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 Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Phillips, R. L. Search for Related Content PubMed PubMed Citation Articles by Phillips, R. L. Agricola Articles by Phillips, R. L. Related Collections Biogeochemical Processes Nutrient Management Soil Biology

TECHNICAL REPORTS

Organic Agriculture and Nitrous Oxide Emissions at Sub-Zero Soil Temperatures

USDA-ARS, Northern Great Plains Research Lab., Box 459, Mandan, ND 58554

^* Corresponding author (Phillips{at}mandan.ars.usda.gov)

Received for publication May 26, 2006.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
In the Red River Valley of the upper midwestern United States, soil temperatures often remain below freezing during winter and N₂O emissions from frozen cropland soils is assumed to be negligible. This study was conducted to determine the strength of N₂O emissions and denitrification when soil temperatures were below zero for a manure-amended, certified organic field (T2O) compared with an unamended, conventionally managed field (T2C). Before manure application, both fields were similar with respect to autotrophic and heterotrophic N₂O production and N₂O flux at the soil surface (0.15 ± 0.05 mg N₂O–N m^–2 d^–1 for T2O and 0.12 ± 0.06 mg N₂O–N m^–2 d^–1 for T2C). After application of pelletized, dehydrated manure, average daily flux (based on time-integrated fluxes from 20 November to 8 April), was 1.19 ± 0.34 mg N₂O–N m^–2 d^–1 for T2O and 0.47 ± 0.37 mg N₂O–N m^–2 d^–1 for T2C. Denitrification for intact cores measured in the laboratory at –2.5°C was greater for organically managed soils, although only marginally significant (p < 0.1). Cumulative emissions for all winter measurements (from 16 November to 8 April) averaged 1.63 kg N₂O–N ha^–1 for T2O and 0.64 kg N₂O–N ha^–1 for T2C. Biological N₂O production was evident at sub-zero soil temperatures, with winter emissions exceeding those measured in late summer. Late autumn manure application enhanced cumulative N₂O–N emissions by 0.9 kg ha^–1.

Abbreviations: POM, particulate organic matter • RRV, Red River Valley • WFPS, water-filled pore space

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
ORGANIC agriculture currently represents 0.5 million hectares of U.S. cropland and interest in conversion from synthetic to organic-based fertilizer amendments is expected to rise with the cost of fossil fuel-based fertilizers. Agricultural fertilization worldwide contributes 6.2 Tg N₂O–N yr^–1 to a total global source strength of 17.7 Tg N₂O–N yr^–1 (Kroeze et al., 1999), and considerable effort has been devoted toward identifying how fertilizers influence the net flux of N₂O (Mosier et al., 1997; Mummey et al., 1998; Flessa et al., 2002; Scott et al., 2002). Nitrous oxide is a concern due to its global warming potential (Robertson et al., 2000), yet data are lacking for winter N₂O emissions in agriculturally productive areas of the upper midwestern United States, where sub-zero soil temperatures persist over a prolonged winter.

