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Soil Biology & Biochemistry 39 (2007) 2362–2370

Dinitrogen and N2O emissions in arable soils: Effect of tillage,N source and soil moisture$

Xuejun J. Liua,�, Arvin R. Mosierb, Ardell D. Halvorsonc, Curtis A. Reulec, Fusuo S. Zhanga

aCollege of Resources and Environmental Sciences, China Agricultural University, Beijing 100094, ChinabUSDA-ARS Retired, 2150 Centre Ave, Bldg. D, Suite 100, Fort Collins, CO 80526 8119, USA

cUSDA–ARS, 2150 Centre Ave, Bldg. D, Suite 100, Fort Collins, CO 80526 8119, USA

Received 25 October 2006; received in revised form 8 April 2007; accepted 12 April 2007

Available online 15 May 2007

Abstract

A laboratory investigation was performed to compare the fluxes of dinitrogen (N2), N2O and carbon dioxide (CO2) from no-till (NT)

and conventional till (CT) soils under the same water, mineral nitrogen and temperature status. Intact soil cores (0–10 cm) were

incubated for 2 weeks at 25 1C at either 75% or 60% water-filled pore space (WFPS) with 15N-labeled fertilizers (100mgNkg�1 soil). Gas

and soil samples were collected at 1–4 day intervals during the incubation period. The N2O and CO2 fluxes were measured by a gas

chromatography (GC) system while total N2 and N2O losses and their 15N mole fractions in the soil mineral N pool were determined by a

mass spectrometer. The daily accumulative fluxes of N2 and N2O were significantly affected by tillage, N source and soil moisture. We

observed higher (Po0.05) fluxes of N2+N2O, N2O and CO2 from the NT soils than from the CT soils. Compared with the addition of

nitrate (NO3�), the addition of ammonium (NH4

+) enhanced the emissions of these N and C gases in the CT and NT soils, but the effect

of NH4+ on the N2 and/or N2O fluxes was evident only at 60% WFPS, indicating that nitrification and subsequent denitrification

contributed largely to the gaseous N losses and N2O emission under the lower moisture condition. Total and fertilizer-induced emissions

of N2 and/or N2O were higher (Po0.05) at 75% WFPS than with 60%WFPS, while CO2 fluxes were not influenced by the two moisture

levels. These laboratory results indicate that there is greater potential for N2O loss from NT soils than CT soils. Avoiding wet soil

conditions (460% WFPS) and applying a NO3� form of N fertilizer would reduce potential N2O emissions from arable soils.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: No-till; WFPS; 15N-labeled fertilizer; Denitrification; Nitrification; Nitrous oxide emission

1. Introduction

Denitrification is a major microbial-based process thatcompletes the nitrogen (N) cycle by returning N2 to theatmosphere. Loss of N from agricultural soils by deni-trification is an important pathway for fertilizer N loss(Aulakh et al., 1992). In a review of 15N field N-balancestudies, Hauck (1981) estimated that, on average, 30% of

fertilizer 15N was unaccounted for, as a result ofdentrification. A product of the denitrification process,nitrous oxide (N2O), is an important greenhouse gas(GHG), which is emitted from N-fertilized agriculturalsoils and its contribution to the anthropogenic greenhouseeffect has been estimated at 5% (IPCC, 2001). N2O isan obligatory intermediary product of denitrification(Crutzen, 1981). The N2O molecule is also produced as aby-product of nitrification (Firestone and Davidson, 1989).Under specific soil conditions, coupled nitrification–deni-trification may occur (Bateman and Baggs, 2005). Thishappens when nitrite or nitrate produced by nitrifiers isused by denitrifiers (Wrage et al., 2001).According to a model of Davidson (1991), N2O is

primarily derived from nitrification at low and moderatesoil moistures while denitrification becomes more important

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0038-0717/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.soilbio.2007.04.008

$The US Department of Agriculture offers its programs to all eligible

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E-mail address: xuejun.13500@gmail.com (X.J. Liu).

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at soil moisture contents greater than 60% water-filled porespace (WFPS) due to a decreased O2 supply. Bateman andBaggs (2005) also found that all of the N2O emitted at 70%WFPS was produced during denitrification, but nitrificationwas the main process producing N2O at 35–60% WFPS.However, the relative contributions of denitrification andnitrification to the emissions of N2O are still not welldocumented (Arah, 1997; Bateman and Baggs, 2005;Stevens et al., 1997).

