Intensive measurement of nitrous oxide emissions froma corn–soybean–wheat rotation under two contrastingmanagement systems over 5 years

C L A U D I A WA G N E R - R I D D L E , A D R I A N A F U R O N , N I C O L E L . M C L A U G H L I N , I VA N L E E ,

J O H N B A R B E A U , S U S A N T H A J AYA S U N D A R A , G A R Y PA R K I N , P E T E R V O N B E R T O L D I

and J O N WA R L A N D

Department of Land Resource Science, University of Guelph, Guelph, ON, Canada N1G 2W1

Abstract

No-tillage (NT), a practice that has been shown to increase carbon sequestration in soils,

has resulted in contradictory effects on nitrous oxide (N2O) emissions. Moreover, it is not

clear how mitigation practices for N2O emission reduction, such as applying nitrogen (N)

fertilizer according to soil N reserves and matching the time of application to crop

uptake, interact with NT practices. N2O fluxes from two management systems [conven-

tional (CP), and best management practices: NT 1reduced fertilizer (BMP)] applied to a

corn (Zea mays L.), soybean (Glycine max L.), winter-wheat (Triticum aestivum L.)

rotation in Ontario, Canada, were measured from January 2000 to April 2005, using a

micrometeorological method. The superimposition of interannual variability of weather

and management resulted in mean monthly N2O fluxes ranging from �1.9 to

61.3 g N ha�1 day�1. Mean annual N2O emissions over the 5-year period decreased

significantly by 0.79 from 2.19 kg N ha�1 for CP to 1.41 kg N ha�1 for BMP. Growing

season (May–October) N2O emissions were reduced on average by 0.16 kg N ha�1 (20% of

total reduction), and this decrease only occurred in the corn year of the rotation.

Nongrowing season (November–April) emissions, comprised between 30% and 90% of

the annual emissions, mostly due to increased N2O fluxes during soil thawing. These

emissions were well correlated (r2 5 0.90) to the accumulated degree-hours below 0 1C at

5 cm depth, a measure of duration and intensity of soil freezing. Soil management in

BMP (NT) significantly reduced N2O emissions during thaw (80% of total reduction) by

reducing soil freezing due to the insulating effects of the larger snow cover plus corn and

wheat residue during winter. In conclusion, significant reductions in net greenhouse gas

emissions can be obtained when NT is combined with a strategy that matches N

application rate and timing to crop needs.

Keywords: crop and soil management, freeze-thaw cycles, nitrogen fertilization, nitrous oxide flux

Received 10 November 2006; revised version received 16 February 2007 and accepted 9 March 2007

Introduction

Increased concentrations of nitrous oxide (N2O) since

preindustrial times are of concern as N2O contributes to

the greenhouse effect in the troposphere (IPCC, 2001),

and plays a role in the destruction of beneficial ozone in

the stratosphere (Cicerone, 1989). N2O is produced by

the microbiological processes of nitrification and deni-

trification (Firestone & Davidson, 1989), and approxi-

mately 60% of the total N2O emitted to the atmosphere

is derived from soils, roughly a third of which is

produced in agricultural soils (IPCC, 2001).

The complex interplay of microbiological processes

and soil conditions, such as water content, carbon (C)

and nitrogen (N) content, temperature and pH (Granli

& B�ckman, 1994) regulates N2O dynamics in the soil

profile, and how and when N2O is released from the soil

surface. Management practices such as soil tillage, crop

type, and the application of nitrogen fertilizers influ-

ence the physical and hydrological condition of the soil

and the timing and distribution of nutrient inputs. ThisCorrespondence: Claudia Wagner-Riddle, fax 11 519 824 5730,

e-mail: cwagnerr@uoguelph.ca

Global Change Biology (2007) 13, 1722–1736, doi: 10.1111/j.1365-2486.2007.01388.x

r 2007 The Authors1722 Journal compilation r 2007 Blackwell Publishing Ltd

in turn affects the size, composition and activity

of the soil microbial population, and therefore, the

extent of N2O production and emission from agricul-

tural soils.

Studies consistently have shown that by providing

additional N, fertilization can greatly increase N2O

emissions (Conrad et al., 1983; Bouwman, 1990; Aulakh

et al., 1992; Maggiotto et al., 2000). Therefore, applying N

according to soil N reserves and matching the time of

application to crop uptake, may increase N use effi-

ciency and potentially reduce N2O emissions (Mosier,

1994).

Storage of C in agricultural soils, through manage-

ment practices such as no-tillage (NT), has been identi-

fied as a potential measure to offset increasing global

CO2 levels (Lal et al., 1998). However, carbon and N

dynamics in soils are intrinsically linked, and soil

management practices to increase C in soils may actu-

ally increase N2O emissions, pointing to a need to

identify practices that minimize net greenhouse gas

emissions (Follett et al., 2005). This means quantifying

net fluxes of CO2 and N2O, and expressing them in

terms of CO2 equivalent by considering the global

warming potential of N2O, which is 296 times larger

than that for CO2 (IPCC, 2001).