Agricultural fertilization influences N₂O fluxes, but the response varies with soil properties, fertilizer type, application timing, and rate (Breitenbeck and Bremmer, 1986; Parton et al., 1996; Mosier et al., 1998; Bouwman et al., 2002). Intensive laboratory studies indicate significant response in N₂O flux when soils are amended with synthetic fertilizers, composts, and animal manures (Breitenbeck and Bremmer, 1986; Paul et al., 1993; Akiyama et al., 2004). The effects of organic fertilizer on N₂O fluxes are complicated by the wide variety of manure used (Akiyama and Tsuruta, 2003) and by gaps in the knowledge of biological N₂O production at sub-zero soil temperatures (Rover et al., 1998). Soil temperatures in the Red River Valley (RRV) typically remain below zero (to 1-m soil depth) between December and March. Consequently, manures are commonly applied to fields in the autumn before soil freezing with the expectation that nutrients are not transformed in frozen soil and are therefore available to crops the following spring. Conventional synthetic fertilizers, such as urea, are applied more often in the spring, just before planting. The effect of autumn-applied manure on winter N₂O fluxes for agricultural soils that remain below 0°C is largely unknown. The goals of this study were to determine if autumn manure amendment would influence N₂O flux and if denitrification contributed to N₂O flux at sub-zero soil temperatures in RRV (47°01' N; 96°36' W) production agriculture systems.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field Sites
Two 30-ha crop fields were selected: one managed conventionally for >50 yr (T2C) and one managed organically for 5 yr (T2O) in Clay County, Minnesota (Fig. 1). Before conversion to organic production, nutrient amendment and pest control practices were similar. An uncultivated site located in a tree-row [Populus (L.)] was selected as background reference (Fig. 1). This reference site is densely covered by grasses [Bromus tectorum (L.) and Elymus repens (L.)], and has remained undisturbed for over 30 yr. Soils are classified as silt loams, specifically Colvin (fine-silty, mixed, superactive, frigid Typic Calciaquoll) for the organic and reference sites and Bearden (fine-silty, mixed, superactive, frigid Aeric Calciaquoll) for the conventional site (Soil Survey Staff, 2006). Agronomic inputs (Table 1) represent actual product applications required to produce high yields in a production environment. The organic management treatment began in 1998, when landowners converted management practices to meet USDA-certified organic standards.

View larger version (82K): [in this window] [in a new window]   Fig. 1. Overall map of the field site area (47°01' N; 96°36' W), located in Clay County, MN where soil and gas flux sample were collected in the Red River Valley (RRV) agricultural region. Weather station location is also shown, from which continuous soil temperature data were collected. Above the regional map is an aerial photograph (2004) indicating proximity of organic field (T2O), conventional field (T2C), and reference site to each other.

Soil Properties
Before the autumn 2004 organic manure application, soils were collected to partition sources of N₂O production and to evaluate field-pair suitability. Ten cores (5-cm diam. x 15-cm depth) were collected with a hammer auger from each management treatment and two cores were collected from the uncultivated reference site in July 2004. Cores were kept in cold storage (4°C) until experiments commenced the following day. Samples were obtained at low soil moisture (23% WFPS) to facilitate diffusion of C₂H₂ into the soil pore spaces, as required for selective inhibition of autotrophic nitrification from denitrification and heterotrophic nitrification (Klemedtsson et al., 1988). Field-moist soils were weighed, sieved, homogenized, and analyzed at 20°C for N₂O to determine the relative source strength of autotrophic nitrification vs. heterotrophic processes to N₂O production using sequential application of H₂SO₄–scrubbed C₂H₂. Briefly, N₂O production was first measured (a) from soils under ambient headspace, then (b) with headspace amended with 10 Pa C₂H₂, then (c) with headspace amended with 10 K Pa C₂H₂, to determine the relative contribution of autotrophic nitrification to total N₂O production (Phillips et al., 2000) and total N emitted from denitrification (Paul and Beauchamp, 1989). Autotrophic nitrification is calculated by taking the difference between total N₂O produced (ambient conditions) and N₂O produced by denitrification and heterotrophic nitrification (under a 10 Pa C₂H₂ headspace). Total denitrified N is determined by incubating soils under a 10 K Pa C₂H₂ headspace, which blocks the reduction of N₂O to N₂.

Before gas sampling, C₂H₂–amended soils were incubated for 4 h and equilibrated with the ambient atmosphere. Headspace pressure was equalized during sampling by injecting an equal volume of N₂ into vessel headspace. Sample aliquots were immediately injected into 15-mL exetainers (Labco Unlimited, Buckinghamshire, UK) and analyzed for N₂O with a Varian Model 3800 Gas Chromatograph and Combi-Pal auto-sampler. In this system, sample is auto-injected into a 1-mL sample loop, then loaded onto columns and routed through a ⁶³Ni electron-capture detector (ultra-pure 95% Argon/5% CH₄ carrier gas) detector. The gas chromatograph was calibrated with a commercial blend of CO₂, CH₄, and N₂O balanced in N₂ (Scott Specialty Gases) following verification of stated concentrations with standards from the National Institute of Standards and Technology. The precision of analysis expressed as a coefficient of variation for 10 replicate injections of 360 and 500 nL N₂O–N L^–1 standards was <1.5%. The minimum detectable concentration change was ± 4 nL L^–1; values within 4 nL L^–1 were not different from each other. Gas samples were stored <8 h before analysis, and tests showed no change in N₂O concentration during storage. The mass of N₂O–N produced was calculated after accounting for soil mass, the mass of N₂O in the aqueous phase, and headspace dilution with N₂. Vessels were sampled four times over a 0.75-hr time course and rates were calculated from the time linear change in headspace N₂O concentration. Incubation data were analyzed separately with a mixed analysis of variance (Littell et al., 1996) to test for differences between organic and conventional soils for N₂O–N emissions, autrotrophic nitrification, and total denitrified N (N₂O + N₂).