No-till (NT) conservation tillage is commonly practicedin order to reduce soil erosion and energy consumption inNorth America (Holland, 2004; Halvorson et al., 2006).Reducing the intensity of soil cultivation under NT lowersthe emissions of CO2, while C sequestration is raised viaincreasing soil organic matter. Thus global warmingpotential (GWP) and GHG flux intensity can be reducedif the cropping system management practices are altered(Mosier et al., 2006). In general, NT is found to beeffective in mitigating N2O or GHG emissions in dry andhumid climate soils due to reduced surface disturbance(Kaharabata et al., 2003; Lemke et al., 2004; Mosier et al.,2006; Lee et al., 2006). However, some studies have shownNT to produce larger (Baggs et al., 2003; Liu et al., 2006;Six et al., 2004) or similar (Grandey et al., 2006; Liu et al.,2005) N2O emissions compared with CT. Using theDAYCENT ecosystem model, Del Grosso et al. (2002)observed that during the first few years of NT, the soil N2Oemissions and thus the net GWP decreased. Over time, asthe rate of increase in soil organic carbon (SOC) declinedand N2O emissions increased because of increased Navailability, the net GWP increased relative to CT soils.This simulation suggests that the impact of NT on N2Oemission and net GWP decreases over time in a dryagroecosystem. But there are great uncertainties regardingthe impact that NT has on nitrification and denitrificationrates and N2O emissions in irrigated and fertilized soils.

In the present study, we selected the 15N tracer methodto quantify the effect of tillage, N source and moisturecontent on gaseous N losses and N2O emissions from soilscollected in a northern Colorado maize field. The mainobjectives of the present study were: (1) to determine howNT affects the fluxes of N2O and N2 compared with CT;(2) to determine the relative contributions of nitrificationand denitrification to N2O emissions in both NT- andCT-treated soils; and (3) to gain a critical insight as to howN2O emissions might be reduced by integrating tillage, Nsource and soil water management.

2. Materials and methods

2.1. Soils

Soil samples were collected from two typical tillagesystems (CT and NT) in a tillage by N rate experiment(Halvorson et al., 2006) that was initiated in 1999 at theAgricultural Research Development and Education Center(ARDEC) in northeastern Colorado near the city of Fort

Collins (401390N, 1041590W; 1530m above mean sea level).The soil was a clay loam soil (fine-loamy, mixed, super-active, mesic Aridic Haplustalfs, according to US SoilTaxonomy) and its related properties are listed in Table 1.Corn had been continuously planted under the CT or NTtreatments with no fertilizer N since 1999.

2.2. Consecutive incubation

Intact soil cores (diameter 5.4 cm, height 10 cm) werecollected from the 0 to 10 cm soil depth from the NT andCT plots in the Summer of 2004 (maize growing season)and immediately stored at 4 1C until start of the incubationstudy (the storage period was 5–7 days). The treatmentsapplied to the soil cores included N fertilizer form and soilmoisture content. N fertilizer was applied either as 15NH4

+

(99 at% excess 15N ammonium sulfate) or 15NO3� (99 at%

excess 15N potassium nitrate). The incubation study wasconducted at two soil moisture contents: 75% and 60%WFPS, respectively. There were four treatments repre-sented as CT NH4, CT NO3, NT NH4, and NT NO3 at thetwo soil moisture levels. Glass canning jars (0.43L), fittedwith air-tight metal lids, were used as incubation vessels.There was a 1-cm diameter butyl rubber septum (thickness1.5 cm) inserted into a hole that was punched in the centerof the lid for gas sampling. Each incubation vesselcontained one soil core. The headspace of each incubationvessel was 0.20 L. The 15NH4

+ or 15NO3� solution was

homogeneously injected in to both the CT and NT soilprofiles with three triangle injection points per soil core viaa 10 cm length needle (as shown in Fig. 1). The resulting Nconcentration in either the CT or NT treatment was100mgNkg�1 soil. The rate of N applied to each soil corewas equivalent to 150 kg Nha�1, a moderate rate whencompared with local farming practices. The incubationvessels were sealed immediately after N addition. Eachtreatment was replicated 18 times to provide 6 dynamicsampling times (three replications each sampling) duringthe entire incubation period. Prior to the incubation, initialsoil moisture contents were measured and expressed asWFPS according to the following Eq. (1):

WFPS;% ¼ Soil water contentð%Þ � rv=ð1� rv=2:65Þ.(1)

In the equation, rv represents soil bulk density (g cm�3),while soil water content is calculated based on oven-dry

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Table 1

Selected soil (0–10 cm) chemical and physical properties of the study site

Tillage pH Bulk density SOCa TSNb Sand Clay

(0.01M CaCl2) (g cm�3) (g kg�1) (g kg�1) (g kg�1) (g kg�1)

CT 7.77 1.38 11.9 1.15 402 334

NT 7.66 1.42 12.8 1.48 402 334

aSOC represents soil organic carbon.bTSN represents total soil nitrogen.

