HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Vourlitis, G. L. Articles by Mustard, R. Search for Related Content PubMed Articles by Vourlitis, G. L. Articles by Mustard, R. Agricola Articles by Vourlitis, G. L. Articles by Mustard, R. Related Collections Dryland Soils Nitrogen Soil Organic Matter Global Change Nutrient Cycling

FOREST, RANGE & WILDLAND SOILS

Chronic Nitrogen Deposition Enhances Nitrogen Mineralization Potential of Semiarid Shrubland Soils

Dep. of Biological Sciences, California State Univ., San Marcos, CA 92096

^* Corresponding author (georgev{at}csusm.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Semiarid chaparral and coastal sage shrublands of southern California have been exposed to high levels of atmospheric N for decades, which has the capacity to increase both N and C storage and cycling in these N-limited systems. Thus we hypothesize that soil C and N mineralization will be higher in areas that have been exposed to high atmospheric N deposition. This hypothesis was tested in a 50-wk laboratory incubation experiment where the inorganic N (NH₄ + NO₃) and CO₂ production of chaparral and coastal sage soils were repeatedly measured. Soil was incubated in the dark at a constant temperature of 25°C and a soil moisture of 0.25 kg H₂O kg^–1 dry soil (65% water-filled pore space). Relative differences in N deposition exposure between the study sites were quantified by repeatedly rinsing and collecting the N accumulated on branch surfaces during 1 yr. Temporal trends in cumulative C and N mineralization were best described by single-pool first-order and zero-order models, respectively. Total N mineralization, but not C mineralization, increased linearly with relative N deposition, and NO₃ accounted for 95% of the total inorganic N accumulated during the 50-wk incubation. The soil ¹⁵N natural abundance increased with relative N deposition (r = 0.85, P < 0.05) and the soil C/N ratio declined with relative N deposition (r = –0.74, P < 0.05), suggesting that N deposition exposure enhanced N mineralization in part because of increases in the soil organic matter quality (i.e., lower C/N ratio). Furthermore, soil C storage declined as a function of relative N deposition exposure, indicating that high atmospheric N inputs are not likely to stimulate soil C storage in these semiarid ecosystems.

Abbreviations: ¹⁵N, ratio of ¹⁵N/¹⁴N of a sample relative to a standard • MRR, Motte Rimrock Reserve • SDEF, San Dimas Experimental Forest • SLW, specific leaf weight • SMER, Santa Margarita Ecological Reserve • SOFS, Sky Oaks Field Station

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Anthropogenic N deposition represents a significant input of N into southern Californian semiarid shrublands, including evergreen chaparral and drought-deciduous coastal sage (Westman, 1981; Bytnerowicz and Fenn, 1996; Padgett et al., 1999; Fenn et al., 2003b). Concentrations of atmospheric N in urban areas of Riverside and San Bernardino counties are 20 times higher than in remote areas, resulting in 25 to 50 kg N ha^–1 to be deposited to urban shrublands annually (Bytnerowicz and Fenn, 1996; Meixner and Fenn, 2004). Most atmospheric N (85%) accumulates as dry deposition on vegetation and soil surfaces during the summer and fall when primary production is limited by drought and becomes available as a large and ephemeral pulse after the first rainfall event (Bytnerowicz and Fenn, 1996; Fenn et al., 2003a).

Nitrogen enrichment of soil and shrub tissue has been reported in semiarid forests and shrublands exposed to chronic, high N deposition (Fenn et al., 1996; Padgett et al., 1999; Korontzi et al., 2000; Vourlitis and Zorba, 2007; Vourlitis et al., 2007). Soil N enrichment has the potential to stimulate N mineralization through direct fertilization and by altering soil organic matter and litter quality (Marion and Black, 1988; Fenn et al., 1996, 2003a; Vitousek et al., 1997; Currie, 1999; Padgett et al., 1999; Korontzi et al., 2000; Li et al., 2006; Vourlitis and Zorba, 2007). Nitrification is rapid in western soils, however (Fenn et al., 1996; Vourlitis and Zorba, 2007), and because NO₃ is highly mobile in soil (Paul and Clark, 1989), shrublands and forests exposed to high N deposition can also be large sources of NO₃ to aquatic systems (Riggan et al., 1985; Fenn and Poth, 1999; Meixner and Fenn, 2004).

