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Comment on "A Reservoir of Nitrate Beneath Desert Soils"
Walvoord et al. (1) reported a large nitrate pool located deep(>1 m) beneath desert soils. Two aspects of this work wereparticularly surprising: the large pool size, estimated to beup to 104 kilograms of nitrogen per hectare, as nitrate (kgN ha–1); and the shape of the nitrate profiles, whichresembled the conservative solute-accumulation profiles of Cl–more than typical nutrient depletion profiles. In view of independentdata and additional issues discussed below, however, we questionthe generality of these results.
Soil nitrate values can vary greatly over short temporal andspatial scales. We recently investigated 16 desert soil profilesto 10 m depth in paired grassland and woody sites at the Jornadaand Sevilleta long-term ecological research stations (2). Analysesof eight Chihuahuan Desert cores at Jornada showed nitrate valuesthat ranged from 4 µg N g–1 of soil near the surfaceto 0.1 µg N g–1 in subsurface soils (Fig. 1). Theobserved values were consistent with data from surface soilsin other desert studies [e.g., (3–8)] and were an orderof magnitude lower than the Chihuahuan Desert values reportedin (1). Our total pool estimates to 10 m depth, at 50 to 100kg N ha–1, were also substantially lower.
Fig. 1. Soil NO3– and Cl– values to 10 m depth for adjacent Jornada grassland and shrubland sites (mean plus standard error). In each of the two communities per site, four cores of 6 cm diameter were taken with an environmental drilling rig to 10 m depth in 61 cm increments, with additional sampling at smaller intervals in the top 50 cm of soil. Soils were analyzed for exchangeable Cl– and extractable NO3– using standard soil protocols (12, 13), including distilled-water extracts for Cl– and 2 M KCl extracts for NO3–. Exchangeable Cl– was determined colorimetrically at the Utah State University Analytical Soil Laboratory on a Lachat Quickchem FIA+8000; extractable NO–3 was determined at Duke University on a Bran and Luebbe TRAACS800. The Jornada site (32°36'16.6'' N, 106°56'46.3'' W) compared adjacent grassland (Bouteloua eriopoda) and shrubland (Prosopis glandulosa) communities where management practices contributed to woody plant encroachment into the native grassland (2). The Cl– data for the grassland and shrubland sites were pooled for clarity; values were similar in both sites. Detailed descriptions of the study sites and sampling protocols are available in (2).
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The shape of the nitrate profiles was the second surprisingresult in (1). Our Jornada nitrate data followed the patternof nutrient depletion profiles (9) and bore no resemblance toCl– profiles at the site (Fig. 1). The one clear differencebetween the two nitrate profiles we studied—the smallernitrate pool in the top meter of soil in the grassland comparedto the shrubland, which is dominated by the nitrogen-fixinghoney mesquite, Prosopis glandulosa (Fig. 1)—was mostlikely biological.
Soil nitrate concentrations from Sevilleta were of similar magnitudeand profile shape to those at Jornada (Fig. 2). The nitrateconcentrations for Sevilleta soils were also an order of magnitudelower than values in (1), with total pool estimates of only60 to 90 kg N ha–1 to 10 m depth. A lack of correspondencebetween the shape of the NO–3 curve and the Cl–peak at 2 to 3 m depth was particularly clear at Sevilleta (Fig. 2).
Fig. 2. Soil NO3– and Cl– values to 10 m depth for adjacent Sevilleta grassland and shrubland sites (mean plus standard error). The Sevilleta site (34°20'05.7'' N, 106°41'59.9'' W) compared adjacent grassland (Bouteloua eriopoda) and shrubland (Larrea tridentata) communities, where L. tridentata was expanding into the native grassland. Additional information on methods can be found in (2) and in the caption for Fig. 1; note difference in scales on x-axis compared with Fig. 1.
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Vegetation and the timing of precipitation—whether rainfalloccurs primarily in the growing season—may play importantroles in determining the variability of nitrate at depth. Inthe Chihuahuan desert, P. glandulosa often grows roots to 5or 10 m (2, 10), as deep as or deeper than the NO–3 andCl– peaks observed in the Chihuahuan data of (1). Directevidence of nutrient uptake from at least 3 to 4 m depth byP. glandulosa and the grass Bouteloua eriopoda has been shownat Jornada (2). We speculate that the large variability in nitrateconcentrations and profile shapes in the soil cores studiedin (1) may be partly caused by plant activity. We further speculatethat the possible uptake of nitrate in such deep pools couldaid the success of invasive woody shrubs in deserts, a mechanismthat to our knowledge has not been considered previously.
The nitrate values we observed and the questions raised abovedo not negate the results of (1). However, the nitrate reservoirproposed for desert soils is large—up to 13,600 kg N ha–1,far larger than values in agricultural soils (11). The extrapolationof these results to 16% of global vadose-zone nitrate and 71%of warm desert nitrate is questionable. Until confirmation ofa large deep-soil nitrate pool exists generally, such regionaland global extrapolations must be treated with caution.
R. B. Jackson* S. T. Berthrong C. W. Cook E. G. Jobbágy R. L. McCulley
Department of Biology and Nicholas School of the Environment and Earth Sciences Duke University Durham, NC 27708–0340, USA
* To whom correspondence should be addressed. E-mail: jackson{at}duke.edu
11. S. A. Barber, Soil Nutrient Bioavailability (Wiley & Sons, New York, 1984).
12. J. D. Rhoades, in Methods of Soil Analysis, Part 2: Chemical and Microbiological Properties, A. L. Page, Ed. (Soil Science Society of America, Madison, WI, 1982), pp. 167–179.
13. D. R. Keeney, D. W. Nelson, in Methods of Soil Analysis, Part 2: Chemical and Microbiological Properties, A. L. Page, Ed. (Soil Science Society of America, Madison, WI, 1982), pp. 643–698.
14. This research was supported by the Biological and Environmental Research (BER) Program, U.S. Department of Energy, through the Southcentral Regional Center of NIGEC, and by NSF and the Andrew W. Mellon Foundation. A. T. Austin and W. H. Schlesinger provided helpful suggestions on the manuscript.
Received for publication 3 December 2003. Accepted for publication 12 February 2004.
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TECHNICAL COMMENTS
Michelle A. Walvoord, Fred M. Phillips, David A. Stonestrom, R. Dave Evans, Peter C. Hartsough, Brent D. Newman, and Robert G. Striegl (2 April 2004) Science304 (5667), 51c.
[DOI: 10.1126/science.1095033] |Full Text »|PDF »
REPORTS
Michelle A. Walvoord, Fred M. Phillips, David A. Stonestrom, R. Dave Evans, Peter C. Hartsough, Brent D. Newman, and Robert G. Striegl (7 November 2003) Science302 (5647), 1021.
[DOI: 10.1126/science.1086435] |Abstract »|Full Text »|PDF »|Supporting Online Material »
104 kilograms of nitrogen per hectare, as nitrate (kg N ha–1); and the shape of the nitrate profiles, which resembled the conservative solute-accumulation profiles of Cl– more than typical nutrient depletion profiles. In view of independent data and additional issues discussed below, however, we question the generality of these results. 