X.J. Liu et al. / Soil Biology & Biochemistry 39 (2007) 2362–2370 2363

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py(weight) method and 2.65 is constant, representing theparticle density of the soil (2.65 g cm�3 on average).

Soil moisture contents of each soil core were adjusted toeither 75% (first incubation experiment) or 60% (secondincubation experiment) WFPS with 15N solution (adding1.5–5.4ml per core). Soil temperature was kept at2570.3 1C during the entire incubation period (2 weeks).On day 1, 3, 5, 7, 10 and 14 after initiating the incubation,gas samples from three jars of each treatment werecollected. Before collecting gas samples, we injected 60mlof fresh air into the headspace of each jar and mixed thiswith the original air in the jar. Then gas samples from theheadspace were collected by syringe (35ml). Twenty-fivemilliliter of each sample in the syringe was injected into a12-ml evacuated tube (to insure over pressure of sample inthe tubes). Tubes were sealed with butyl-rubber septa andanalyzed within 24 h. After each gas sampling soil core wasmixed thoroughly and prepared for further analysis. Theremaining jars were opened and flushed with compressedair for 30 s, and then covered again until the next samplingtime during the entire incubation (Cai and Mosier, 2000).

2.3. Measurements of gas and soil samples

An automatic GC system was used for measuring CO2,CH4 and N2O concentrations in the gas samples (Liu et al.,2005). Briefly, the GC used was a fully automatedinstrument (Varian 3800) equipped with thermoconductiv-ity, flame ionization and electron capture detectors to

quantify CO2, CH4 and N2O, respectively. Analyticalprecision was o71% for each gas. The fluxes of CO2

and N2O were calculated from the production of the twogases in the jar headspace within a certain period (1–4days), based on comparison of samples to certifiedstandard gases that were prepared for analysis in thesame way as the gas samples, and soil dry weight in a jar(Eq. (2)). The dilution effects of added 60ml air wereconsidered when calculating the production of eitherCO2 or N2O in the jar headspace in the Eq. (2).

Fluxes of CO2 or N2Oðmg C or Nkg�1 day�1Þ

¼ CO2 or N2O productionðmg C or N jar�1Þ=

soil DWðkg jar�1Þ=incubation days: ð2Þ

To determine the 15N enrichment of the N2 and N2O gassamples, the inlet system of the MS (Mass Spectrometer,VG-903) was arranged to permit introduction of a gassample directly from a sample syringe into an evacuated 5-ml gas sample loop that was isolated from the remainder ofthe inlet system by a high vacuum valve. This sample loopwas immersed in a liquid N2 trap that removed N2O, CO2

and water vapor from the gas sample prior to the samplepassing through a 250 1C copper-filled tube. This heatedcopper tube removed O2 from the gas sample and reducedN2O to N2 when the liquid N2 trap was subsequentlywarmed to release the trapped N2O gas. From the O2 trap,the gas sample was routed through another liquid N2 trapand then into the sample inlet of the MS. The fluxes of N2Oand N2 were then calculated as described by Hutchinsonand Mosier (1981) using the ratios of 29/28 and 30/28.Firstly we checked the separate 15XN (15N mole fractionfrom soil mineral N or nitrate N pool) of both N2 and N2Oby the MS for a number of gas samples. But the 15XN

values of either N2 or N2O were quite similar to the 15XN

values of total N2+N2O in the same samples (data notshown), suggesting most N2 and N2O came from the samelabeled 15N pool. Thus, we did not distinguish 15Nenrichment of N2 or N2O and use the same 15N enrichmentto calculate the fertilizer-induced N2 and/or N2O emissionsin this study. The detailed calculation was based on theequations proposed by Hutchinson and Mosier (1981) andMulvaney and Boast (1986). Measurement precision waso72%.The fresh soil samples were extracted with 1M KCl

solution and analyzed for NH4+-N and NO3

�-N using acontinuous flow analyzer (Lachat QuickChem FIA+ 8000Series, Loveland, CO, USA). The remaining soil wasair dried and ground to pass 0.15mm sieve. Total soil Nand its 15N enrichment were then measured directly by aC–N analyzer combined MS system using the air-dry soilsamples.