In N-limited systems, such as chaparral and coastal sage (Mooney and Rundel, 1979; Westman, 1981; Gray and Schlesinger, 1983), increases in atmospheric N input and retention can also cause an increase in C storage and cycling because tight coupling between C and N cycles causes changes in N storage to be matched by changes in C storage (Vitousek and Howarth, 1991; Rastetter et al., 1992; Asner et al., 1997). The soil and vegetation C/N ratios of semiarid forests and shrublands is often observed to decline, however, following exposure to N deposition (Fenn et al., 2003a; Korontzi et al., 2000; Vourlitis and Zorba, 2007; Vourlitis et al., 2007), suggesting that atmospheric N inputs have little effect on soil C storage in these systems. Rather, declines in soil C/N ratios imply higher rates of soil organic matter decomposition and CO₂ emission (Micks et al., 2004), which could potentially offset any gain in soil C in response to atmospheric N deposition.

Given the significant inputs of atmospheric N (Bytnerowicz and Fenn, 1996; Meixner and Fenn, 2004) and the potential for these inputs to alter C and N storage and cycling (Fenn et al., 1996, 2003a; Padgett et al., 1999; Vourlitis and Zorba, 2007; Vourlitis et al., 2007), we hypothesized that N and C mineralization of semiarid shrubland soils will increase as a function of N deposition. This hypothesis was tested in a 50-wk laboratory incubation experiment where the C and N mineralization was quantified from chaparral and coastal sage soils exposed to varying N deposition levels.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Soil samples were obtained in September 2002 from chaparral and coastal sage sites located in southern California along a N pollution gradient described by Padgett et al. (1999) and Fenn et al. (2003b). The chaparral sites consisted of the Sky Oaks Field Station (SOFS) and the San Dimas Experimental Forest (SDEF) while the coastal sage sites consisted of the Santa Margarita Ecological Reserve (SMER) and Motte Rimrock Reserve (MRR). Each study site had four 10- by 10-m plots, and selected characteristics of these sites are displayed in Table 1. Chaparral sites were 50 (SOFS) and 42 (SDEF) yr old at the time the soil samples were obtained, but both have since burned.

Soil pH was measured 1 to 4 d after collection in 1:2 (w/v) soil slurries, where 15 g of fresh soil was added to 30 mL of deionized water and pH was measured after 30 min using a standard pH meter (Model MP 220, Mettler-Toledo, Columbus, OH). Approximately 6.5 to 7.0 mg of oven-dried litter material and 45 to 50 mg of air-dried soil were analyzed for ¹³C and ¹⁵N natural abundance and total C and N at the Kansas State University Stable Isotope Mass Spectrometry Laboratory using a mass spectrometer (ThermoFinnigan Delta Plus, Thermo Electron Corp., Bremen, Germany) coupled to an elemental analysis system (CE 1110, Carlo-Erba, Milan, Italy).

Potential Mineralization Laboratory Incubation Experiments
Potential N mineralization laboratory incubations were conducted using sequential leaching techniques (Stanford and Smith, 1972; Nadelhoffer, 1990). Triplicate samples of air-dried soil (50 g) were placed into microlysimeters (Falcon Filter Unit 7102, Becton Dickinson, Lincoln Park, NJ) and incubated in the dark at a constant temperature of 25°C during a 50-wk period. Soils were leached weekly (first 4 mo), bimonthly (Months 4–6), and approximately every 3 mo thereafter by adding 100 mL of N-free nutrient solution (Stanford and Smith, 1972; Nadelhoffer, 1990). Soil saturated with N-free nutrient solution was left to stand for 30 min, and leachate was removed using a 0.06-MPa vacuum pump and stored at 4°C until analysis for extractable NH₄ and NO₃ using an autoanalyzer (Lachat Quikchem 3000, Lachat Instruments, Milwaukee, WI). Mean (±1 SD) soil moisture was maintained at 0.25 ± 0.05 kg H₂O kg^–1 dry soil by weighing the soil + lysimeter units weekly and adding distilled H₂O as needed. This soil moisture corresponded to a 65% water-filled pore space, which is considered optimal for microbial activity (Robertson et al., 1999).