NT soils are characterized by the surface accumula-

tion of crop residues and tend to be less aerated, have

greater C, N and water contents, and therefore higher

denitrification rates, particularly in the surface layer,

when compared with tilled soils (Staley et al., 1990;

McKenney et al., 1993). In addition, the amplitude in

temperature variation is decreased in NT soil, resulting

in cooler temperatures early in the growing season (GS)

(Drury et al., 1999), but warmer minimum winter

temperatures when compared with conventional

tillage (Gauer et al., 1982). Some of the changes in soil

conditions under NT, such as soil organic matter in-

crease, occur over long (410 years) time periods; other

changes associated with surface placement of residue

and lack of fragmentation due to tillage occur in the

short term (days or months) (Kay & VandenBygaart,

2002).

The complex interaction of soil factors affected by NT

likely explains the contradictory results obtained in

investigations into effects on N2O emissions. Some

studies have found that NT decreased N2O emissions

(Kessavalou et al., 1998; Lemke et al., 1999; Liebig et al.,

2005), whereas others reported no change (Robertson

et al., 2000; Grandy et al., 2006; Parkin & Kaspar, 2006) or

increased N2O emissions under NT (Aulakh et al., 1984;

McKenney et al., 1993; Ball et al., 1997; Mackenzie et al.,

1997). In a recent compilation of Canadian studies,

Helgason et al. (2005) observed that in humid regions

NT tended to increase N2O emissions, while in arid

regions emissions were reduced sometimes. Most of

these studies have used long-term (410 years) NT

plots, but as discussed above, N2O flux also could be

affected by short-term changes in soil conditions due

to NT.

Given that NT has the potential for carbon sequestra-

tion, as well as several other environmental benefits

(Holland, 2004), it is imperative to determine if match-

ing N application rate and timing to crop needs under

NT can reduce N2O emissions when compared with

conventional practices (CP). Indeed, it has been sug-

gested that N management strategies for long-term N2O

flux reduction under NT conditions should be the focus

of additional research (Six et al., 2004).

N2O fluxes from soil are highly episodic with peak

emissions occurring after wetting of a dry soil (Jorgensen

et al., 1998), following the application of ni-

trogen fertilizers (Maggiotto et al., 2000) and during

the thaw of frozen soils (Wagner-Riddle et al., 1997).

Thus, continuous and intensive sampling are re-

quired to quantify annual emissions. Soil freezing and

thawing occurs over 35% of the earth’s land area,

mainly in the Northern Hemisphere, including large

areas in Russia, middle North America, Northern

Europe (Williams & Smith, 1989) and Northern China

(Jin et al., 2000). Significant N2O losses have been

observed from cultivated soils over winter (Rover

et al., 1998; Teepe et al., 2000; van Bochove et al., 2000)

and during spring thaw (Bremner et al., 1980;

Wagner-Riddle & Thurtell, 1998; Dorsch et al., 2004).

In addition, a seasonal response of N2O emissions

to NT has been observed, with increased emissions

during summer but decreased emissions during

spring thaw in comparison with cultivated soil (Lemke

et al., 1998), emphasizing the need for year-round

measurements.

Here, we report on N2O fluxes from two management

systems applied to a corn (Zea mays L.), soybean (Gly-

cine max L.), winter-wheat (Triticum aestivum L.) rotation

in Ontario, Canada, measured from January 2000 to

April 2005, using a micrometeorological method. One

system, termed the conventional system, employed

conventional tillage and fertilization based on general

recommendations for each crop. The other system,

named the best management system, employed NT

(initiated in May 1999), fertilization based on soil tests

and timed to crop uptake, and use of a cover crop when

possible. The objectives of this study were (1) to eval-

uate the magnitude of N2O emission reduction due to

best management practices (BMP) for greenhouse gas

mitigation in comparison with CP; (2) to study the

seasonal variability in reduction of emissions due to

the interaction between management and weather over

5 years.

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Materials and methods

Site description

Flux measurements were performed at the Elora Re-

search Station (431390N 801250W, 376 m elevation),

Ontario, Canada, from January 2000 to April 2005. The

soil at the site is classified as an imperfectly drained

Guelph silt loam (29% sand, 52% silt, 19% clay), with

average pH of 7.6 (water), organic carbon of 27 g kg�1,

total N of 2 g kg�1, available P of 24 mg kg�1 and avail-

able K of 146 mg kg�1, in the 0–15 cm soil layer. The

experiment consisted of two management systems, one

using CP and one using BMP. Four plots were mon-

itored, each 150 m� 100 m (1.5 ha) in size, two plots for

each system (plots 1 and 4 for CP; 2 and 3 for BMP).

These plots were within a level and aerodynamically

homogeneous, 30 ha area, which was planted with the

same crop as used in the experimental area. Corn,

soybean and winter-wheat or barley had been grown

in rotation in the experimental plots over the previous

8 years. Crops had been fertilized with inorganic N

fertilizer at the recommended rates (70–90 kg N ha�1 for

winter-wheat or barley; 150 kg N ha�1 for corn). Solid

beef manure (16 Mg ha�1) had been applied to the site,

once every 3 years, following cereal harvest (last in fall

of 1998).

The crop sequence during the experiment, corn in

2000, soybean in 2001, winter wheat in 2002, corn in

2003, and soybean in 2004, was common for both

systems. Depending on the year, corn and soybeans

were planted in May or June. Soybeans were harvested

in September, and corn in October or November. Win-

ter-wheat was planted in October after soybean harvest

in 2001, and harvested in August 2002.