Cores were analyzed for total N and total C by dry combustion (Carlo Erba NA 1500 Elemental Analyzer). Soil moisture was measured gravimetrically (oven-dried at 105°C). Soil particle density was measured pycnometrically, texture was assessed hydrometrically, and bulk density computed as the quotient of oven-dried mass divided by field volume. Percentage water-filled pore space (%WFPS) was calculated as the ratio of volumetric soil water content to total soil porosity. Soil pH was measured potentiometrically on 1:2 soil/deionized water slurries equilibrated for 24 h. Ground soil was used to measure inorganic C on soils after application of dilute hydrochloric acid stabilized with FeCl₂ by measuring the amount of CO₂ produced by gas chromatography (Loeppert and Suarez, 1996). Soil organic carbon was calculated as the difference between total C and inorganic C. Particulate organic matter (POM) was estimated using the C content of material retained on a 0.053-mm sieve (Gregorich and Ellert, 1993). Soil characteristics were normalized by soil weight, due to differences in bulk density, and analyzed separately with a mixed analysis of variance to determine if soil variables (pH, bulk density, organic carbon, nitrogen, inorganic carbon, % sand, % silt, % clay, % POM, and soil C/N ratio) were similar for conventional and organic fields. For the soil incubation experiments, differences in %WFPS were also tested similarly.

Organic Amendment Application and Core Sampling
On 20 Nov. 2004, the 10 organic field sites were amended with 67.5 kg N ha^–1 of a pelletized, dehydrated manure product (Creekwood Farms, Lake Mills, WI) and homogenized into the top 5 cm of soil. Amendment was applied to only 1 m² since the field was too wet for heavy equipment. The landowner assisted with amendment application to ensure manure was worked into the soil to resemble typical homogenization following disking. Manure product chemical analyses are given in Table 2. Soil temperatures below freezing were recorded 1 wk following manure application, 17 d after manure application. Five intact soil cores (5-cm diam. x 15-cm depth) were extracted from manure-amended sites in the organic field, five from the unamended conventional field, and one from the uncultivated reference site. Cores were kept in cold storage (–4°C) and remained below freezing for the duration of the experiment. Air temperature was –5.1°C and soil temperature (13-cm depth) was –2.0°C at the time of core collection.

To simulate field conditions and minimize soil disturbance, cores were analyzed the following day and kept intact as they were placed inside a 1-L glass Mason jar equipped with a septum for gas sampling. A laboratory-grade, temperature-controlled (± 0.5°C) freezer (Scientemp, Adrian, MI) was used to maintain cores at field temperature in the laboratory. Samples remained vented to the atmosphere except when performing time-course measurements. Cores were allowed to equilibrate at –2.5°C for 4 h before each experiment. Included in the analysis were two empty jars, or blanks, which were treated as samples throughout the experiment. Before sampling, the freezer was opened briefly and jar covers removed to introduce fresh atmosphere into the headspace for 1 h. Jars were then capped while in the freezer, and a 15-mL headspace sample was drawn and replaced with an equal volume of N₂ to maintain constant pressure. The headspace was sampled every 0.25 h to complete a 0.75-hr time course and analyzed as above. Denitrification was also measured on the same soil cores at –2.5°C using the acetylene inhibition technique (Paul and Beauchamp, 1989), and denitrified N (N₂O + N₂) measured following the protocol described above. Following incubations, percentage water-filled pore space was determined for each core. Effects of management treatment on denitrification were tested with the mixed analysis of variance described previously.