Soil respiration (hereafter referred to as C mineralization) was determined by measuring the change in CO₂ concentration in the lysimeter head space during a 2- to 4-h period (Nadelhoffer, 1990). Carbon dioxide evolution was measured weekly (first 4 mo), bimonthly (Months 4–6), and approximately every 3 mo thereafter. Lysimeters were purged of CO₂ by flowing 1.5 L min^–1 of N₂ for 3 to 5 min and were immediately sealed and incubated at 25°C for 2 to 4 h; gas samples were obtained using a 10-mL syringe immediately after purging and after the incubation period. The CO₂ concentration of the lysimeter head space was measured using an infrared gas analyzer (LI-6200, LI-COR Inc., Lincoln, NE) equipped to integrate the CO₂ concentration of the injected air sample (LI-COR, 1998).

Relative Nitrogen Deposition
Published estimates of atmospheric N pollution and deposition exposure are not available for the research sites, and estimates derived from high-resolution (4-km) models (Tonnesen, unpublished data, 2004) or sparse measurement networks may be too coarse to provide point estimates required for the study sites (Bytnerowicz and Fenn, 1996; Padgett et al., 1999; Fenn et al., 2003b; Fenn and Poth, 2004; Meixner and Fenn, 2004). Thus, an estimate of relative N deposition was derived for each study site using branch rinsing techniques (Bytnerowicz and Fenn, 1996) where inorganic N (NH₄ + NO₃) that accumulated on vegetation surfaces was collected every 3 mo for 1 yr. Between 8 and 16 apical branches of the dominant shrubs at each site (one branch per shrub) were rinsed with 30 mL of distilled H₂O, and the inorganic N collected from the branch rinsing was analyzed using an autoanalyzer (Lachat Quikchem 3000, Lachat Instruments, Milwaukee, WI). Apical shoots approximately 5 cm in length were selected for sampling, and after rinsing the shoot was collected and oven dried at 70°C for 1 wk to obtain the shoot dry weight.

Concentration data (mg N L^–1) were converted to a mass per unit ground area estimate (mg N m^–2) by (1) converting the concentration data to a mass of N per unit dry leaf mass (mgN/g dry weight), (2) multiplying the mass of N per unit dry leaf mass by the specific leaf weight (SLW; g m^–2) to derive the mass of N per unit leaf area (mg N m^–2), and (3) multiplying the mass of N per unit leaf area by the leaf area index (LAI; m² leaf area m^–2 ground area). SLW was determined by measuring the leaf area of a fresh apical shoot using a portable leaf area meter (CI-202, CID, Inc., Camas, WA, USA), oven drying the shoot at 70°C for 1 wk to obtain the dry weight, and dividing the leaf area by the leaf dry weight. LAI was measured using a portable PAR-ceptometer (AccuPAR PAR-80, Decagon, Inc., Pullman, WA, USA) which calculates LAI based on the extinction of photosynthetically-active radiation (PAR) by the shrub canopy. Inorganic N collected from each branch rinsing event was summed over a 1 yr period to obtain an annual estimate of the relative N deposition for each study site.

Estimates of relative N deposition are clearly not appropriate for estimating the absolute value of N deposition for the study sites because (i) a significant amount of accumulated N could have been lost from the vegetation surface from rainfall between each sampling event, (ii) ground-area-based estimates of relative N deposition derived from apical shoots assume that shrub biomass composition is uniform and similar to the apical shoot, and (iii) branch rinsing techniques fail to account for N deposited as wet deposition, which accounts for 10 to 15% of the total annual N deposition in these shrublands (Bytnerowicz and Fenn, 1996), and the N that may be directly taken up by leaves (Padgett and Bytnerowicz, 2001). Thus, the estimates of N deposition reported here are best viewed as an index of relative N exposure for the study sites.

Statistical Analysis
Cumulative potential C mineralization (C₀), the total cumulative N mineralization (N_min), and the C and N mineralization rate constants (k_C and k_N) for the 50-wk incubation were estimated using regression. Comparisons of response variables within a given vegetation type (chaparral or coastal sage) were not possible because only one high- and low-deposition stand was sampled in each vegetation type, and such comparisons would have amounted to simple pseudoreplication (Hurlbert, 1984). Rather, linear regression and correlation were used to assess the relationship between relative N deposition and the bulk soil C and N properties and C and N mineralization kinetics for both vegetation types combined. Data were tested for normality and heteroscedasticity before analyses, and response variables violating these assumptions were lognormally transformed (Zar, 1984).