2.4. Statistics

Statistical analyses of the data including gas flux rates andtotal emissions, soil NH4

+-N and NO3�-N concentrations

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15N solution container

Long syringe needle (10cm)

Injection point

Injection profile

Soil core

Fig. 1. The diagram of intact soil core and the needle for 15N solution

injection.

X.J. Liu et al. / Soil Biology & Biochemistry 39 (2007) 2362–23702364

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and the residual 15N in different treatments were conductedusing a two- or three-factor analysis of variance (ANOVA)with tillage, N addition and soil WFPS in this study. Theleast significant difference at the 5% level (LSD0.05) orotherwise stated value was listed if significant differencesbetween WFPS, tillage and N source appeared.

3. Results

3.1. Total emissions of N2+N2O, N2O and CO2

Dinitrogen plus nitrous oxide (N2+N2O) emissionsfrom either fertilized CT or NT soils at 75% WFPS werehigher (Po0.05) than those at 60% WFPS during the14-day incubation period (Table 2). At 75% WFPS,total emissions of N2+N2O were higher (Po0.05) inthe NT soils (19.6mgNkg�1) than in the CT soils(14.0mgNkg�1); whereas the form of N fertilizer (NH4

or NO3) did not significantly affect the total N2+N2Ofluxes (Table 2). At 60% WFPS, the cumulative N2+N2Ofluxes were consistently higher (Po0.01) in the NT soilsand where NH4

+ addition was made (Po0.01, Table 2).Daily N2+N2O fluxes as affected by tillage and N source

at 75% and 60% WFPS are shown in Fig. 2. At 75%WFPS, the daily N2+N2O fluxes in the CT NO3 treatmentwere consistently higher than in the CT NH4 treatmentduring the whole incubation period while the dailyN2+N2O fluxes in the NT NO3 treatment were similar orlower than those in the NT NH4 treatment except forDay 3 (Fig. 2). The average daily N2+N2O fluxes inNT (1.4mgNkg�1) were higher (Po0.05) than in CT(1.0mgNkg�1) soils across N source. At 60% WFPS, theNH4

+ addition led to higher daily N2+N2O evolution ratesfrom both the CT and NT soils in the first 5 days of theincubation relative to the NO3

� addition (Fig. 2). After Day7, the difference in the daily N2+N2O fluxes between theCT NH4 and CT NO3 treatments or the NT NH4 and NTNO3 treatments tended to be smaller.

Similar to the N2+N2O emissions, the cumulative N2Ofluxes were higher (Po0.05) from the NT soil than fromthe CT soil but the addition of NH4

+ and NO3� did not

produce consistent effects on the N2O emissions at two soil

moisture levels (Table 2). At 75% WFPS, the cumulativeN2O fluxes were highest from the NT NO3 treatment thenfollowed by the NT NH4, CT NH4 and CT NO3

treatments, respectively. At 60% WFPS, however, thecumulative N2O fluxes were significantly higher from soilstreated with NH4

+ compared with the soils treated withNO3�. Cumulative N2O fluxes were significantly higher at

75% WFPS (average 4.9mgNkg�1) than those at 60%WFPS (average 2.6mgNkg�1) across tillage and N sourceduring the entire incubation (Table 2).Daily fluxes of N2O (0.04–0.75mgNkg�1 day�1) varied

greatly in both the CT and NT soils treated with NH4+ or

NO3� at the two soil moisture levels (Fig. 2). Under higher

WFPS (75%), the daily N2O fluxes in soils treated withNO3� were higher (Po0.05) than those in soils treated with

NH4+ on Day 1 and 3 of incubation. After Day 5, the

differences of daily N2O fluxes between the soils treatedwith NO3

� and NH4+ tended to be smaller or the trend was

reversed (e.g., CT NO3 vs. CT NH4). Under the lowerWFPS (60%), in contrast, the daily N2O fluxes in the soilstreated with NH4