Surface Flux Field Samples
Gas fluxes within each management treatment were repeatedly measured using the static chamber method (Whalen and Reeburgh, 1992). Polyvinyl chloride collars (20-cm diam. x 11-cm height) were deployed in the soil 10 d before the first flux measurement. Polyvinyl chloride covers fitted with butyl O-rings were placed onto the soil collars during each gas flux measurement. Covers included a capillary bleed to equalize pressure and an O-seal fitting and septa for syringe sampling. Samples of the headspace gas were withdrawn from each chamber at 0.25-h intervals during a 0.75-h time course. Air temperature near the soil surface and soil temperature at 13-cm depth were measured in conjunction with gas flux measurements for each date and field with a Type K temperature probe (Cole-Parmer Instrument, Vernon Hills, IL). Soil moisture content was determined at each point in time until 6 December for the 0- to 15-cm depth zone.

Fluxes were measured only four times between August and September 2004 before organic amendment, and time-integrated, daily average values by treatment are reported. Winter field fluxes were measured between 16 November and 8 April and time-integrated, daily average values by treatment are reported. Post-amendment winter fluxes were also analyzed statistically with a mixed, repeated measures analysis of variance to test for effects of amendment on N₂O flux with soil temperature as a covariate. Cumulative N₂O flux by chamber for each treatment were calculated (Phillips et al., 2000), and daily average fluxes by treatment reported. A test for significant differences between treatments for cumulative flux was performed as described previously with the mixed analysis of variance.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field data collected before the organic amendment indicate soil properties are similar between conventional and organic fields with respect to soil texture, pH, bulk density, soil organic C, soil inorganic C, soil N, and POM (Table 3). Although the organic field received 1.86 Mg ha^–1 of organic C inputs over the previous 5 yr (Table 1), evidence of greater organic carbon or greater POM in the top 15-cm of soil was not found. Average N₂O–N fluxes collected from soybean fields on 4 August, 24 August, 31 August, and 30 September ranged from 0.05 to 0.34 mg m^–2 d^–1, with an overall average (± SD) of 0.12 ± 0.12 for T2C and 0.16 ± 0.15 for T2O. Moreover, N₂O production and the relative contribution of N₂O from autotrophic nitrification and heterotrophic processes were also similar (Fig. 2). There was no significant difference in %WFPS among soils, with an average for all soils of 23 ± 3.3%. At this %WFPS, autotrophic nitrification accounted for approximately 40% of the total N₂O produced. Bateman and Baggs (2005) found approximately 50% of the N₂O–N produced in arable soils occurred during nitrification at 20% WFPS, while the remaining 50% occurred during denitrification. Results here do not differentiate denitrification from heterotrophic nitrification; however, heterotrophic nitrification in arable soils at low %WFPS has not been reported to date. Total denitrified N (N₂O + N₂) was, on average, <10% greater than the rate of N₂O–N produced during denitrification and/or heterotrophic nitrification. If, as in Bateman and Baggs (2005), N₂O produced after incubation with 10 Pa C₂H₂ occurred during denitrification, then microbial reduction of N₂O to N₂ for soils incubated at 23% WFPS was incomplete (Davidson, 1991) and aerobic denitrification likely (Patureau et al., 2000; Bateman and Baggs, 2005). In summary, pre-amendment N₂O emissions and soil core data (field fluxes under soybean and N₂O source partitioning) suggest physical and microbiological controls on N₂O emissions were similar before 20 November organic amendment application.

View larger version (13K): [in this window] [in a new window]   Fig. 2. Nitrous oxide flux partitioning for comparison between organically and conventionally managed soils collected in July and at 23% WFPS. Soils were incubated under ambient air, then under 10 Pa C2H2, then under 10 K Pa C2H2 at field moisture and temperature to determine sources of rates of N2O emissions and N produced during denitrification. Mean (n = 10) ± SEM are shown for each treatment and incubation.