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Cumulative Carbon and Nitrogen Mineralization Kinetics
Cumulative C mineralization was best described by a single-pool, first-order model, C_t = C₀[1 – exp(–k_Ct)], where C₀ = cumulative potential C mineralization, k_C = the C mineralization rate constant, and t = time with a mean (±1 SD) coefficient of determination (r²) of 0.987 ± 0.015 (Fig. 1a , Table 2). While single-pool models have been used extensively to describe cumulative C mineralization dynamics for a variety of soils (Stanford and Smith, 1972; Zak et al., 1994; Updegraff et al., 1995), multiple pool models may be more appropriate given the heterogeneous nature of soil organic matter (Parton et al., 1987; Paul et al., 2006; Piñeiro et al., 2006). The r² was higher and the standard error of the regression was lower, however, with the single-pool model and the coefficients for the "slower" C pools calculated from the multiple-pool model were highly variable and not significantly different from zero.

View larger version (21K): [in this window] [in a new window]   Fig. 1. The mean (±1 SE, n = 4 plots per site) cumulative (a) C and (b) N mineralization for the Sky Oaks Field Station (SOFS, open circles and short-dashed line), San Dimas Experimental Forest (SDEF, closed circles and dotted line), Motte Rimrock Reserve (MRR, closed squares and long-dashed line), and the Santa Margarita Ecological Reserve (SMER, open squares and solid line). Lines were fit to data using (a) nonlinear and (b) linear regression.

The rate constant of cumulative C mineralization (k_C) ranged between 0.052 wk^–1 for the chaparral stand at SOFS and 0.022 wk^–1 for the coastal sage stand at the MRR, and in contrast to the trend in C₀ observed between the vegetation types, the average k_C was twofold higher for chaparral soil than for coastal sage soil (Table 2). Differences in k_C between chaparral and coastal sage may be due to differences in soil microbial activity or community composition, or to differences in the content of labile C from dead microbial cells that is rapidly mineralized following soil rewetting (Zak et al., 1994; Miller et al., 2005). Regardless, k_C values reported here are lower than those reported by Zak et al. (1994) for comparable vegetation types; however, this difference is presumably due to the 10°C lower incubation temperature used here.

Cumulative N mineralization was best described by a zero-order model (Fig. 1b, Table 2), and while most investigators have used first-order models to describe potential cumulative N mineralization (i.e., Stanford and Smith, 1972; Juma et al., 1984; Wang et al., 2004), the increase in performance of a zero-order model (i.e., higher r² and lower standard error) may arise because of low or variable first-order rate constants, which can produce a linear response if N is mineralized from stable organic matter (Zak et al., 1994; Springob and Kirchmann, 2003).

The average (±SE, n = 4) total amount of N mineralized during the 50-wk incubation (N_min) ranged between 4.5 ± 0.4 g N m^–2 for the chaparral stand at SOFS to 10.2 ± 0.8 g N m^–2 for the coastal sage stand at MRR, and N_min tended to be higher for the sites that experienced higher relative N deposition (MRR and the SDEF) (Tables 1 and 2). These values are comparable to those reported for a variety of Mediterranean-type shrublands (Marion and Black, 1988; Monokrousos et al., 2004), desert shrublands of the arid Southwest (Zak et al., 1994), and Great Basin sagebrush (Artemisia tridentata Nutt.) scrub (Chen and Stark, 2000). Most of the accumulated N (95%) was in the form of NO₃, and there was no statistically significant difference in the amount of NO₃–N produced between the study sites, indicating a high nitrification potential for the semiarid shrublands studied here (Fenn et al., 1996; Vourlitis and Zorba, 2007). Similar trends were observed with k_N (Table 2), which is expected given that N_min and k_N were estimated from a zero-order model and were highly correlated (r = 0.99).

Relationships between Relative Nitrogen Deposition and Carbon and Nitrogen Mineralization
There was no significant trend between C₀ (Fig. 2a ) or k_C (Fig. 2b) and relative N deposition, implying that microbial respiration was not limited by available N (Micks et al., 2004). Nitrogen addition has been found to stimulate the decomposition of labile soil C while stabilizing more recalcitrant C fractions (Neff et al., 2002), and given the inability to differentiate between various C fractions using the single-pool model, it is unclear whether differences in C pool decomposition for the soils studied here were significantly correlated with relative N deposition. The fraction of potential C mineralization to total organic carbon (C₀/C) was between 16 and 47% and, in general, C₀ comprised a larger fraction of the total organic C for coastal sage shrublands than chaparral (Table 2) and was not significantly related to relative N deposition (Table 3).