In the CP system, seedbed preparation at planting

consisted of disking and application of fertilizer N

according to the local, general recommendations

(OMAFRA, 2002). For corn (in May 2000 and 2003),

fertilizer N was supplied as granular urea broadcasted

at planting at a rate of 150 kg N ha�1. For winter wheat

(in April 2002) fertilizer N was applied as granular urea

broadcasted before the start of stem elongation at a rate

of 90 kg N ha�1. Fertilizer N was not applied to soy-

beans. Conventional tillage was practiced by mold-

board ploughing to a depth of 20 cm after harvest,

except for fall 2000, when ploughing was delayed until

spring 2001 due to weather conditions, and before

winter-wheat planting (after soybean harvest) in 2001,

when plots were only disked. Crop residue was

chopped after corn harvest (2000 and 2003) in CP plots

in order to facilitate residue incorporation.

In the BMP system, a combination of BMP was

performed. They were: NT, N fertilizer rate based on a

soil N test applied at a later date than in CP for corn,

and inclusion of a cover crop when possible. The BMP

plots were last tilled in May 1999, when the 30 ha area

was seeded to corn. For corn, a side-dress injection of

28% N (urea-ammonium nitrate solution) occurred at

the six-leaf stage with rate of N determined according to

a soil NO3� test (OMAFRA, 2002) on soil samples taken

at planting. Application rates were 50 and 60 kg N ha�1,

applied 30 and 45 days after planting, respectively, in

2000 and 2003. For wheat in 2002, consideration of

30 kg N ha�1 credits from the previous year’s soybean

crop allowed reduction of N applied as urea to

60 kg N ha�1 applied at the same time as for CP treat-

ment. Red clover (Trifolium pratense L.) was used as a

cover crop during winter of 2002/2003 by underseeding

it to winter wheat in March 2002, and chemically killing

in April 2003. Details on the timing of different manage-

ment practices in the two systems are given in Jayasun-

dara et al. (2006).

Micrometeorological measurements

The vertical flux of N2O was obtained using the flux-

gradient method:

FN2O ¼ �K@C

@z; ð1Þ

where K is the eddy diffusivity of N2O at height z and

@C/@z is the N2O concentration gradient at height z.

Assumption of similarity between sensible heat and

N2O turbulent transport, and integration of Eqn (1)

between heights z1 and z2 results in

FN2O ¼u�kDC

ln z2�dz1�d

� �� ch2

þ ch1

h i ; ð2Þ

where u�is the friction velocity, k is the von Karman

constant (50.41), DC is the N2O concentration differ-

ence between heights z2 and z1, d is the displacement

height, and ch2and ch1

are the integrated Monin–

Obukhov similarity functions for heat for both sampling

heights. These functions were calculated using the

stability parameter (z�d)/L, where L is the Obukhov

length, according to empirical expressions derived by

Dyer & Hicks (1970) and Paulson (1970), as reported in

Wagner-Riddle et al. (1996).

The concentration of N2O was measured at heights z2

and z1, spaced 40 cm apart, using a tunable diode laser

trace gas analyzer (TGA100, Campbell Scientific, Logan,

UT, USA). An air intake made of copper tubing

(12.7 mm i.d.) was placed at each height and connected

to one of the two ports of a solenoid valve (Mark 15 line,

Numatics, Highland, MI, USA) (Fig. 1). Switching be-

tween the bottom and the top air intake occurred by

activating the solenoid valve at 20 s intervals. The out-

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put of the solenoid valve was connected to a filter

holder (47 mm in-line, Gelman Sciences, Ann Arbor,

MI, USA) with a 2mm Teflo filter, followed by a multi-

tube dryer (Perma Pure products, Toms River, NJ,

USA), and a severe service needle valve (Whitey, High-

land Heights, OH, USA). The sampled air then traveled

through polyethylene tubing (12.7 mm i.d.) to a three-

way valve (Ascoelectric Ltd, Brantford, ON) (Fig. 1)

placed inside an instrumentation trailer. The tubing

length varied between plots, and was 60, 95, 88, and

180 m for plots 1, 2, 3, and 4, respectively, due to the

asymmetrical layout of the experimental site. Each one

of the four 1.5 ha plots studied had its own air sampling

system, comprised of two air intakes, solenoid valve,

filter, drier, needle valve, tubing and three-way valve.

Each drier was fixed to a stake approximately in the

center of each plot, except during the corn GS when the

drier was placed at the edge of the plots. The four three-

way valves (one for each plot) were mounted on a

manifold inside the instrumentation trailer, with all

‘off’ positions connected to a vacuum pump (RB 0040,

Busch, Virginia Beach, VA, USA) (pump A in Fig. 1),

and all ‘on’ positions connected to the TGA100. A

second vacuum pump (RB 0021, Busch) (pump B, Fig.