Biological denitrification occurred for both conventionally and organically managed soils at temperatures below freezing in the laboratory. Average N₂O emission for soils amended with dehydrated manure at –2.5°C (Fig. 3) was 16.30 ng N₂O–N g dw^–1 d^–1. Average emission for unamended soil cores was 5.63 ng N₂O–N g dw^–1 d^–1. Nitrous oxide emission for soil with no known history of cultivation or fertilization was 0.24 ng N₂O–N g dw^–1 d^–1. When calculated as fluxes of N₂O–N m^–2 d^–1 (using jar volume and soil core surface area) average emissions for T2O was 1.51 mg m^–2 d^–1, 0.41 mg m^–2 d^–1 for T2C, and 0.22 mg m^–2 d^–1 for reference. Total denitrification (N₂O + N₂) for T2O soil cores averaged 19.90 ng (N₂O–N + N₂) g dw^–1 d^–1, while average denitrification for T2C soil cores was 1.68 ng (N₂O–N + N₂) g dw^–1 d^–1 (Fig. 3). Denitrification measured for intact cores was highly variable and statistically only marginally different between treatments (F = 4.75; p < 0.1), with a tendency for greater rates in T2O, compared with T2C (Fig. 3). There was no evidence of denitrification for the uncultivated reference soil. Water-filled pore space was similar for both T2O and T2C field soils, with an average %WFPS of 41.4 ± 5.0 and 39.1 ± 5.8, respectively. While denitrification occurred in both treatments, greater N₂O emissions following incubation with 10 K Pa C₂H₂ suggest complete reduction to N₂ was favored in the manure-amended soil, compared with unamended soil.

View larger version (12K): [in this window] [in a new window]   Fig. 3. Nitrous oxide flux and denitrification for intact core incubations at field moisture (40% WFPS) and temperature (–2.5°C) 17 d following T2O manure amendment. Soil cores were incubated under ambient air, then under 10 K Pa C2H2 to determine rates of N2O emissions and N produced during denitrification. Mean (n = 5) ± SEM are shown for each treatment and incubation.

View larger version (17K): [in this window] [in a new window]   Fig. 4. Nitrous oxide fluxes (mean ± SEM) measured at the soil-atmosphere interface between 4 Aug. 2004 and 8 Apr. 2005 with respect to number of days following manure amendment application for organic (T2O), conventional (T2C), and reference sites. Soybeans were in both fields between days –112 and –55. Fields were fallowed between days –8 and 135.

View larger version (22K): [in this window] [in a new window]   Fig. 5. Mean (± SD) for air and soil temperature collected manually at the field site for each field during flux sampling and soil temperature collected continuously at the weather station located north of the field site (see Fig. 1).

Time-integrated winter N₂O fluxes were significantly different between conventional and organic soils (F = 36.24; p < 0.05). Previous agricultural field studies suggest effects of manure on N₂O flux persist 42 to 56 d when applied during the growing season and emissions amount to 1% of the manure N applied (Lessard et al., 1996; Petersen, 1999). Here, integrated cumulative winter flux measured for the first 35 d since fertilization totaled 0.49 kg ha^–1, or 0.7% of the manure N applied. Integrated, postamendment fluxes measured between 22 November and 8 April averaged 1.63 kg ha^–1 for T2O and 0.64 kg ha^–1 for T2C. By taking the difference between treatments as the effect of manure on N₂O–N emissions, 1.3% of the manure N applied was emitted as N₂O–N, which is slightly higher than the 0.9% global mean for fertilizer-induced N₂O emissions (Bouwman et al., 2002). Data reported here represents only one winter in the RRV and one type of manure (pelletized, dehydrated). Further study under variable conditions and with alternative amendments is needed to constrain estimates of N₂O–N emissions during winter in RRV agricultural soils.