View larger version (19K): [in this window] [in a new window]   Fig. 2. The (a) mean (±1 SE, n = 4 plots per site) cumulative C mineralization potential and (b) the C mineralization rate constant (kC) for the Sky Oaks Field Station (SOFS, open circles), San Dimas Experimental Forest (SDEF, closed circles), Motte Rimrock Reserve (MRR, closed squares), and the Santa Margarita Ecological Reserve (SMER, open squares). Lines were fit to data using linear regression; also shown is the equation, coefficient of determination (r2), and probability of type-I error (P) for the regression. NS = P > 0.05.

View larger version (19K): [in this window] [in a new window]   Fig. 3. The (a) mean (±1 SE, n = 4 plots per site) cumulative N mineralization and (b) the N mineralization rate constant for the Sky Oaks Field Station (SOFS, open circles), San Dimas Experimental Forest (SDEF, closed circles), Motte Rimrock Reserve (MRR, closed squares), and the Santa Margarita Ecological Reserve (SMER, open squares). Lines were fit to data using linear regression; also shown is the equation, coefficient of determination (r2), and probability of type-I error (P) for the regression. NS = P > 0.05.

The soil C/N ratio declined as a function of relative N deposition (r = –0.74, P < 0.01), and the decline in soil C/N was due to a decline in total soil C (r = –0.52, P < 0.05) and not an increase in total N (r = 0.33, P > 0.05; Table 3). A decline in the soil C/N ratio with an increase in N deposition has been widely observed, but N deposition usually leads to soil N enrichment, not soil C loss (Aber et al., 1998; Gundersen et al., 1998; Baron et al., 2000). Assuming that C₀ is not significantly affected by N deposition (Fig. 2), C input to soil organic matter must also decline with N deposition for a decline in total soil C to be realized. Research in deciduous and evergreen coniferous forest indicates that chronic N deposition can inhibit root production and have little overall effect on aboveground primary production and litterfall (Aber et al., 1998; Grulke and Balduman, 1999; Bauer et al., 2004; Magill et al., 2004), suggesting that long-term N deposition exposure can result in a decline in soil C inputs. Litter produced under high N deposition is also N enriched, with a low C/N ratio (Fenn et al., 1996; Korontzi et al., 2000; Magill et al., 2004), which enhances litter decomposition (Gundersen et al., 1998) and further reduces soil C storage. Thus, while potential C mineralization was not significantly related to relative N deposition in this laboratory incubation (Fig. 2), long-term exposure to high N deposition may cause a decline in soil C input, and thus soil C storage. Unfortunately, this hypothesis cannot be assessed with the data provided.

In conclusion, the N mineralization potential of semiarid shrubland soil increased linearly with relative N deposition exposure, and the increase in N mineralization was apparently due in part to a concomitant increase in soil organic matter quality (i.e., decline in soil C/N ratio) with relative N deposition. Nitrogen mineralization increased as a function of N deposition; however, C mineralization was not significantly affected by N deposition, suggesting that C mineralization was not N limited. While the laboratory incubations may not provide realistic estimates of in situ C or N mineralization, the long incubation period (50 wk) and simultaneous determination of C₀ and N_min provides a powerful tool for assessing the potential effects of atmospheric N deposition on soil C and N cycling in southern Californian semiarid shrublands.

	ACKNOWLEDGMENTS

 
This research was supported in part by the NSF-CAREER (DEB-0133259) and NIH-NIGMS-SCORE (S06 GM 59833) programs. We thank David Faber for elemental analysis, R. Fagan of the Kansas State University Stable Isotope Mass Spectrometry Laboratory for conducting the stable isotope analyses, and the >25 graduate and undergraduate student assistants whose effort made this research possible. Permission to use the SOFS and SMER (SDSU Field Station Programs), SDEF (M. Oxford, U.S. Forest Service), and the MRR (B. Carlson, Univ. of California, Riverside) field sites is gratefully appreciated.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication September 27, 2006.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 

