Soil Sci. Plunt Nutr., 5 1 ( S ) , 719-723, 2005 719

Geologic Nitrogen as a Source of Soil Acidity

Randy A. Dahlgren

University of California, Land, Air and Water Resources, One Shields Avenue, Davis, CA 95616. USA

Received April 1, 2004; accepted in revised form November 2, 2004

The origin of highly acidic (pHC4.6) 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-', which corresponds to 7.1 Mg N ha-' contained in a 10-cm thickness of bed- rock. In situ soil solutions were dominated by H+, labile-monomeric A13+ and NO,-, indicat- ing that the barren area soils were nitrogen saturated-more mineral nitrogen available than required by biota. Leaching of excess NO,- 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-' d-'. Nitrogen release rate from fresh ground rock was 0.0466 mg N kg-' d-'. This study demonstrates that geologic nitrogen may represent a large and reac- tive nitrogen pool that can contribute significantly to soil acidification.

Key Words: acidification, aluminum toxicity, nitrification, nitrogen saturation, weathering.

While the role of reduced sulfur (e.g., sulfides) in soil acidification is well known, soil acidification by reduced nitrogen compounds (e.g., ammonium, amino com- pounds) contained in geologic materials has received lit- tle attention. Geologic nitrogen is distributed globally and about 20% of the global nitrogen pool is contained in rocks (Schlesinger 1997; Holloway and Dahlgren 2002). Because standard analytical methods for elemen- tal analysis of rocks and minerals do not typically deter- mine nitrogen content, its presence and potential importance are often overlooked. Elevated nitrogen con- centrations (>lo0 mg kg-') have been found in meta- morphic, igneous, and, particularly in sedimentary and metasedimentary rocks (Stevenson 1962; Holloway and Dahlgren 2002). Geologic nitrogen has been shown to contribute to ecosystem nitrogen saturation (more nitro- gen available than required by biota), which may lead to nitrogen leaching and elevated concentrations of nitrate in surface and ground waters (Holloway et al. 1998). When reduced forms of nitrogen occur in poorly buff- ered geologic materials, there is the potential for appre- ciable soil acidification (Dahlgren 1994).

Nitrogen may be incorporated in rock in several forms, including relict organic matter (e.g., carbo- naceous shale), fixed ammonium in silicate clays (e.g., NH,+ interlayered mica) or salts of ammonium or nitrate (Holloway and Dahlgren 2002). Some bedrock samples have nitrogen concentrations in excess of 2,000 mg N kg-' (Krohn et al. 1988; Williams et al. 1987; Dahlgren 1994). The ultimate source of nitrogen in rock is organic matter deposited in sediments (Krohn et al. 1988; Ram- seyer et al. 1993). Nitrogen contained in organic matter is converted to ammonium during diagenesis as temper-

ature exceeds about 150°C (Williams and Ferrell 1991). Sediment cores collected from the North Atlantic and southwest Africa as part of the Ocean Drilling Project yielded pore fluids with maximum ammonium concen- trations ranging from 310 to 700 mg NH,+-N L-' (Wefer et al. 1998; Borowski and Paul1 2000). The liber- ated ammonium may substitute for potassium in silicate minerals or may be transported in hydrothermal fluids.

The primary purpose of this paper is to demonstrate the role of ammonium-bearing mica schist in contribut- ing to ecosystem nitrogen saturation and soil acidifica- tion. In situ soil solutions from the field site and simulated weathering in laboratory incubations were used to assess the release of nitrogen from soils and their parent bedrock. Management strategies for amelio- ration of soil acidity and reestablishment of conifer veg- etation are discussed based on field observations.

MATERIALS AND METHODS

The study area was located along South Fork Moun- tain in the Klamath Mountains, northwestern California (40"15'N, 123"15'W). The elevation of the study site was about 1,675 m and slopes ranged from 3 to 8%. The climate is Mediterranean with cold, wet winters and hot, relatively dry summers. Mean annual air temperature and precipitation were estimated to be about 7°C and 1,500 mm, respectively. Most of the precipitation falls as snow between November and March, although thun- derstorms provide some summer precipitation.

Soils (Xerumbrepts in Soil Taxonomy) formed on South Fork Mountain schist having a mineralogical assemblage consisting of quartz-albite-mica-chlorite

720 R.A. DAHLGREN

that formed from low-grade metamorphism of carbona- ceous, argillaceous mudstone. We focused this study on small barren areas (< 10 ha) devoid of coniferous vege- tation. The only vegetation existing on these barren areas is a scattering of the prostrate herb, Culyptridium umbellutum (Torr.) Greene. These barren areas were believed to have formed from small lightening induced forest fires followed by erosion of the litter layer and organic-rich topsoil.