1) was used to draw sampled air from the valve mani-

fold into the TGA100, which was operated at a pressure

of about 50 mb. One three-way valve was activated for

1 h in sequence so that air originating from one plot at a

time was analyzed over 1 h. Air samples from the other

three plots (i.e. three-way valves that were not acti-

vated) were discarded. The purge flow of each drier

was directed to a manifold via 60–180 m of vinyl tubing

(19-mm, Rubberline, Guelph, ON, USA) which in turn

was connected to the RB 0040 pump. A flowmeter

(Visiflow, Dwyer Instruments, Michigan City, IN,

USA) was used to regulate the purge air flow in each

drier (1.5 L min�1).

The liquid nitrogen cooled laser (serial number 1085–

14, Laser Analytics Inc., Bedford, MA, USA) in the

TGA100 was operated at a temperature of 89.2 K, a

current of 257 mA, and a wavenumber of 2233.3 cm�1.

Switching between the two intake heights over each

plot was controlled by the analyzer’s software, which

also calculated N2O concentrations at a frequency of

10 Hz. The software selected 10 Hz data for each intake

height to be used in calculating hourly concentration

values after accounting for air sample travel time and

removing data points which consisted of an air mixture

of both sample heights. The average hourly difference in

N2O concentration between the two heights was then

calculated using the remaining data points (Wagner-

Riddle et al., 2005).

Hourly friction velocity and sensible heat flux,

needed for calculating the Obukhov length and estimat-

ing the stability functions, were measured with two

sonic anemometers (CSAT3, Campbell Scientific, Logan,

UT, USA), one for each treatment. A tower with four

cup anemometers (F460, Climatronics Corp., Newton,

PA, USA), was placed in one plot of each treatment, and

hourly wind speed profiles were recorded. Data from

sonic and cup anemometers were used in the logarith-

mic wind profile equation to iteratively solve for dis-

placement height (d) and roughness length (zo). Mean d

and zo for specific periods, taking into account changes

in surface conditions over the year, were then calcu-

lated. These means were directly used to calculate flux,

and to estimate u� with measured wind speeds in the

logarithmic wind profile equation (d and zo). The latter

calculation was necessary when the sonic anemometer

did not yield data (e.g. due to rainy or foggy condi-

tions). For those periods, stability corrections in the

wind profile equation were calculated using an estimate

of sensible heat flux based on measured net radiation

(CNR1, Kipp and Zonen, Delft, the Netherlands). This

approach yielded an N2O flux that related linearly

(r2 5 0.95) with flux values calculated using a directly

measured u�, and allowed for calculation of fluxes for

rainy periods, which are extremely important for esti-

mation of total annual N2O emissions.

Measured hourly N2O concentrations that met the

following criteria were selected for concentration dif-

ference calculations: hourly SD o100 ppb; analyzer

operating pressure 430 and o70 mb; difference in

Pump B

TGA

Pump A

Valve A

Valve B

Drier

Air intakes

Filter

Needle valve

Needle valve and flowmeter

Purge line

Sample tubing

Drier purge flow manifold

Fig. 1. Diagram of the air sampling system used over four

1.5 ha plots at the Elora Research Station from January 2000 to

April 2005. Valve A denotes solenoid valve used for switching

between upper and lower air intake. Valve B denotes three-way

valve used for selecting air over 1 h from plots 1–4 in sequence.

Vacuum pump A was used for purging driers and bringing

sampled air to three-way valves, while pump B was used to

draw sampled air through the trace gas analyzer (TGA).

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analyzer pressure when measuring air sampled from

upper and lower intakes o2 mb; and number of sam-

ples per hour 480% of maximum possible number of

data points. Hourly wind speed and friction velocity

41.0 and 40.1 m s�1, respectively, were selected for

turbulence calculations. The stability parameter (z�d)/

L was constrained to values o2 and 4�5, as these were

the conditions for which the empirical stability func-

tions were derived (Arya, 1988). In addition, wind

direction recorded at the Elora weather station (100 m

from the experimental site) was used in conjunction

with air intake and cup anemometer heights and tower

position within plots, to select periods when the wind

direction allowed for a fetch-to-height ratio of at least

50 : 1 (horizontal distance to height of measurement

ratio). According to theory developed by Leclerc &

Thurtell (1990), this criterion assured that 480% of

the flux measured originated within the experimental

plots monitored, depending on atmospheric stability

conditions, and height of the crop.

Filtered data were used in Eqn (2) to calculate hourly

N2O flux for each plot. Owing to the sequential air

sampling setup used, a maximum of six hourly flux

values were calculated for each plot per day (e.g. at 1:00,

5:00, 9:00, 13:00, 17:00, 21:00 h for plot 1). Daily flux

means for each plot were obtained by averaging hourly

values collected during a day (minimum of two hourly

values). Fluxes for days with missing data were inter-

polated by averaging means occurring on days adjacent

to the missing value. For long periods with missing data

(41 month) an average obtained from data in adjacent

months for all plots was used, in order to not bias the

overall comparison of treatment means. The daily inter-

polated flux data set expressed in kg N2O–N ha�1 day�1

was then summed to yield monthly and annual N2O

emission.