• Aber, J., W. McDowell, K. Nadelhoffer, A. Magill, G. Berntson, M. Kamakea, S. McNulty, W. Currie, L. Rustad, and I. Fernandez. 1998. Nitrogen saturation in temperate forest ecosystems. Bioscience 48:921–934.[CrossRef][ISI]
• Asner, G.P., T.R. Seastedt, and A.R. Townsend. 1997. The decoupling of terrestrial carbon and nitrogen cycles. Bioscience 47:226–234.[CrossRef][ISI]
• Baron, J.S., H.M. Rueth, A.M. Wolfe, K.R. Nydick, E.J. Allstott, J.T. Minear, and B. Moraska. 2000. Ecosystem responses to nitrogen deposition in the Colorado Front Range. Ecosystems 3:352–368.[CrossRef]
• Bauer, G.A., F.A. Bazzaz, R. Minocha, S. Long, A. Magill, J. Aber, and G.M. Berntson. 2004. Effects of chronic N additions on tissue chemistry, photosynthetic capacity, and carbon sequestration potential of a red pine (Pinus resinosa Ait.) stand in the NE United States. For. Ecol. Manage. 196:173–186.[CrossRef]
• Bytnerowicz, A., and M.E. Fenn. 1996. Nitrogen deposition in California forests: A review. Environ. Pollut. 92:127–146.[CrossRef][Medline]
• Chen, J., and J.M. Stark. 2000. Plant species effects and carbon and nitrogen cycling in a sagebrush–crested wheatgrass soil. Soil Biol. Biochem. 32:47–57.[CrossRef]
• Currie, W.S. 1999. The responsive C and N biogeochemistry of the temperate forest floor. Trends Ecol. Evol. 14:316–320.[CrossRef][Medline]
• Emmett, B.A., O.J. Kjønaas, P. Gundersen, C. Koopmans, A. Tietema, and D. Sleep. 1998. Natural abundance of ¹⁵N in forests across a nitrogen deposition gradient. For. Ecol. Manage. 101:9–18.[CrossRef]
• Fenn, M.E., J.S. Baron, E.B. Allen, H.N. Rueth, K.P. Nydick, L. Geiser, W.D. Bowman, J.O. Sickman, T. Meixner, D.W. Johnson, and P. Neitlich. 2003a. Ecological effects of nitrogen deposition in the western United States. Bioscience 53:404–420.[CrossRef][ISI]
• Fenn, M.E., R. Haeuber, G.S. Tonnesen, J.S. Baron, S. Grossman-Clarke, D. Hope, D.A. Jaffe, S. Copeland, L. Geiser, H.N. Rueth, and J.O. Sickman. 2003b. Nitrogen emissions, deposition, and monitoring in the western United States. Bioscience 53:391–403.[CrossRef][ISI]
• Fenn, M.E., and M.A. Poth. 1999. Temporal and spatial trends in streamwater nitrate concentrations in the San Bernardino Mountains, southern California. J. Environ. Qual. 28:822–836.[Abstract/Free Full Text]
• Fenn, M.E., and M.A. Poth. 2004. Monitoring nitrogen deposition in throughfall using ion exchange resin columns: A field test in the San Bernardino Mountains. J. Environ. Qual. 33:2007–2014.[Abstract/Free Full Text]
• Fenn, M.E., M.A. Poth, and D.W. Johnson. 1996. Evidence for nitrogen saturation in the San Bernardino Mountains in southern California. For. Ecol. Manage. 82:211–230.[CrossRef]
• Gray, J.T., and W.H. Schlesinger. 1983. Nutrient use by evergreen and deciduous shrubs in Southern California. II. Experimental investigations of the relationship between growth, nitrogen uptake, and nitrogen availability. J. Ecol. 71:43–56.[CrossRef]
• Grulke, N.E., and L. Balduman. 1999. Deciduous conifers: High N deposition and O₃ exposure effects on growth and biomass allocation in Ponderosa pine. Water Soil Air Pollut. 116:235–248.[CrossRef]
• Gundersen, P., B.A. Emmett, O.J. Kjønaas, C.J. Coopmans, and A. Tietema. 1998. Impact of nitrogen deposition on nitrogen cycling in forests: A synthesis of NITREX data. For. Ecol. Manage. 101:37–55.[CrossRef]
• Heaton, T.H.E. 1986. Isotopic studies of nitrogen pollution in the hydrosphere and atmosphere: A review. Chem. Geol. 59:87–102.[CrossRef][ISI]
• Hurlbert, S.H. 1984. Pseudoreplication and the design of ecological field experiments. Ecology 54:187–211.
• Johannisson, C., and P. Högberg. 1994. ¹⁵N abundance of soils and plants along an experimentally induced forest nitrogen supply gradient. Oecologia 97:322–325.[ISI]