+ were greater than those in the soilstreated with NO3

� in the first 5 days of the incubation, thenthey were comparable and dropped to near backgroundlevels (o0.08mgNkg�1 day�1) for both N sources with theexception of the NT NH4 treatment. In addition, averagedaily N2O fluxes were consistently higher (Po0.05) in theNT soils than in the CT soils and also higher (Po0.05) at75% WFPS than at 60% WFPS irrespective of N source.The cumulative CO2 fluxes in the NT soils were much

higher (Po0.01) than in the CT soils while the addition ofNH4

+ led to greater (Po0.05) cumulative CO2 fluxescompared with the addition of NO3

� at both the soilmoisture contents (Table 2). But no significant differencesin total emissions of CO2 were found between 75% and60% WFPS. The highest cumulative CO2 emissions (645and 450mgCkg�1) were from the NT NH4 treatment atboth the soil moisture contents (Table 2). The daily CO2

fluxes from the NT soils were generally greater than fromthe CT soils while the addition of NH4

+ led to higher(Po0.05) peak daily fluxes of CO2 evolution comparedwith the addition of NO3

� at 75% and 60% WFPS (Fig. 2).The daily CO2 fluxes showed an increasing trend duringthe entire incubation period at 75% WFPS while theypeaked on Day 5 or 7 then dropped drastically after Day 7at 60% WFPS (Fig. 2), indicating different soil respirationpatterns.

3.2. Emission of fertilizer-derived N2+N2O and N2O

The mole fraction of 15N in the soil nitrate pool (15XN),which determined the ratio of fertilizer derived N2+N2Oand N2O to total emissions of N2+N2O and N2O, waslower (Po0.05) in the NH4

+-based treatments than theNO3�-based treatments in the first 5 days across soil

moisture level (Fig. 3). Then the difference in the 15XN

between the soils treated with NH4+ and NO3

� was smallerafter Day 7 particularly at 75% WFPS (Fig. 3). Lower

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Table 2

Total emissions of N2+N2O, N2O and CO2 (mgN or Ckg–1)71 SEM

(n ¼ 3) in CT and NT soils treated with 15NH4+ or 15NO3

� at two soil

moisture contents (75% and 60% WFPS) after 14-day incubation

CT NH4 CT NO3 NT NH4 NT NO3

75% WFPS

N2+N2O 10.570.76 17.570.65 22.172.93 17.272.80

N2O 4.1870.21 3.0570.41 5.2270.45 7.1070.11

CO2 261717.5 14174.91 645729.3 372770.6

60% WFPS

N2+N2O 4.7071.00 1.8870.29 16.372.07 5.8972.36

N2O 2.8570.41 1.1970.26 4.4770.77 2.0070.68

CO2 243723.9 155730.8 450751.4 267717.3

X.J. Liu et al. / Soil Biology & Biochemistry 39 (2007) 2362–2370 2365

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15XN values were observed at 60% WFPS (average 0.75)compared to 75% WFPS (average 0.85) because of agreater soil nitrate N pool at 60% WFPS. The NT-treatedsoils had higher (Po0.05) 15XN values than the CT-treatedsoils during the incubation (Fig. 3).

Total emissions of fertilizer-derived gaseous N losses(expressed as 15N2+

15N2O) and nitrous oxide (expressedas 15N2O) over the 14-day incubation period at the twosoil moisture levels (Table 3) followed similar patterns tothe total N2+N2O and N2O emissions (Table 2). Total

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75% WFPS

0.0

0.2

0.4

0.6

0.8

1.0

1 3 5 7 10 14

N2O

flu

x(m

g N

kg-1

day-1

)

60% WFPS

0.0

0.2

0.4

0.6

0.8

1.0

1 3 5 7 10 14

N2O

flu

x(m

g N

kg-1

day-1

)

75% WFPS

0.0

0.8

1.6

2.4

3.2

4.0

1 5 7 10 14

N2+

N2O

flu

x(m

g N

kg-1

day-1

)60% WFPS

0.0

0.8

1.6

2.4

3.2

4.0

1 3 5 7 10 14

N2+

N2O

flu

x(m

g N

kg-1

day-1

)

75% WFPS

0

15

30

45

60

75

1 3 5 7 10 14

CO

2 fl

ux(m

g C

kg-1

day-1

)

60% WFPS

0

15

30

45

60

75

1 3 5 7 10 14

CO

2 fl

ux(m

gC k

g-1 da

y-1)

CT NH4CT NO3NT NH4NT NO3

3

Days of incubation Days of incubation

Days of incubation Days of incubation

Days of incubation Days of incubation

Fig. 2. Daily fluxes of N2+N2O, N2O and CO2 from CT and NT soils treated with 15NH4+ and 15NO3

� at 75% and 60% WFPS. Bars denote standard

error of means (SEM) (n ¼ 3).