Continuous periods with missing data occurred

mostly when instrumentation was removed from the

plots for field operations such as soil tillage, planting

and harvest. These data gaps varied from 6 to 40 days,

the latter in fall 2000 when weather conditions delayed

soil tillage until a decision was made to defer ploughing

until spring. In addition, problems with the trace gas

analyzer from end of August to October 2004 resulted in

a 70 days data gap. The percent of daily data capture for

May to October was 50% in 2000, 80% in 2001, 60% in

2002, 32% in 2003, 49% in 2004. For the November to

April period data capture was 66% in 2000/2001, 94% in

2001/2002, 92% in 2002/2003, 79% in 2003/2004 and

87% in 2004/2005. The difference between periods was

mostly due to fetch to height requirements that were met

more frequently during the nongrowing season (NGS).

Over the 5 years, 5015 daily fluxes were used in this

study, obtained from a total of 22 214 hourly fluxes.

Supporting data

Air temperature, precipitation and snow depth on the

ground were obtained from the Elora weather station.

In addition, snow depth on the ground was measured

on each plot at several points (at least five) using a ruler.

Soil temperature was measured at 5 cm depth using

thermistors (Model #107, Campbell Scientific), and soil

water content in the 0–10 cm layer using reflectometers

(Model CS615, Campbell Scientific) (McCoy et al., 2006).

Five soil samples from the 0 to 30 cm soil layer were

taken per plot on a biweekly basis during the GS and

then monthly after harvest. The five sampling points

were selected at 25 m intervals along a transect in the

middle of each 1.5 ha plot. Samples were kept on ice

in a cooler until transported to the laboratory. Ex-

changeable NH41 and NO3

� nitrogen were extracted

from the field moist soil with 2 M KCl (at 1 : 5 soil : ex-

tractant ratio) within 24 h after sampling. Concentra-

tions of NO3–N in soil KCl extracts were measured by

copper cadmium reduction to nitrite (Technicon Indus-

trial Method No. 824–89T), and of NH4–N by the

indophenol blue method (Technicon Industrial Method

No. 820–89T).

Statistical analyses

The probability distribution of N2O fluxes has been

shown to follow log normal or highly skewed (e.g.

reverse J shaped) distributions (Yates et al., 2006). This

was also the case for daily flux values observed in this

study. Hence, treatment effects were tested by paired

comparison of daily medians using the Wilcoxon signed

rank-test (Steel et al., 1997). This statistical test was

performed using measured values, that is, without

considering the interpolated daily means, so as not to

bias treatment comparisons. The experimental area was

arranged in a randomized complete block design,

where plot 1 was paired with plot 2, and plot 3 was

paired with plot 4.

Results and discussion

Overview of experimental conditions

During the experimental period, mean monthly air

temperature varied between –10.7 1C in January 2004

and 21.1 1C in July 2002, and total monthly precipitation

between 17 mm in March 2005 and 180 mm in June 2000

(Fig. 2a). The GS, defined here as the May–October

period, was warmest in 2001 and 2002, with mean

temperature around 15.5 1C, and slightly cooler during

the other years at 14.5 1C. The wettest GS occurred in

2000 with 570 mm, and the driest in 2002 with 411 mm.

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Crop yields were not significantly affected by treatment

except for higher soybeans yields for BMP in 2001 (4.9

vs. 3.4 Mg ha�1 for CP). Cool and wet conditions in 2000

decreased mean corn yields for both treatments to

4.1 Mg ha�1 compared with 8.7 Mg ha�1 in 2003, while

2002 and 2004 were favorable for crop growth, yielding

an average of 7.2 and 3.7 Mg ha�1 for winter-wheat and

soybeans, respectively.

The NGS, here defined as the November to April

period, presented large interannual variability in mean

air temperature varying from 0.4 1C in 2001/2002 (No-

vember 2001 to April 2002) to –3.4 1C in 2002/2003.

Warmer conditions during 2001/2002 resulted in the

smallest depth of snow cover over the study period

(mean o5 cm), while it reached a maximum of 35 cm in

2000/2001 (Fig. 2a). Precipitation was highest in the

NGS of 2003/2004 with 437 mm, and lowest in 2002/

2003 with 314 mm.

Soil and crop management resulted in substantial

variations in mineral N in the 0–30 cm soil layer over

May Ju

l

Sep

Nov Jan

Mar

May Ju

l

Sep

Nov Jan

Mar

May Ju

l

Sep

Nov Jan

Mar

May Ju

l

Sep

Nov Jan

Mar

May Ju

l

Sep

Nov Jan

Mar

0

40

80

120

160

Min

eral

N (

kg h

a–1)

F P F P F P P

May Ju

l

Sep

Nov Jan

Mar

May Ju

l

Sep

Nov Jan

Mar

May Ju

l

Sep

Nov Jan

Mar

May Ju

l

Sep

Nov Jan

Mar

May Ju

l

Sep

Nov Jan

Mar

–10

0

10

20

30

40

Tem

pera

ture

(°C

)

Sno

w d

epth

(cm

)

0

40

80

120

160

200(a)

(b)

(c)

Pre

cipi

tatio

n (m

m)

2000/2001 2001/2002 2002/2003 2003/2004 2004/2005

May Ju

l

Sep

Nov Jan

Mar

May Ju

l

Sep

Nov Jan

Mar

May Ju

l

Sep

Nov Jan

Mar

May Ju

l

Sep

Nov Jan

Mar

May Ju

l

Sep

Nov Jan

Mar

0

20

40

60

N2O

flux

(g

N h

a–1 d

ay–1

)

T T T T T

corn Soy Wheat Clover (BMP) Corn Soy

Fig. 2. Mean monthly (a) air temperature (solid line), daily snow depth (dashed line), and monthly total precipitation (bars) recorded at

the Elora research station, (b) mineral nitrogen content (NO3�1 NH4

1 ) in the 0–30 cm soil layer, and (c) nitrous oxide flux for plots under

conventional (4) and best management practices (BMP) (�) measured from May 2000 to April 2005. Horizontal arrows in (b) indicate the

crop grown, which were common for both practices, except in August 2002–April 2003 when red clover was grown in the BMP plots only.