Six soil profiles were randomly sampled from two barren areas located about 1 km apart. In situ soil solu- tions were extracted by centrifugation using double-bot- tom tubes at a relative centrifugal force of 15,530 times gravity; about -6,500 kPa assuming attainment of equi- librium (Dahlgren 1993). No water was added to the soil before centrifugation. Soil solutions were extracted monthly during the snow-free period. Solutions were fil- tered through a 0.2 k m membrane filter and analyzed for pH, major cations and anions (ion chromatography) and labilehon-labile monomeric A1 (pyrocatechol violet method; McAvoy et al. 1992).

To determine nitrogen release rates, batch weathering/ leaching columns were established for individual soil horizons and bedrock. Soil samples were sieved to pass a 2-mm screen. Soil samples were treated with 5% hydrogen peroxide to oxidize soil organic matter. Fresh interior samples of rock were ground and the 50 to 250 p.m fraction isolated by sieving. Fine particles were removed by sonication in distilled-deionized (DDI) water. Total nitrogen on soil and rock samples was mea- sured by pyrolysis at 1,020"C using a Carlo Erba C/N analyzer (Schroeder and Ingall 1994).

Batch weatherinfleaching columns (n =6) were con- structed using 60-mL polypropylene syringe barrels and leaching was performed using a Centurion mechanical vacuum extractor (Centurion International, Lincoln, NE, USA, Model 24). Thirty grams of soil materials and 10 g of ground rock were added to each syringe. Quartz sand columns were similarly established as an experi-

mental blank. Samples were initially leached with 30 mL of 1 M KCl solution followed by 20 mL of DDI water to displace NH,' and NO,- from mineral surfac- es. An inoculum of soil microorganisms was added to the samples using 2 mL of in situ soil solution collected by centrifugation. Samples were allowed to drain freely prior to incubation. Samples were regularly flushed with air to prevent development of anaerobic conditions and denitrification. They were stored in the dark at 202 1°C in a constant-temperature room. The columns were leached every 2 months for a 1-year period as described above. The KCl extracts were combined with the DDI water rinse and analyzed for NH4+ and NO,- using a conductimetric nitrogen analyzer (Carlson et al. 1990).

RESULTS AND DISCUSSION

Soil characterization data show that the soils are extremely acidic (pH<4.5) with low concentrations of exchangeable base cations and high exchangeable A13+ (Table 1). In contrast, soils in the adjacent forested area had a higher pH and base saturation, especially in the upper soil horizons most strongly affected by nutrient cycling. Nitrogen concentrations are high leading to low C/N ratios (<5 in barren area soils). A previous study attributed the high nitrogen concentrations to the pres- ence of nitrogen in the bedrock (Dahlgren 1994). Soil solutions collected in September showed that anions were dominated by NO,- (>87%) and cations by H+ and A13+ (Table 2). Extracted soil solutions had a pH ranging between 3.4 and 3.5. This soil solution signature indicates that soil acidity was largely derived from nitric acid and that organic acids (as estimated from the anion charge deficit) were only a minor contributor to the charge balance. Dissolved aluminum was predominantly labile monomeric A13+ (A13+ and its complexes with OH-, F- and SO:-), as defined by its rapid reactivity with pyrocatechol violet reagent. Labile monomeric A P is considered to be among the most toxic forms of

Table 1. Selected soil characterization data for the barren area soils and adjacent forested soils.

Barren area soils Forest area soils Horizon

A AB Bw BC Oi/Oa A AB Bw BC Depth (cm) Coarse fragments (>2-mm volume %) Bulk density (g cm-')

Exch-Ca (cmol, kg-I) Exch-Mg (cmol, kg-I) Exch-K (cmol, kg-I) Exch-Na (cmol, kg-I) Exch-Al (cmol, kg-I) Organic C (g kg-') Total N (g kg-I) C/N ratio (mass)