75% WFPS

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1 5 3 7 10 14

Mol

e fr

acti

on o

f15

N in

soi

l nit

rate

poo

l

60% WFPS

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1 3 5 7 10 14

Mol

e fr

acti

on o

f15

N in

soi

l nit

rate

poo

l CT NH4 CT NO3 NT NH4 NT NO3

Days of incubation Days of incubation

Fig. 3. The mole fraction of 15N in soil nitrate pool in CT and NT soils treated with 15NH4+ and 15NO3

� at 75% and 60% WFPS. Bars denote 7SEM

(n ¼ 3).

X.J. Liu et al. / Soil Biology & Biochemistry 39 (2007) 2362–23702366

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emissions of 15N2+15N2O and 15N2O in NT soils (17.8%

and 5.6% of applied N, respectively) were significantlyhigher (Po0.05) than in CT soils (12.4% and 3.1% ofapplied N, respectively) at 75% WFPS. The 15N2+

15N2Oand 15N2O emissions were lower in both CT (2.0% and1.2% of applied N, respectively) and NT (9.5% and 2.8%of applied N, respectively) soils at 60% WFPS comparedwith 75% WFPS, although the tillage effect on the totalemissions was the same as at 75% WFPS (Table 3). Theaddition of NH4

+ significantly increased total 15N2+15N2O

and 15N2O fluxes from both the CT and NT soils at 60%WFPS but this phenomenon was not evident at 75%WFPS (Table 3). Soil derived gaseous N losses as N2 and/or N2O emissions were also increased significantly(Po0.05) by the addition of NH4

+ at 60% WFPS butnot at 75% WFPS (calculated from the difference betweenTables 2 and 3).

3.3. Mineral N and remained 15N in soils

Changes in NH4+-N and NO3

�-N concentrations duringthe course of the incubation in the CT and NT soils showedthe N transformation (esp. mineralization and nitrification)after fertilization. At 75% WFPS, soil NH4

+-N concentra-tions decreased steadily while NO3

�-N increased corre-spondingly in the CT NH4 and NT NH4 treatments(Fig. 4). In the CT NO3 and NT NO3 treatments, NH4

+-Nconcentrations were low (o 4mgNkg�1) during the entireincubation period. In contrast, NO3

�-N was as high as100mgNkg�1 at the beginning of the incubation andshowed a decreasing trend in the CT NO3 and NT NO3

treatments (Fig. 4). At 60% WFPS, the dynamics ofmineral N were similar to those at 75% WFPS. However,soil NH4

+-N in the CT NH4 and NT NH4 treatmentsdeclined faster while NO3

�-N accumulated quicker at60% WFPS than at 75% WFPS (Fig. 4). The changes inNH4

+-N and NO3�-N in the NT NH4 treatment were much

smaller compared to the CT NH4 treatment at both soilmoisture contents.Total labeled 15N in soils showed a declining trend

during two incubation periods with a few exceptions (datanot shown). The averaged labeled 15N contents in NT soils(109mgNkg�1 at 75% WFPS and 113mgNkg�1 at 60%WFPS) were generally lower than those in CT soils(126mgNkg�1 at 75% WFPS and 121mgNkg�1 at60% WFPS), while the addition of NH4

+ led to lessresidual fertilizer N or more fertilizer N loss (averaged22mgNkg�1) compared with the addition of NO3

� ateither soil moisture level.

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Table 3

Fertilizer induced emissions of 15N2+15N2O and 15N2O (mgNkg–1)71

SEM (n ¼ 3) in CT and NT soils treated with 15NH4+ or 15NO3

� at two soil

moisture contents (75% and 60% WFPS) after 14-day incubation

CT NH4 CT NO3 NT NH4 NT NO3

75% WFPS15N2+

15N2O 8.8671.22 15.970.74 19.470.74 16.172.7415N2O 3.4770.24 2.8170.37 4.5770.29 6.6270.13

60% WFPS15N2+

15N2O 2.5070.62 1.4070.16 13.971.68 5.1672.1515N2O 1.5270.26 0.8870.16 3.7970.59 1.7170.62

75% WFPS

0

20

40

60

80

100

120

0 2 4 6 8 10 12 14

Days of incubation

NH

4+ -N (

mg

N k

g-1)

60% WFPS

0

20

40

60

80

100

120

0 2 4 6 8 10 12 14

NH

4+ -N (

mg

N k

g-1) CT NH4

CT NO3NT NH4NT NO3

75% WFPS

0

30

60

90

120

150

0 2 4 6 8 10 12 14

NO

3- -N (m

g N

kg-1

)

60% WFPS

0

30

60

90

120

150

0 2 4 6 8 10 12 14

NO

3- -N (

mg

N k

g-1)

Day of incubation

Days of incubation Days of incubation

Fig. 4. Dynamics of NH4+-N and NO3

�-N in CT and NT soils treated with 15NH4+ and 15NO3

� at 75% and 60% WFPS. Bars denote 7SEM (n ¼ 3).