Vertical arrows in (b) show when conventional plots were ploughed or disked (P), and when nitrogen fertilizer (F) was applied. Closed

diamonds in (c) show when soil thawing (T) took place each year.

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the 5 years, with highest content following application

of N fertilizer to corn in 2000 and 2003 (Fig. 2b). Lower

N application rates in the BMP treatment (50–60 vs.

150 kg N ha�1 for CP) resulted in lower mineral N

content in those plots during the ‘corn’ years. But,

differences were much smaller after reduced N applica-

tion to wheat at the end of April 2002, likely due to the

smaller difference in N rate between treatments

(60 kg N ha�1 for BMP vs. 90 kg N ha�1 for CP). Soil

mineral N decreased to o40 kg ha�1 for both treatments

after July or September, when maximum crop N uptake

had occurred, in all years except 2000, when corn

growth and yield were poor due to unfavorable grow-

ing conditions (Jayasandura et al., 2007). This relatively

higher N content in fall 2000 was still present in both

treatments in spring 2001, when soybeans were planted.

The superimposition of interannual variability of

weather and management resulted in mean monthly

N2O fluxes ranging from �1.9 to 61.3 g N ha�1 day�1

(Fig. 2c). Fluxes increased after fertilizer application to

corn and to a lesser degree to wheat, and during the

event of thaw in March or April of each year. However,

the magnitude of monthly fluxes varied substantially

between years, and between treatments within a given

year. Statistical comparison of daily N2O flux medians

between management practices for the GS and NGS

periods indicated significant reductions in fluxes for

BMP compared with CP except November 2001 to April

of 2002, and May 2004 to April 2005 (Table 1). Upon

inspection of Fig. 2c, it is clear that significant differ-

ences were associated with the differing response of

BMP and CP plots during the events of N fertilization

and thaw.

Nitrogen fertilization events

The N management practices studied, that is, reduced

fertilizer rates for wheat and reduced rate plus applica-

tion at a later date for corn, significantly decreased N2O

fluxes from BMP compared with CP during the GS

(Table 1). This effect dominated any potential trends

for higher N2O fluxes associated with NT in BMP plots,

such as caused by higher soil water content.

For corn in 2000, application of N fertilizer to CP plots

was followed by frequent and heavy rainfalls (158 mm

from day 160 to 180), which resulted in emission events

for both treatments (Fig. 3a and d). Increased soil

mineral N content related to N fertilization at planting

in CP plots in June 2000 (Fig. 2b), probably resulted in

N2O fluxes from CP that were larger for a longer time

when compared with BMP (Fig. 3a). On the other hand,

in the 2 weeks that followed fertilizer application to the

BMP plots (day 186–200), the corn plants were accumu-

lating dry matter and nitrogen at a maximum rate

(2 kg N ha�1 day�1, data not shown) and a relatively

dry period occurred (11 mm from day 181 to 194),

potentially explaining the absence of a flux event re-

lated to N fertilization in both BMP plots (Fig. 3d).

Table 1 N2O fluxes (median, and 25th and 75th percentile in brackets) from two plots managed using conventional and best

management practices for the GS and NGS during the study period spanning 2000–2005

Yearw Periodz/crop

Median N2O flux§ (g N ha�1 day�1)

n P}Conventional Best management

2000/2001 June–Julyk (corn) 5.76 (2.46, 13.9) 4.26 (0.54, 8.88) 44 0.008*

November–April 4.71 (2.60, 7.34) 2.77 (1.35, 5.28) 219 o0.0001*

2001/2002 May–October (soybean) 3.59 (1.22, 6.16) 2.39 (0.45, 4.36) 264 o0.0001*

November–April 1.78 (0.39, 3.22) 1.98 (0.21, 3.84) 334 0.403

2002/2003 May–October (wheat) 1.73 (�0.16, 4.60) 1.17 (�0.47, 3.23) 185 o0.002*

November–April 4.18 (1.41, 10.33) 1.94 (0.58, 3.86) 322 o0.0001*

2003/2004 May–Julyk (corn) 9.39 (5.32, 20.27) 2.42 (0.74, 5.30) 82 o0.0001*

November–April 2.69 (0.59, 5.47) 1.79 (�0.38, 4.84) 271 o0.004*

2004/2005 May–August (soybean) 1.42 (0.38, 3.48) 0.864 (�0.86, 4.64) 169 0.691

November–April 2.23 (0.33, 6.52) 2.32 (0.014, 6.13) 305 0.273

Number of daily flux values (n) and probability level (P) of Wilcoxon signed rank test carried out for paired comparison of medians.