• Juma, N.G., E.A. Paul, and B. Mary. 1984. Kinetic analysis of net nitrogen mineralization in soil. Soil Sci. Soc. Am. J. 48:753–757.[Abstract/Free Full Text]
• Knecht, A.A. 1971. Soil survey for western Riverside area, California. Soil Conserv. Serv., Washington, DC.
• Korontzi, S., S.A. Macko, I.C. Anderson, and M.A. Poth. 2000. A stable isotopic study to determine carbon and nitrogen cycling in a disturbed southern Californian forest ecosystem. Global Biogeochem. Cycles 14:177–188.[CrossRef][ISI]
• Li, X., T. Meixner, J.O. Sickman, A.E. Miller, J.P. Schimel, and J.M. Melack. 2006. Decadal-scale dynamics of water, carbon and nitrogen in a California chaparral ecosystem: DAYCENT modeling results. Biogeochemistry 77:217–245.
• LI-COR. 1998. Measuring small volumes of CO₂ with the LI–COR LI-6200 system. LI–COR Application Note 121. LI–COR Inc., Lincoln, NE.
• Magill, A.H., J.D. Aber, W.S. Currie, K.J. Nadelhoffer, M.E. Martin, W.H. McDowell, J.M. Melillo, and P. Steudler. 2004. Ecosystem response to 15 years of chronic nitrogen additions at the Harvard Forest LTER, Massachusetts, USA. For. Ecol. Manage. 196:7–28.[CrossRef]
• Marion, G.M., and C.H. Black. 1988. Potentially available nitrogen and phosphorus along a chaparral fire cycle chronosequence. Soil Sci. Soc. Am. J. 52:1155–1162.[Abstract/Free Full Text]
• Meixner, T., and M.E. Fenn. 2004. Biogeochemical budgets in a Mediterranean catchment with high rates of atmospheric N deposition: Importance of scale and temporal asynchrony. Biogeochemistry 70:331–356.
• Micks, P., J.D. Aber, R.D. Boone, and E.A. Davidson. 2004. Short-term soil respiration and nitrogen immobilization response to nitrogen applications in control and nitrogen-enriched temperate forests. For. Ecol. Manage. 196:57–70.[CrossRef]
• Miller, A.E., J.P. Schimel, T. Meixner, J.O. Sickman, and J.M. Melack. 2005. Episodic rewetting enhances carbon and nitrogen release from chaparral soils. Soil Biol. Biochem. 37:2195–2204.[CrossRef]
• Monokrousos, N., E.M. Papatheodorou, J.D. Diamantopoulos, and G.P. Stamou. 2004. Temporal and spatial variability of soil chemical and biological variables in a Mediterranean shrubland. For. Ecol. Manage. 202:83–91.[CrossRef]
• Mooney, H.A., and P.W. Rundel. 1979. Nutrient relations of the evergreen shrub, Adenostoma fasciculatum, in the California chaparral. Bot. Gaz. 140:109–113.
• Moreno, J.M., and W.C. Oechel. 1992. Factors controlling postfire seedling establishment in southern California chaparral. Oecologia 90:50–60.[CrossRef][ISI]
• Nadelhoffer, K.J. 1990. Microlysimeter for measuring nitrogen mineralization and microbial respiration in aerobic soil incubations. Soil Sci. Soc. Am. J. 54:411–415.[Abstract/Free Full Text]
• Neff, J.C., A.R. Townsend, G. Gleixnerk, S.J. Lehman, J. Turnbull, and W.D. Bowman. 2002. Variable effects of nitrogen additions on the stability and turnover of soil carbon. Nature 419:915–917.[CrossRef][Medline]
• Padgett, P.E., E.B. Allen, A. Bytnerowicz, and R.A. Minnich. 1999. Changes in soil inorganic nitrogen as related to atmospheric nitrogenous pollutants in southern California. Atmos. Environ. 33:769–781.[CrossRef]
• Padgett, P.E., and A. Bytnerowicz. 2001. Deposition and adsorption of the air pollutant HNO₃ vapor to soil surfaces. Atmos. Environ. 35:2405–2415.[CrossRef]
• Parton, W.J., D.S. Schimel, C.V. Cole, and D.S. Ojima. 1987. Analysis of factors controlling soil organic matter levels in Great Plains grasslands. Soil Sci. Soc. Am. J. 51:1173–1179.[Abstract/Free Full Text]
• Paul, E.A., and F.E. Clark. 1989. Soil microbiology and biochemistry. Academic Press, San Diego.