Previous studies confirm increased biological N₂O production and denitrification enzyme activity for soils exposed to freeze–thaw cycles (Christensen and Tiedje, 1990; Rover et al., 1998; Jacinthe et al., 2002), for diffusion of trace gases through snowpack (Sommerfeld et al., 1993), and for soil slurries incubated below zero in the laboratory (Dorland and Beauchamp, 1991). In this study, N₂O emissions measured in the field may have originated in soil microsites where temperatures were above freezing. For example, N₂O produced below the frost zone may have diffused upward and/or N₂O may have been produced near the surface on those days when there was sufficient radiation to raise surface temperatures above freezing. However, for laboratory incubations, microbial denitrification occurred below 0°C. Both amended and unamended soils emitted N₂O, with at least a portion of N₂O flux from denitrification. Trapped gas in soil pore spaces may explain N₂O emissions to some extent (Huttunen et al., 2003), but it does not account for differences between treatments. For organic soils amended with dehydrated manure, denitrification rates were likely stimulated by mineralization of readily available C and N provided by the substrate (Ginting et al., 2003). Microbial ammonification and denitrification of supercooled soils have been reported for slurries in the laboratory, with temperature thresholds strongly influenced by organic substrate supply (Dorland and Beauchamp, 1991). In this study, soil C was relatively high (Table 2), and denitrification occurred for both T2O and T2C soils. Mineralization of the C- and N-rich manure likely began shortly after incorporation into soils, despite subsequent low soil temperatures (Dorland and Beauchamp, 1991; Mikan et al., 2002). The resultant increase in substrate supply from mineralization to the microbial heterotroph population should have enhanced denitrification among microbial communities well-adapted to soil temperatures below freezing, compared with unamended soil.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Late autumn manure application influenced N₂O fluxes at sub-zero soil temperatures. Denitrification contributed to winter N₂O fluxes, even at –2.5°C. Although an additional 1 kg ha^–1 of N₂O–N was emitted from the surface of manure-amended soils, it remains to be shown if application of dehydrated manure in the spring would result in lower N₂O emissions. More importantly, the question of how well manure N mineralization rates track rapid plant growth is not entirely certain (Paul and Beauchamp, 1994), which would support winter manure application recommendations. Clearly identification of interactions between timing of organic amendment application, gaseous N emissions, and mineralization rates are critical for constructing strategies for optimum N conservation in organic production agriculture ecosystems.

	ACKNOWLEDGMENTS

 
Thank you to the farmers who participated in this study: Noreen and Lee Thomas, Jim Renners, and Kevin Nelson. Special thanks to Mark Wutske, Varion Corporation, for his unfailing customer service. Thanks to the University of North Dakota School of Aerospace personnel and students, including Dean Smith, Professor Tony Grainger, Blake Mozer, Jared Clayburn, and the Biology Department's Bob Sheppard. Special thanks to Mark Liebig for soil processing assistance. This work would not have been possible without support from Dr. Ofer Beeri and Scott Bylin. Comments by Ron Follett, Steve Whalen, and anonymous reviewers greatly improved this manuscript. This project was supported by U.S. Department of Agriculture, Agreement No. 58-5445-3-314.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

