ammonium is partitioned preferentially into biotite over muscovite [Boyd and Philippot, 1998; Duit et al., 1986; Sadofsky and Bebout, 2000].

Table 4. Nitrogen Concentration and δ15N Composition of Metasedimentary Rocks
OccurrenceN, mg kg−1δ15N, ‰MethodaReference
  • a

    K, Kjeldahl extract; P, pyrolysis.

  • a

    Methods: C combustion in sealed quartz glass tube with Cu and CuO.

Erzegeberge, Germany, phyllite (greenschist)223–6503.3–7.8K[Haendel et al., 1986]
  Mica schist/gneiss (lower amphibolite)14–5023.4–10.8  
  Gneiss (upper amphibolite)25–826.8–17.0  
Erzegeberge, Germany, schist, low-grade620–7381.2–3.2K[Mingram and Brauer, 2001]
  Phyllite460–8751.7–2.6  
  Garnet-phyllite430–8452.3–4.2  
  Mica schist/eclogite150–5392.5–10.3  
Catalina Schist, CA, USA, lawsonite-albite360–11901.0–2.7C[Bebout and Fogel, 1992] and Bebout, 1997]
  Lawsonite-blueschist16–14802.0–2.7  
  Blueschist100–10752.0–3.1  
  Greenschist65–8101.1–4.0  
  Epidote-amphibolite30–7403.8–5.3  
  Amphibolite35–2603.6–5.9  
Catalina Schist, mineral separates    
  Lawsonite/albite fuchsite1840–21401.8–2.3C[Bebout, 1997]
  Lawsonite-blueschist fuchsite168–17702.5–2.6  
  Epidote-blueschist/amphibolite fuchsite1050–12293.2–3.5  
  Epidote-blueschist/amphibolite white mica510–14502.5–5.9  
  Amphibolite white mica275–3503.8–5.1  
Moine metasediments, Scotland    
  Amphibolite facies, whole rock140–4228.4–16.6C[Boyd and Philippot, 1998]
  Biotite365–6337.4–16.2  
  White Mica845–17397.7–16.1  
Dôme d'Agout, France, muscovite0–1140 P[Duit et al., 1986]
  Biotite90–1880   
Amphibolite schist, VT, USA, biotite13–12703.3–11.9C[Sadofsky and Bebout, 2000]
  White mica9–4013.8–10.9  
Biotite schist, Mojave Desert, CA, USA677.7C[Densmore and Böhlke, 2000]

[23] Nitrogen concentrations for metasedimentary rocks vary widely, with some concentrations in excess of 2000 mg N kg−1 (Table 4). The origin of nitrogen in metasedimentary rock suites is generally the sedimentary protolith. The δ15N values (−1.5 and −6.9‰) for N2-rich fluid inclusions were used to support a sedimentary organic matter origin for nitrogen in slates from north Wales [Bottrell et al., 1988]. Wide variability in nitrogen concentration and δ15N in a suite of metasedimentary rocks in Maine was interpreted to be a combination of protolith organic matter variation, meter-scale devolatilization, and infiltration by externally derived fluids during metamorphism [Sadofsky and Bebout, 2000].

3.5. Ore Deposits

[24] Ore deposits are remnants of ancient hydrothermal systems and, like their modern counterparts, some are associated with elevated nitrogen concentration (>1000 mg N kg−1; Table 5), likely the result of hydrothermal mobilization of nitrogen from organic matter associated with sedimentary host rock [Krohn et al., 1993; Kydd and Levinson, 1986]. The 15‰ range (−0.9 to +14.2‰) of δ15N in ammonium minerals in the western United States does not indicate any single source for the nitrogen [Krohn et al., 1993]. Ammonium appeared to be preferentially partitioned into white mica and fuchsite, a chromium mica, relative to biotite and vanadium mica during hydrothermal alteration [Jia and Kerrich, 1999]. Ammonium illite was found in gold [Wilson et al., 1992] and base metal deposits associated with black shale (Table 5) [Sterne et al., 1982, 1984]. Studies from a black shale-hosted gold deposit in north Wales, U.K., indicated an association between N2 in fluid inclusions and gold mineralization [Bottrell et al., 1988], resulting from hydrothermal leaching of nitrogen from shale during mineralization [Bottrell and Miller, 1990].

Table 5. Nitrogen Concentration and δ15N Composition of Hydrothermal Systems and Ore Deposits
OccurrenceN, mg kg−1δ15N, ‰MethodaReference
  • a

    IR, infrared spectrometry; P, pyrolysis.