PH

0-10 25

1.04 4.3 1 <o. I <o. 1

0.1 <o. 1

4.1 40 8.6 4.7

10-22 32

1.15 4.32

<o. I <o. 1

0.1 <o. 1

4.5 35 8.8 4.0

22-46 45

1.18 4.45 <o. 1

0.1 0. I

<o. 1 4.1 25 6.5 3.9

46-66 50

1.24 4.46 <o. I

0.1 0.1

<o. 1 3.9 23

5.7 4.0

12-0 0

0.15 4.63 10.9 6.1 1.6 0.6 0.1 320

24.4 13.1

0-18 15

1.08 4.80

3.9 1.5 0.4

<o. 1 4.1 157 18.7 8.4

18-32 25

1.10 4.63 0. I 0.1 0.2

<o. 1 4.3 36

9.2 3.9

32-55 40

1.18 4.75 0.1 0.1 0.2

<o. 1 4.6 33 8.0 4.1

55-80 50

1.28 4.76 0.1 0.1 0.1

<o. 1 3.8 31

5.6 5.6

~

Results are the mean of six replicate soil profiles. Analysis methods are: pH. 1:l soi1:water; exchangeable base cations. 1 M NH,C1 extraction; exchangeable Al' + , 1 M KCI extraction: C and N, Carlo Erba C/N analyzer.

Geologic Nitrogen as a Source of Soil Acidity 72 1

Table 2. Chemistry of in situ soil solutions extracted by centrifugation from barren area soils. Non-labile Labile

H+ monomeric AI monomeric AI pH C1- K+ NH,' Mg2+ Caz+ A13+ Organic NO,- Horizon anions

kmol, L-l pmol L-'

A 4 1 515 45 40 5 10 8 4 180 440 12 51 3.45 Bw 56 540 54 30 5 8 4 4 240 420 29 51 3.40 BC 20 600 I0 18 4 10 2 2 250 440 29 60 3.40

Organic anion contribution is estimated from the anion deficit. All values are the mean of six samples.

Y 0

5.0

3.0 I J u n July Aw. Sep. on.

Fig. 1. Nitrate concentration and pH of soil solutions collected by centrifugation from barren area soils during the snow-free period. All values are the mean of six samples; coefficient of variability (standard deviationtmeanx100) was less than 12% for all data points.

dissolved aluminum for sensitive plants (Stevenson and Vance 2000).

In situ soil solutions extracted from soil samples dur- ing the snow-free period showed a rapid increase in nitrate concentrations between June and August fol- lowed by and a plateau (Fig. 1). Solution pH was inversely related to NO,- concentrations, decreasing from about 4.2 to 3.4 from June to September. The nitrate builds up in the pore waters over the summer dry period and appears to be flushed during the melting of the snowpack. Ammonium concentrations were general- ly less than 10 p , ~ , indicating nearly complete conver- sion of ammonium to nitrate by the nitrification process. Soil temperatures at a depth of 30 cm were 4, 12, 19 and 15°C at the time of soil collection during June, July, August and September, respectively. The soluble NO,-N pool increased from 0.56 kg ha-' in June to 6.96 kg ha-' in September, an increase of 6.4 kg ha-'. While we are not able to determine the total amount of nitrate pro- duced due to possible plant uptake and nitrate leaching with summer thunderstorms, this amount of NO,-N serves as a minimum estimate for the amount of nitrate produced and potentially leached during the wet winter months. This nitrogen may originate from weathering of the inorganic soil fraction, as well as from mineralization of soil organic matter. The nitrification of 6.4 kg ha-' of NH,+-N (NH,++20,=NO,-+2Ht +H,O) would con- tribute 0.91 kmol H+ ha-' yr-'. This amount of acidity is about equal to the acidic deposition load in eastern North America (1 kmol, ha-' yr-'; NADP/NTN 2004). Due to charge balance requirements, the leaching of

0 M 60 90 120 I50 180 210 240 270 300 330 360 Days

Fig. 2. Cumulative release of mineral nitrogen (NH,' +NO,-) from laboratory weathering study over a 1-year period. Slope of the regression line is equal to the nitrogen release rate. All values are the mean of six samples; coefficient of variability (standard deviation+ meanX 100) was less than 9% for all data points.

NO,- results in the leaching of base cations, which low- ers the base saturation and results in soil acidification. In the adjacent coniferous forest, the uptake of NO,- by the vegetation attenuates acidification by either simulta- neous uptake of H' or release of bicarbonate or organic alkalinity to maintain charge balance within the plant (Dahlgren 1994). Therefore the loss of vegetation from these barren area sites greatly enhances soil acidifica- tion. The leaching of base cations (e.g., Ca2+, Mg2+ and K') further leads to very low base cation concentrations on cation exchange sites and in soil solution suggesting that base cation deficiencies may contribute to the lack of conifer establishment on the barren areas.