• Akiyama, H., I.P. McTaggert, B.C. Ball, and A. Scott. 2004. N₂O, NO, and NH₃ emissions from soil after the application of organic fertilizers, urea, and water. Water Air Soil Pollut. 156:113–129.
• Akiyama, H., and H. Tsuruta. 2003. Effect of organic matter application of N₂O, NO, and NO₂ fluxes from an Andisol field. Global Biogeochem. Cycles 17:1101–1116.[CrossRef]
• Bateman, E.J., and E.M. Baggs. 2005. Contributions of nitrification and denitrification to N₂O emissions from soils at different water-filled pore space. Biol. Fertil. Soils 41:379–388.[CrossRef]
• Bouwman, A.F., L.J.M. Boumans, and N.H. Batjes. 2002. Emissions of N₂O and NO from fertilized fields: Summary of available measurement data. Global Biogeochem. Cycles 16:1058–1071.[CrossRef]
• Breitenbeck, G.A., and J.M. Bremmer. 1986. Effects of rate and depth of fertilizer application on emission of nitrous oxide from soil fertilized with anhydrous ammonia. Biol. Fertil. Soils 2:201–204.
• Christensen, S., and T. Tiedje. 1990. Brief and vigorous N₂O production by soil at spring thaw. Soil Sci. Soc. Am. J. 41:1–4.
• Davidson, E.A. 1991. Fluxes of nitrous oxide and nitric oxide from terrestrial ecosystems. p. 219–235. In J.E. Roger and W.G. Whitman (ed.) Microbial Production and Consumption of Greenhouse Gases: Methane, Nitrogen Oxides, and Halomethanes. American Society for Microbiology, Washington, DC.
• Dorland, S., and E.G. Beauchamp. 1991. Denitrification and ammonification at low soil temperatures. Can. J. Soil Sci. 71:293–303.
• Flessa, H., R. Ruser, P. Dorsch, T. Kamp, M.A. Jimenez, J.C. Munch, and F. Beese. 2002. Integrated evaluation of greenhouse gas emissions (CO₂, CH₄, N₂O) from two farming systems in southern Germany. Agric. Ecosyst. Environ. 91:175–189.[CrossRef]
• Ginting, D., A. Kessavalou, B. Eghball, and J.W. Doran. 2003. Greenhouse gas emissions and soil indicators four years after manure and compost applications. J. Environ. Qual. 32:23–32.[Abstract/Free Full Text]
• Gregorich, E.G., and B.H. Ellert. 1993. Light fraction and macroorganic matter in mineral soils. p. 397–407. In M.R. Carter (ed.) Soil Sampling Methods and Analysis. Canadian Society of Soil Science, Lewis Publ., Boca Raton.
• Huttunen, J.T., J. Alm, E. Saarijarvi, K. Matti Lappalainen, J. Silvola, and P.J. Martikainen. 2003. Contribution of winter to the annual CH₄ emission from a eutrophied boreal lake. Chemosphere 50:247–250.[Medline]
• Jacinthe, P.A., W.A. Dick, and L.B. Owens. 2002. Overwinter soil denitrification activity and mineral nitrogen pools as affected by management practices. Biol. Fertil. Soils 36:1–9.
• Klemedtsson, L., B.H. Svensson, and T. Rosswall. 1988. A method of selective inhibition to distinguish between nitrification and denitrification as sources of nitrous oxide in soil. Biol. Fertil. Soils 6:112–119.
• Kroeze, C., A. Mosier, and A.F. Bouwman. 1999. Closing the global N₂O budget: A retrospective analysis 1500–1994. Global Biogeochem. Cycles 13:1–8.[Medline]
• Lessard, R., P. Rochette, E.G. Gregorich, E. Pattey, and R.L. Desjardins. 1996. Nitrous oxide fluxes from manure-amended soul under maize. J. Environ. Qual. 25:1371–1377.[Abstract/Free Full Text]
• Littell, R.C., G.A. Milliken, W.W. Stroup, and R.D. Wolfinger. 1996. SAS system for mixed models. SAS Inst., Cary, NC.
• Loeppert, R.H., and D.L. Suarez. 1996. Carbonate and gypsum. p. 437–474. In Sparks D.L. (ed.) Methods of Soil Analysis. Part 3. Chemical Methods. SSSA and ASA, Madison, WI.
• Mikan, C.J., J.P. Schimel, and A.P. Doyle. 2002. Temperature controls of microbial respiration in arctic tundra soils above and below freezing. Soil Biol. Biochem. 34:1785–1795.[CrossRef]
• Mosier, A.R., J.A. Delgado, V.L. Cochran, D.W. Valentine, and W.J. Parton. 1997. Impact of agriculture on soil consumption of atmospheric CH₄ and a comparison of CH₄ and N₂O flux in sub-arctic, temperate, and tropical grasslands. Nutr. Cycling Agroecosyst. 49:71–83.[CrossRef]