  • a

    Methods: C combustion in sealed quartz glass tube with Cu and CuO.

Hydrothermal ammonium minerals    
  Feldspar, Sulphur Bank, CA, USA9900–19,000−0.6–+4.1C[Krohn et al., 1993]
  Feldspar, Ivanhoe, NV, USA3800–15,000−0.9–+12.3  
  Feldspar, McLaughlin, CA, USA3200–83001.5–8.4  
  Feldspar, Cedar Mt, CA, USA27008.0  
  Feldspar, Phosphoria Fm, ID, USA12,00014.2  
  Sal-ammoniac, Mt. Vesuvius and Mt Etna, Italy230,00011.0–11.5K[Parwel et al., 1957]
  Ammonioalunite, The Geysers, CA, USA14,000−0.6–+4.1P[Altaner et al., 1988]
Mesothermal gold deposits    
  Superior Province, Canada fuchsite35–1916.8–19.8C[Jia and Kerrich, 1999]
  White mica19–12810.4–21.0  
  Vanadium Mica19–2012.8–20.7  
  Biotite28–3013.9–14.3  
  Western Australia biotite10–708.7–23.7  
Black shale-hosted Zn-Pb-Ag deposit, AK, USA    
  Barren shale870–4610 IR[Sterne et al., 1984]
  Ore horizons1570–7070  [Sterne et al., 1982]
  Illite11,200–13,000   
Shale-hosted Zn-Pb deposits, Yukon, Canada96–2000 IR[Williams et al., 1987]
Black shale-hosted Au deposit, Wales    
Greenstone wall rock - mineralized89–649 P[Bottrell and Miller, 1990]
  Shale - unmineralized322–1467   
  Altered shale207–1669   
Horse Canyon gold deposit, NV, USA    
  Carbonates, unoxidized zone0–11,200 P[Kydd and Levinson, 1986]
  Oxidized zone0–960   

[25] Elevated nitrogen concentrations were reported for sedimentary-hosted ore deposits, reflecting the elevated nitrogen concentrations in the host rock (Table 5), although not all ore deposits will have nitrogen concentrations in the hundreds to thousands of mg N kg−1 [e.g., Jia and Kerrich, 1999]. Nitrogen concentrations in altered rock may be elevated relative to the unaltered host-rock [Sterne et al., 1982, 1984; Bottrell and Miller, 1990], possibly through concentration of ammonium in hydrothermal fluids. The oxidized zone of the carbonate-hosted Horse Canyon deposit had lower nitrogen concentrations than the unoxidized zone [Kydd and Levinson, 1986], indicating a loss of nitrogen through weathering.

3.6. Nitrate Mineral Deposits

[26] Nitrate deposits in arid regions result from the accumulation of atmospheric deposition, including wet fall and dry fall (Figure 1), in the absence of sufficient precipitation to export accumulated nitrate salts from soils. Nitrate associated with dry fall can be significant, with a mean monthly deposition of 43 mg NO3-N and 1 mg NH4−N per 100 g of loess in the Negev desert of Israel [Offer et al., 1992]. Caliche-type deposits in the Atacama Desert of Chile ranged from 15,000 to 163,000 mg N kg−1 [Ericksen, 1981] and 12,800 to 73,300 mg N kg−1 in the Mojave Desert of California [Ericksen et al., 1988], with the upper value range measured for essentially pure sodium nitrate. Isotopic data (δ15N predominantly near 0‰, δ18O between +31 and 50‰, consistent with nitrate in precipitation) suggest that the caliche-type nitrate deposits in these deserts originate from long-term (104–107 year) accumulation of atmospheric deposition with relatively little soil leaching or biological cycling of nitrogen [Böhlke et al., 1997]. Regions in California with pronounced wet-dry cycles may produce caliche-nitrate deposits, suggested by site descriptions of the Sierra Pelona basin [Williams et al., 1998] and the San Joaquin Valley [Strathouse et al., 1980].

[27] Nitrate minerals nitratine, nitre, darapskite, and humberstonite were identified in evaporite deposits in the Atacama Desert, Chile [Ericksen, 1981], the Mojave Desert, California [Ericksen et al., 1988], and other arid regions throughout the western United States [Mansfield and Boardman, 1932] (Table 1). Nitre was also identified in cave evaporites, as were the nitrate minerals nitrocalcite [Hill, 1999] and sveite [Martini, 1980], and the ammonium phosphate mineral mundrabillaite [Bridge and Clark, 1983]. Tschermigite and lecontite were also associated with evaporite deposits.