• Paul, E.A., S.J. Morris, R.T. Conant, and A.F. Plante. 2006. Does the acid hydrolysis–incubation method measure meaningful soil organic carbon pools? Soil Sci. Soc. Am. J. 70:1023–1035.[Abstract/Free Full Text]
• Piñeiro, G., M. Osterheld, W. Batista, and J.M. Paruelo. 2006. Opposite changes of whole-soil vs. pools C:N ratios: A case of Simpson's paradox with implications on nitrogen cycling. Global Change Biol. 12:804–809.[CrossRef]
• Rastetter, E.B., R.B. McKane, G.R. Shaver, and J.M. Melillo. 1992. Changes in C storage by terrestrial ecosystems: How C–N interactions restrict responses to CO₂ and temperature. Water Air Soil Pollut. 64:327–344.[CrossRef]
• Riggan, P.F., R.N. Lockwood, and E.N. Lopez. 1985. Deposition and processing of airborne nitrogen pollutants in Mediterranean-type ecosystems of southern California. Environ. Sci. Technol. 19:781–789.
• Robertson, G.P., D.C. Coleman, C.S. Bledsoe, and P. Sollins. 1999. Standard soil methods for long-term ecological research. Oxford Univ. Press, New York.
• Schlesinger, W.H., and M.M. Hasey. 1981. Decomposition from chaparral shrub foliage: Losses of organic and inorganic constituents from deciduous and evergreen leaves. Ecology 62:762–774.[CrossRef][ISI]
• Springob, G., and H. Kirchmann. 2003. Bulk soil C to N ratio as a simple measure of net N mineralization from stabilized soil organic matter in sandy arable soils. Soil Biol. Biochem. 35:629–632.[CrossRef]
• Stanford, G., and S.J. Smith. 1972. Nitrogen mineralization potentials of soils. Soil Sci. Soc. Am. Proc. 36:465–472.
• Turner, C.L., J.M. Blair, R.J. Schartz, and J.C. Neel. 1997. Soil N and plant responses to fire, topography, and supplemental N in tallgrass prairie. Ecology 78:1832–1843.[CrossRef][ISI]
• Updegraff, K., J. Pastor, S.D. Bridgham, and C.J. Johnson. 1995. Environmental and substrate controls over carbon and nitrogen mineralization in northern wetlands. Ecol. Appl. 5:151–163.[CrossRef]
• Vitousek, P.M., J.D. Aber, R.W. Howarth, G.E. Likens, P.A. Matson, C.W. Schindler, W.H. Schlesinger, and D.G. Tilman. 1997. Human alteration of the global nitrogen cycle: Sources and consequences. Ecol. Appl. 7:737–750.[CrossRef]
• Vitousek, P.M., and R.W. Howarth. 1991. Nitrogen limitation on land and in the seas: How can it occur? Biogeochemistry 13:87–115.
• Vourlitis, G.L., S. Pasquini, and G. Zorba. 2007. Plant and soil N response of southern Californian semi-arid shrublands after one year of experimental N deposition. Ecosystems (in press).
• Vourlitis, G.L. and G. Zorba. 2007. Nitrogen and carbon mineralization of semi-arid shrubland soil exposed to long-term atmospheric nitrogen deposition. Biol. Fertil. Soils, doi:10.1007/s00374-006-0137-y.
• Wang, W.J., C.J. Smith, and D. Chen. 2004. Predicting nitrogen mineralization dynamics with a modified double exponential model. Soil Sci. Soc. Am. J. 68:1256–1265.[Abstract/Free Full Text]
• Wedin, D., and D. Tilman. 1990. Species effects on nitrogen cycling: A test with perennial grasses. Oecologia 84:433–441.[ISI]
• Westman, W.E. 1981. Factors influencing the distribution of species of Californian coastal sage scrub. Ecology 62:439–455.[CrossRef][ISI]
• Zak, D.R., D. Tilman, R.R. Parmenter, C.W. Rice, F.M. Fisher, J. Vose, D. Michunas, and C.W. Martin. 1994. Plant production and soil microorganisms in late-successional ecosystems: A continental-scale study. Ecology 75:2333–2347.[CrossRef][ISI]
• Zar, J.H. 1984. Biostatistical analysis. 2nd ed. Prentice Hall, Englewood Cliffs, NJ.

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Vourlitis, G. L. Articles by Mustard, R. Search for Related Content PubMed Articles by Vourlitis, G. L. Articles by Mustard, R. Agricola Articles by Vourlitis, G. L. Articles by Mustard, R. Related Collections Dryland Soils Nitrogen Soil Organic Matter Global Change Nutrient Cycling