• Mosier, A.R., J.M. Duxbury, J.R. Freney, O. Heinemeyer, and K. Minami. 1998. Assessing and mitigating N₂O emissions from agricultural soils. Clim. Change 40:7–38.[CrossRef]
• Mummey, D.L., J.L. Smith, and G. Bluhm. 1998. Assessment of alternative soil management practices on N₂O emissions from US agriculture. Agric. Ecosyst. Environ. 70:79–87.
• North Dakota Agricultural Weather Network. 2000. [Online]. NDAWN hourly data. Available at http://ndawn.ndsu.nodak.edu/get-table.html?ttype=hourly&begin_date=2006-10-04&end_date=2006-10-04&station=2 (accessed 20 Aug. 2006; verified 4 Oct. 2006). NDSU-NDAWN, Fargo, ND.
• Parton, W.J., A.R. Mosier, D.W. Ojima, D.S. Schimel, K. Weier, and A.E. Kulmalla. 1996. Generalized model for N₂ and N₂O production from nitrification and denitrification. Global Biogeochem. Cycles 10:401–412.[CrossRef][ISI]
• Patureau, D., E. Zumstein, J.P. Delgenes, and R. Moletta. 2000. Aerobic denitrifiers isolated from diverse natural and managed ecosystems. Microb. Ecol. 39:145–152.[CrossRef][ISI][Medline]
• Paul, J.W., and E.G. Beauchamp. 1989. Effect of carbon constituents in manure on denitrification in soil. Can. J. Soil Sci. 69:49–61.
• Paul, J.W., and E.G. Beauchamp. 1994. Short-term nitrogen dynamics in soil amended with fresh and composted cattle manures. Can. J. Soil Sci. 74:147–155.
• Paul, J.W., E.G. Beauchamp, and X. Zhang. 1993. Nitrous oxide and nitric oxide emissions during nitrification and denitrification from manure-amended soil in the laboratory. Can. J. Soil Sci. 73:539–553.
• Petersen, S.O. 1999. Nitrous oxide emissions from manure and inorganic fertilizers applied to spring barley. J. Environ. Qual. 28:1610–1618.[Abstract/Free Full Text]
• Phillips, R.L., S.C. Whalen, and W.H. Schlesinger. 2000. Nitrous oxide flux from loblolly pine soil under free-air carbon dioxide enrichment. Global Biogeochem. Cycles 15:671–677.
• Robertson, G.P., E.A. Paul, and R.R. Harwood. 2000. Greenhouse gases in intensive agriculture: Contributions of individual gases to the radiative forcing of the atmosphere. Science 289:1922–1925.[Abstract/Free Full Text]
• Rover, M., O. Heinemeyer, and E.A. Kaiser. 1998. Microbial induced nitrous oxide emissions from an arable soil during winter. Soil Biol. Biochem. 30:1859–1865.[CrossRef]
• Scott, M.J., R.D. Sands, N.J. Rosenberg, and R.C. Izaurralde. 2002. Future N₂O from US agriculture: Projecting effects of changing land use, agricultural technology, and climate on N₂O emissions. Glob. Environ. Change 12:105–115.
• Soil Survey Staff. 2006. Official Soil Series Descriptions [Online]. USDA, Washington, DC. Available at http://soils.usda.gov/technical/classification/osd/index.html (verified 4 Oct. 2006).
• Sommerfeld, R.A., A.R. Mosier, and R.C. Musselman. 1993. CO₂, CH₄, and N₂O flux through a Wyoming snowpack and implications for global budgets. Nature 361:140–142.[CrossRef]
• Venterea, R.T., M. Burger, and K.A. Spokas. 2005. Nitrogen oxide and methane emissions under varying tillage and fertilizer management. J. Environ. Qual. 34:1467–1477.[Abstract/Free Full Text]
• Wagner-Riddle, J.C., G.W. Thurtell, G.K. Kidd, E.G. Beauchamp, and R. Sweetman. 1997. Estimates of nitrous oxide emissions from agricultural fields over 28 months. Can. J. Soil Sci. 77:135–144.
• Whalen, S.C., and W.S. Reeburgh. 1992. Interannual variations in tundra methane emissions: A four-year time series at fixed sites. Global Biogeochem. Cycles 6:139–159.

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 Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Phillips, R. L. Search for Related Content PubMed PubMed Citation Articles by Phillips, R. L. Agricola Articles by Phillips, R. L. Related Collections Biogeochemical Processes Nutrient Management Soil Biology