Note that maximum n for the May–October or November–April periods is �360 (6 months� 30 days� 2 plots per treatment).wStart of year was considered May 1, and end of year April 30 of following year.zGS months: May to October; NGS months: November to April.§25th and 75th percentiles are shown in brackets.}Significant differences at the 0.05 level are marked with*.kMissing data for August–October.

NGS, nongrowing season; GS, growing season; N2O, nitrous oxide.

1728 C . WA G N E R - R I D D L E et al.

r 2007 The AuthorsJournal compilation r 2007 Blackwell Publishing Ltd, Global Change Biology, 13, 1722–1736

In the case of winter-wheat in May 2002, the flux

event timing after application was similar for both

treatments on day 124, coinciding with a rainfall event;

however, the flux magnitude was lower for BMP de-

spite slightly higher overall water content in the 0–

10 cm layer for this period (0.32 vs. 0.30 m3 m�3 for

CP) (Fig. 3b and e).

In contrast to 2000, fertilizer application to corn under

BMP on day 183 of 2003 was followed by a 16 mm

rainfall on day 186, which resulted in a large N2O flux

that was comparable with that observed following

application to CP plots at corn planting (Fig. 3c and f).

The faster response in fluxes to fertilization events in

BMP compared with CP treatment (5 vs. 10 days), and

the comparable peak values despite lower fertilization

rates (60 vs. 150 kg N ha�1) can be ascribed to the form

of fertilizer applied. The 28% urea–ammonium nitrate

solution used for the side-dress application to corn at

the sixth leaf stage under BMP contained nitrogen in

nitrate form (25%) which can be directly denitrified; the

nitrogen broadcast to the CP plots as urea would have

to undergo nitrification first. Overall, N2O fluxes mea-

sured for BMP were significantly lower for May–July

2003 than for CP (Table 1), as the duration of the N

fertilization-related emission event was longer for CP

(25 vs. 15 days).

For the GS without any N fertilizer application (soy-

bean in 2001 and 2004, Fig. 2c) the effect of BMP would

have been limited to factors related to tillage manage-

ment, except that in early 2001 mineral N content was

significantly higher in CP treatment, likely a carryover

effect of previous manure and fertilizer management

(Fig. 2b). Mean soil water content in 0–10 cm layer

during GS was higher for BMP than CP plots in 2004

(0.30 vs. 0.21 m3 m�3), but measurements were not

available for May 2001. The difference in water content

during 2004 did not translate into a treatment effect

on N2O fluxes, but CP presented larger GS fluxes

than BMP in 2001 (Table 1). Hence, if soil water content

effects related to tillage were present in 2001, they

were most likely overridden by mineral N content

differences.

In a 3-year study that quantified fluxes over 60 days

following N application to corn, McSwiney & Robertson

(2005) observed that N rate could be reduced to levels

that satisfied crop needs (�100 kg N ha�1 for their con-

ditions) without compromising yields, and with re-

duced N2O fluxes. For our site, which previously had

received manure N, and cover crop N in the case of

BMP plots, application of reduced N rates was sufficient

to meet the yield goal and also significantly reduce N2O

fluxes, despite higher soil water content due to NT in

BMP. In studying N2O fluxes from plots under NT for

8–9 years in comparison with conventional tillage,

Parkin & Kaspar (2006) concluded that tillage was

not a significant factor, but that N fertilization was a

controlling factor in N2O emissions. In their case, N

was applied at 202 kg ha�1 38 days after corn planting,

and N2O flux peaks following fertilization were

400 g N ha�1 day�1, compared with 100 g N ha�1

day�1 in our study (Fig. 3, corn years). It is not apparent

if corn yields would have been affected if N rate had

been reduced from 202 kg N ha�1 in Parkin and Kas-

par’s study but we suggest management of N rate has a

large potential of significantly reducing N2O emissions.

In a recent study, Mulvaney et al. (2006) identified an

average corn overfertilization of 103 kg N ha�1 in 69% of

studied farms in Illinois, USA, when the prevalent

yield-based method was used to determine N fertiliza-

tion rates. They proposed that environmental conse-

quences of fertilization in humid regions such as Illinois

could be reduced using a soil N test to determine N

application rate. Hence, it appears that significant re-

duction of N2O emissions, while keeping comparable

0

50

100

150

200

(a) (d)

(e)(b)

(c) (f)

250

F F

0

0.2

0.4

0

50

100

150

200

250

N2O

Flu

x (g

N h

a–1 d

ay–1

)

F F

0

0.2

0.4

Wat

er c

onte

nt (

m3

m–3

)

120 140 160 180 200Day of year

0

50

100

150

200

250

F

120 140 160 180 200

F

0

0.2

0.4

42

CP BMP

Corn

Wheat

Corn

Wheat

Corn Corn

Fig. 3. Daily mean N2O fluxes for two 1.5 ha plots (plot 1: dots,

plot 2: open circles) managed according to (a–c) conventional

practices (CP) and (d–f) best management practices (BMP), from

April 20 (day 110) to July 31 (day 213) for corn in 2000 (a, d),

winter-wheat in 2002 (b, e), and corn in 2003 (c, f). Vertical

hanging bars show daily rainfall larger than 5 mm with magni-

tude as indicated by 42 mm label in graph (a). Vertical arrows

show when nitrogen fertilizer (F) was applied to plots (150

and 90 kg N ha�1 for CP corn and wheat, respectively; 50–

60 kg N ha�1 for BMP). Average hourly water content as mea-

sured in each plot is shown when available (solid line).