4. Archean Nitrogen Cycling

[28] Trace nitrogen concentrations (<200 mg N kg−1; Table 6) in Precambrian (>543 Myr) rock have been used to derive information on nitrogen cycling preceding and immediately following the advent of life in the late Archean (3–2.5 Gyr) to early Proterozoic (2.5–1.6 Gyr). Interpretations, however, are limited by the potential loss of nitrogen during diagenesis and metamorphism [Watanabe et al., 1997]. Nitrogen concentrations from Precambrian biotite samples varied with lithology (sedimentary (50–200 mg N kg−1) > igneous (<40 mg N kg−1)) in rocks from cratonic provinces in Australia, Kenya, South Africa, Swaziland, and Finland [Itihara and Suwa, 1985, 1987; Itihara et al., 1986; Itihara and Tainosho, 1989]. Similar partitioning of ammonium into 3.8 Gyr sedimentary biotites from western Greenland was interpreted as an evidence for the fact that clay minerals were a major sink for ammonium and other nitrogen species before the advent of life [Honma, 1996].

Table 6. Nitrogen Concentration and δ15N Composition of Precambrian Rocks
OccurrenceN, mg kg−1δ15N, ‰MethodaReference
  • a

    K, Kjeldahl extract.

  • a

    Methods: C combustion in sealed quartz glass tube with Cu and CuO.

Chert, Precambrian, various sources0.6–15.5−0.07–+9.93C[Sano and Pillinger, 1990]
Kerogen, Precambrian cherts2–106−6.2–+13C[Beaumont and Robert, 1999]
Isua Belt, Greenland 3800 Ma, mica schist6–86 K[Honma, 1996]
  Gneiss9   
Superior Province, Canada    
  Shale, metapellites, meta-argillites27–130 K[Honma, 1996] and [Honma and Schwarcz, 1979]
  Volcanics, gneisses0–27   
  Amphibolite6–14   
Precambrian biotites, Finland    
  Metasediments 2600–2800 Ma22–40 K[Itihara and Suwa, 1985]
  Metasediments 2400–1900 Ma21–158   
  Plutonic rock 1800–1900 Ma10–202   
  Rapakivi granite 1650–1700 Ma14–67   
Kenya 2500–2800 Ma whole rock    
  Shale27–62 K[Itihara et al., 1986]
  Sandstone2–4   
  Conglomerate matrix1–4   
Precambrian biotites, Australia  K[Itihara and Tainosho, 1989]
  Hutchison Group schist 1850–2100 Ma103–117   
  Lincoln Complex granite/gneiss 1700–2100 Ma21–46   

[29] Interpretation of early nitrogen cycling dynamics on earth based on Precambrian rock nitrogen has been enhanced by the measurement of nitrogen isotopes in fractionated organic matter and mineral separates. Archean cherts from cratonic provinces yielded kerogen δ15N values from −6 to +13‰, with most values between −6 and +6‰, values that are depleted relative to early Proterozoic rocks (0.3–10‰) [Beaumont and Robert, 1999] and organic nitrogen in modern marine sediment (2–10‰) [Peters et al., 1978]. Beaumont and Robert interpreted this shift in nitrogen isotope to reflect the absence of nitrification in a hypoxic Archean ocean. Nitrification plays an integral role in nitrogen cycling and results in the enrichment of modern ocean sediment δ15N [Altabet, 1988; Fruendenthal et al., 2001]. Negative δ15N values (approximately −1.7‰) associated with early Archean metasediments were thought to demonstrate that the Archean nitrogen cycle was controlled by nitrogen fixation associated with chemosynthetic bacteria [Pinti and Hashizume, 2001] in addition to fractionation of ammonium from seafloor hydrothermal vents by other chemosynthetic bacteria [Pinti et al., 2001].

5. Influence of Bedrock in Modern Nitrogen Cycling

[30] The conventional view of modern nitrogen cycling has atmospheric nitrogen as the major input to the terrestrial system either by N2 fixation or by wet and dry deposition (Figure 1). A model for the introduction of geologic nitrogen into terrestrial nitrogen cycling has three possible origins: organic matter, ammonium silicates, and nitrate and ammonium salts (Figure 2). Organic nitrogen associated with sedimentary and low-grade metasedimentary rock is mineralized during weathering, converting the organic nitrogen to ammonium, which is more readily utilized by soil biota. Ammonium derived from organic matter mineralization and from silicate minerals and ammonium salts (Table 1) enters the soil nitrogen cycle through dissolution and is either assimilated by biota or oxidized to nitrate through nitrification

equation image

Nitrate from minerals is either assimilated by biota or reduced to nitrogen gasses (N2 and N2O) under anaerobic conditions through denitrification

equation image

[31] Denitrification may be critical in modifying the extent to which nitrogen weathering from bedrock influences the export of nitrate to surface and groundwaters. Various studies indicate that nitrogen released through weathering can have a profound influence on soil and water quality.