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4. Discussion

4.1. Effect of tillage, N source and soil moisture on gaseous

N losses

The present study confirmed that N2 and N2O lossescould vary markedly from 1.4% (CT NO3 at 60% WFPS)to 19.4% of 15N added (NT NH4 at 75% WFPS) when soilconditions were suitable (according to Table 3). These gaslosses were generally higher than those reported in a similarsoil under field conditions (Mosier et al., 1986a). Thedifference is mainly because the controlled laboratoryconditions (e.g. high mineral N concentration, moderatesoil temperature and moisture level) as well as no N uptakeby crop and no leaching loss may favor either nitrification-and/or denitrification-induced gaseous N losses. Soilsunder NT had greater N2 and N2O losses than soils underCT at both 75% and 60% WFPS. This difference betweenthe NT and CT soils was much higher at 60% WFPSthan at 75% WFPS. These data suggest that a significantproportion of fertilizer N was probably lost via gaseous Npathways in NT and may be part of the reason yields werereduced in the NT system (Halvorson et al., 2006). Here weassume that more soluble SOC and relatively higherdenitrifying activity in the NT soil may be responsible forthe higher N losses compared with the CT soil since soilmineral N (approximately 100mgNkg�1) and temperature(25 1C) were not limiting factors for either nitrification ordenitrification processes in the present study. Rice andSmith (1982) reported that the ratio of NT to CT soildenitrifying activity on all sampling dates in their study was4 1 (up to 77). They suggested that higher soil moisturecontents observed in the NT soils, rather than tillage, wereprimarily responsible for higher denitrifying activity.Results from Staley et al. (1990) also suggested that Nlosses via denitrification and nitrification under NT shouldexceed those under CT.

The addition of N fertilizer provided N sources for eithernitrification and/or denitrification. In our study, the formof N fertilizer influenced nitrification and denitrificationrates, which varied markedly with soil moisture content. At75% WFPS, there was no significant difference in the ratesof gaseous N losses between the two N sources (NH4

+ andNO3�). At 60% WFPS, however, the addition of NH4

+

enhanced the fraction of 15N in the soil mineral N pool inboth the CT and NT soils (particularly in the NT soil). Itsuggests a substantial N loss from nitrification and/orcoupled nitrification–denitrification (Abbasi and Adams,2000; Wrage et al., 2001). This was indirectly supported bythe elevated CO2 fluxes (induced greater O2 consumption)when NH4

+ was added to soils in this study (Fig. 2 andTable 2), indicating enhanced microbial activities.

Soil moisture content also markedly affected gaseous Nloss rates in this study. Total and fertilizer-induced N loss(N2+N2O and 15N2+

15N2O) rates were increased byfactors of 1.34 and 1.63 at 75% WFPS compared to 60%WFPS on average (Tables 2 and 3). The results show that

gaseous N losses increased with WFPS irrespective oftillage and N source. This is in agreement with manystudies, which reported greater denitrification rate at highersoil water content (e.g., Duxbury and McConnaughey,1986; Linn and Doran, 1984; Mosier et al., 1986b; Rice andSmith, 1982; Ruser et al., 2006).

4.2. Effect of tillage, N source and soil moisture on N2O

evolution

Our study indicates that tillage practice influences N2Oemissions. Under controlled conditions, NT soils showedsignificantly higher soil respiration fluxes and N2O emis-sions compared with CT soils. This is different from theresults obtained under field condition at the same experi-mental site (e.g., Mosier et al., 2006; Del Grosso andHalvorson, 2006, pers. commun.). Where no significantdifferences in N2O emissions between CT and NT plotsacross 4 years were observed. The reason for these differingfindings could be due to the varying environmentalconditions. Under field conditions, NT soils normally havehigher soil water content and lower soil temperaturebecause of the surface cover provided by crop residuals.Such conditions generally inhibit nitrification but favordenitrification. Furthermore, higher soil moisture willof course decrease the mole ratio of N2O/(N2+N2O)(Firestone and Davidson, 1989; Ruser et al., 2006). Underlaboratory conditions, however, moisture content andtemperature in the NT soils were kept the same as thosein the CT soils. Thus the factors restricting N2O emissionsin the NT soils did not exist anymore. The higher CO2

evolution rates in the NT soils (Table 2) also supported thisexplanation. The results suggest that the advantage of NTpractice for C sequestration could be offset by higher N2Oemissions under specific soil conditions (e.g., high WFPSand high soil temperatures).Using high enrichment 15N-labeled NO3