Average nitrogen concentration in the mica schist bedrock was 2,700 mg N kg-'. This concentration corre- sponds to about 7.1 Mg ha-' of nitrogen contained in each 10-cm thickness of bedrock. Batch weathering in leaching columns was performed to determine whether nitrogen could be released from soil and rock at a rate that could significantly contribute to H+ and NO,- pro- duction. Rates of mineral N (NH,++NO,-) release reached a constant rate after 122 d (Fig. 2). Mineral N was dominated by NO,- (>72%). Steady-state release rates were determined from a linear regression line plot- ted through data from days 122 to 363. Nitrogen release rates for soil materials ranged from 0.01 63 to 0.032 1 mg N kg-' d-' and displayed increasing rates with increas- ing soil depth (Table 3). When standardized for the N content of the soil material, rates ranged from 2,507 to 5,228 mg N kg-' N d-I. Nitrogen release rates were highest from the ground rock materials: 0.0321 mg N

722 R.A. DAHLGREN

Table 3. Nitrogen content, N release rates and fraction of total nitrogen released from <2-mm fraction of organic matter free soil and ground rock (50-250 bm fraction).

Nitrogen release rate Fraction of

Soil or rock (mg N kg-') total Nitrogen

Soil'rock content material (mg kg-') Soil or rock (%I

A horizon 1,687 0.0163 3,507 0.35 AB horizon 1,734 0.0190 3,978 0.40 Bw horizon 1,965 0.0283 5,228 0.52 BC horizon 2,266 0.0321 5,142 0.5 1 Mica schist rock 2.705 0.0465 6,240 0.62

kg-' d-' or 6,240 mg N kg-' N d-I. These data suggest that nitrogen contained within the soil/rock materials displays a range of weatherability. The A horizons being the most highly weathered have released their most labile N fraction while the ground rock has a greater fraction of easily released N, probably associated with broken edge sites of mica that were exposed by grinding of the rock materials. Rates of nitrogen release obtained in this study are similar to those found for greenstone and slate lithologies from the Sierra Nevada foothills of California (Holloway et al. 2001). Over the course of a year, 0.35 to 0.62% of the N contained in the soil/rock materials was released.

Extrapolating laboratory N release rates to a soil pro- file basis yields release of 38.2 kg N ha-' yr-'. When oxidized by nitrification, this amount of nitrogen would contribute 5.5 kmol H+ ha-' yr-I. When comparing lab- oratory to field weathering rates, it is expected that field weathering rates will be considerably lower due to cold- er soil temperatures and suboptimal soil water contents during the summer. However, the amount of nitrogen released under field conditions may account for an appreciable fraction of the net nitrogen retention capaci- ty of coniferous forest, which generally ranges from 5- 30 kg N ha-' yr-' (Johnson 1992).

This study shows that geologic nitrogen can be a large and reactive pool of nitrogen that has the potential to contribute to ecosystem nitrogen saturation and soil acidification. Removal of vegetation decouples the bio- geochemical cycle leading to soil acidification, loss of nutrient pools through erosion and leaching, and poten- tially toxic concentrations of labile monomeric Al". Establishment of coniferous vegetation on the barren area soils represents a mechanism to attenuate leaching of nitrate so that soil acidification and base cation leach- ing are reduced. The only sites with established conifer vegetation on the barren area soils are where a litter lay- er exists beneath existing vegetation and where large woody debris lies at the soil surface. These organic materials have a high C/N ratio and therefore will con- tribute to microbial immobilization of nitrogen. Leach- ing of these organic materials results in release of organic acids that will complex and help detoxify dis- solved A13+ (Stevenson and Vance 2000). Furthermore,

decomposition of these materials results in a slow- release source of nutrient cations (e.g., Ca", Mg*+, K') that have been largely leached from the barren area soils. Amending soils with a high C/N ratio organic material, such as sawdust or wood chips (C/N-400), may provide a local source of organic material to immo- bilize mineral nitrogen. Addition of dolomitic lime and phosphate fertilizer will further reduce acidity and pro- vide nutrients that are in low supply. Extreme care must be exercised in forest management practices in these sensitive ecosystems to preserve the natural nutrient cycle that attenuates soil acidification and maintains the nutrient status.

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