5.1. Soil Quality

[32] 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].

[33] 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.

[34] 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

[35] 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.

[36] 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].

[37] 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.

[38] 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.

[39] The variation in δ15N for rock (Tables 26), 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].

[40] 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].

[41] 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].

[42] 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. [1997] 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‰.

[43] 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].

[44] 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.

[45] 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].

[46] 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

[47] 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.

[48] 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.

[49] 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.

6. Summary

[50] Elevated background nitrogen concentrations in soil, surface water, and groundwater can result from the weathering of nitrogen-bearing bedrock. The potential sensitivity of ecosystems with elevated background nitrogen concentrations to human impacts has generated a growing interest in the role of geologic nitrogen in global nitrogen cycling. Nitrogen in bedrock has only recently been considered as various methodologies for measuring nitrogen in rock are developed. Consequently, global nitrogen cycling models have generally neglected the role of geologic processes.

[51] Nitrogen enters the lithosphere through incorporation of organic matter in sediment and volatilization from the mantle through volcanism. Nitrogen may be retained in bedrock as organic matter (e.g., kerogen) or liberated from organic matter during diagenesis resulting in ammonium incorporation in silicate minerals or transport with hydrothermal waters. Nitrogen occurs in sedimentary, metamorphic, and igneous rocks in a wide variety of environments, with concentrations ranging from a few mg N kg−1 to in excess of 10,000 mg N kg−1 in mineral separates. Both bedrock nitrogen concentrations and δ15N isotopic signatures are dramatically altered during diagenesis and metamorphism, resulting in large variability in nitrogen concentrations and δ15N enrichment over short distances within rock bodies. Wet and dry atmospheric deposition accumulates in arid to semi-arid regions to form nitrate mineral deposits, an additional form of geologic nitrogen that comprises a significant pool of leachable nitrogen.

[52] Nitrogen in the Precambrian rock record has been used to interpret variations of nitrogen cycling from the Archean, through the Proterozoic, to the present. The interest in finding analogs for life on other planets should stimulate further work on nitrogen in the Precambrian rock record.

[53] Geologic nitrogen has been shown to be a significant source of nitrogen in some terrestrial and aquatic ecosystems. Release of nitrogen through weathering occurs at ecologically significant rates; nitrogen release rates ranged from 10−19 to 10−20 mol N cm−2 s−1 corresponding to nitrogen fluxes of 4 to 37 kg N ha−1 yr−1. Nitrification of ammonium released from bedrock can cause strong soil acidification resulting in alteration of plant communities and possible leaching of toxic metals. Geologic nitrogen may contribute to ecosystem nitrogen saturation (more nitrogen available than required by biota), which leads to nitrogen leaching and elevated concentrations of nitrate in surface and groundwaters. A paired watershed approach revealed stream water NO3–N fluxes greater than 10 kg N ha−1 yr−1 (maximum fluxes ≈20 kg N ha−1 yr−1) in watersheds containing geologic nitrogen compared to values <2 kg N ha−1 yr−1 for watersheds with no appreciable geologic nitrogen. These levels of nitrate leaching have the potential to adversely affect human health and the productivity, and functioning and structure of aquatic ecosystems. Given the recent emphasis on developing TMDL criteria for nonpoint source nitrate pollution, it is important to consider the potential of natural sources of nitrogen contributing to high background concentrations of nitrate.

[54] Relating bedrock sources to anomalous concentrations of nitrogen in soils, stream water, and groundwater is challenging. Standard soil and rock mineralogical assessments can be used to determine concentrations and δ15N values of bedrock nitrogen. Multiple fates of nitrogen in the soil environment will significantly alter the form and isotopic signature of nitrogen released from bedrock, limiting the use of nitrogen isotopes in discerning the probable source of excess nitrate in surface and groundwaters to arid regions with limited soil microbial activity. The use of multiple tools of assessment will allow better understanding of the extent to which rock contributes to nitrogen cycling in modern ecosystems.

Acknowledgments

[55] Funding for the senior author was provided by the National Research Program of the United States Geological Survey through the National Research Council. This work has been benefited from the comments of J. K. Böhlke (USGS), S. T. Petsch (WHOI), and R. L. Smith (USGS).

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