� and NH4+,

Master et al. (2004) observed that the majority of N2O wasformed via nitrification and N2O (no detectable N2) wasthe main gas emission form under their experimentalconditions (WFPS ranging from 40% to 70%). In ourstudy, we also found that the addition of NH4

+ inducedmuch higher N2O emission than the addition of NO3

� at60% WFPS compared to 75% WFPS. The uniquedifference between soils treated with NH4

+ and NO3� was

the mode of N2O production. Undoubtedly, fertilizer-induced 15N2O was from both nitrification and denitrifica-tion in the NH4

+-treated soils but it was just fromdenitrification in the NO3

�-treated soils. Background N2Oemission from unfertilized soils at the experimental sitewas small (Liu et al., 2006). Thus, the difference in N2Oevolution between the NH4

+- and NO3�-treated soils (e.g.,

CT NH4 and CT NO3 or NT NH4 and NT NO3) could bemainly attributed to nitrification or the oxidation of NH4

+.From this viewpoint, nitrification may contribute greatly toN2O emission when soil moisture content is suitable (e.g.,60% WFPS). The dynamics of inorganic N during the

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incubation period support that view. At 60% WFPS, forexample, the increase in the daily N2O fluxes in the CTNH4 or NT NH4 treatment in the first 5 days (Days 1, 3and 5) corresponded well with the decline in soil NH4

+-Nconcentrations (Figs. 2 and 4). This suggests a substantialcontribution of nitrification to the N2O fluxes comparedwith those in the CT NO3 or NT NO3 treatment. At75% WFPS, in contrast, N2O was mainly derived fromdenitrification because nitrification was depressed by thehigher soil moisture (as seen in Fig. 4). However, we shouldkeep in mind that the oxygen depletion by nitrification inthe same soil microsite may indirectly favor N2O evolutionfrom denitrification, especially under the NT soils wheregreater soil organic C is available compared to the CT soil(to see Table 1). Azam et al. (2002) reported that theapplication of ammonium helped to develop denitrifyingpopulations, so they suggested that the elevated N2Oemissions induced by addition of ammonium were mainlydue to denitrification rather than nitrification. Clough et al.(2004) reported greater water soluble C combined withhigher N2O emission in soils at 80% WFPS than at 54%WFPS and the decreased flux ratio of N2O/(N2+N2O)over time with the increase in soil moisture content,indicating much higher N2 loss under saturated soilwater condition. In our study, we therefore suspect thatboth nitrifier denitrification and/or coupled nitrification–denitrification contribute to the elevated N2O emission inthe NH4

+-treated soils especially under the NT and lowersoil moisture conditions (e.g. 60% WFPS).

5. Conclusions

Using intact soil cores from CT and NT continuous cornplots and high enrichment 15N-labeled fertilizer, weinvestigated the effect of tillage, N source and moisturecontent on N2 and N2O emission in a northern Coloradomaize field. Higher soil water content (75% WFPS)induced greater fertilizer-derived N2O and N2 fluxescompared with lower soil water content (60% WFPS). Inthe NT soils, both total and fertilizer-derived N2O andN2+N2O fluxes were significantly enhanced across soil theWFPS and N source treatments, suggesting that NT mayincrease the potential of soils to emit N2 and N2O. Theaddition of NH4

+ led to significantly higher N2O andN2+N2O emissions derived from both fertilizer and soilcompared with the addition of NO3

� at 60% WFPS. Theresults reveal the importance of integrating tillage, Nsource, and soil water management practices whenconsidering the mitigation of the N2O and N2 emissionsin agricultural soils.

Acknowledgments

This study was financially supported by USDA-ARS, USDA-CSREES-NRI, USDA-CSREES-CASMGS,the National Natural Science Foundation of China, theProgram for Innovative Research Team in University, and

the Key Import Project of MOA, China. The authors aregrateful to A. Kear, W. Morgan, M. Smith, G. Smith, S.Crookall, P. Norris, C. Cannon, and B. Floyd for theirtechnical assistance. The authors also express their specialthanks to two anonymous reviewers for their critical butvaluable suggestions.

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