N 2 O E M I S S I O N S F R O M C O N T R A S T I N G M A N A G E M E N T 1729

r 2007 The AuthorsJournal compilation r 2007 Blackwell Publishing Ltd, Global Change Biology, 13, 1722–1736

yields as observed in our study, could be achieved in a

large geographical region.

Winter and spring thaw events

N2O fluxes during the November to April period were

significantly affected by treatment in three out of five

study years (Table 1). The treatment differences were

mostly due to the magnitude of fluxes during thawing

events in February, March or April. N2O fluxes were

much larger for CP than BMP during spring thaw of

2003, somewhat larger in 2001 and 2004, but similar in

2002 and 2005 (Fig. 2c).

The increase in N2O fluxes associated with thawing

events preceded increases in soil temperature at 5 cm

depth and water content in the 0–10 cm layer (Fig. 4),

indicating that increased fluxes were associated with

N2O production processes in the thawed layer present

above the recording probes. While the water content

probes used only measure liquid water and not frozen

water, they serve as qualitative indicators of thawing

events. However, the measurement obtained is an aver-

age for the whole 0–10 cm layer, and will increase only

when a significant portion of ice in this layer thaws.

The smallest overall spring thaw N2O fluxes were

observed in February and March 2002, following the

mildest winter of the studied years, as indicated by soil

temperature and water content readings, which were

similar for both treatments (Fig. 4a and d). In contrast,

soil temperature probes indicated more extensive freez-

ing for CP than BMP treatments, particularly in early

2003 (Fig. 4b and e), and to a lesser extent in early 2004

(Fig. 4c and f), and this was associated with higher N2O

fluxes for CP, notably in 2003. These contrasting soil

conditions between treatments were likely caused by

the difference in surface conditions and associated

snow depth (Table 2). Crop residue and stubble were

present on the surface of BMP plots in all years, while

the CP plots were ploughed surfaces, except for fall

2000 and 2001, when chopped corn residue and winter-

wheat, were present on the surface, respectively. Corn

and wheat stubble were effective in trapping snow, as is

evidenced by the larger snow depth in BMP plots, but

soybean stubble, which was shorter and comprised

lower dry matter, was not as effective (Table 2).

Residue and snow have low heat diffusivity and

decrease the transfer of heat from the soil layers to the

atmosphere during fall and winter (Oke, 1990). As a

result, soil temperature at 5 cm depth was higher for

BMP treatment in all years, except in years with soy-

bean residue, which did not trap additional snow when

compared with ploughed plots (Table 2). Indeed, the

difference in snow depth was correlated with the dif-

ference in soil temperature observed between treat-

ments (r2 5 0.98), with years with large difference in

snow depth also presenting large differences in soil

temperature between CP and BMP (e.g. January–Feb-

ruary 2003). Soil temperatures under CP were well

‘coupled’ to atmospheric conditions as evidenced by

linear regression between soil and air temperature

(r2 5 0.82, slope 5 0.9), but BMP soil temperature was

related less to air temperature (r2 5 0.51, slope 5 0.14).

Interestingly, soil temperature did not correlate well

with snow depth as expected (e.g. higher temperature

with larger snow depth), but this was a function of the

snowfall timing and arrival of subzero temperatures in

the fall. For example, air temperatures dropped below

0

40

80

120

160

200

240

280

320

(a)

(b)

(c) (f)

(e)

(d)

0

0.2

0.4

WC

(m

3 m

–3);

T (

°C)

x 10

0

0

40

80

120

N2O

flux

(g

N h

a–1 d

ay

–1)

0

0.2

0.4

330 360

0

40

80

120

30 60 90 120Day of year

330 360 30 60 90 120

0

0.2

0.430

CP BMP

Soy Soy

Corn

Corn

Wheat

Wheat

Fig. 4. Daily mean N2O fluxes for two 1.5 ha plots (plot 1: dots,

plot 2: open circles) managed according to (a–c) conventional

practices (CP) and (d–f) best management practices (BMP), from

November 30 (day 334) to April 30 (day 120) for winter and

spring of 2001/2002 (a, d), 2002/2003 (b, e), 2003/2004 (c, f),

following soybeans in 2001, winter-wheat in 2002 and corn in

2003. Note that winter wheat was planted in fall 2001. Average

hourly water content in 0–10 cm layer (solid line), daily soil

temperature (�100) at 5 cm depth (bold line, shaded when

o0 1C), and snow depth on ground (bars) for each treatment

are shown when available. Scale for snow depth is shown in

graph (f) by labeling a bar corresponding to 30 cm.

1730 C . WA G N E R - R I D D L E et al.

r 2007 The AuthorsJournal compilation r 2007 Blackwell Publishing Ltd, Global Change Biology, 13, 1722–1736

freezing by mid-November in 2000, but significant

snow depth on the ground only started mid-December.

Hence, soil temperature was lower than expected from

the snow cover present in 2000/2001 when compared

with other years (Table 2).

Soil management in BMP (NT) changed the degree of

freezing of the soil profile when compared with CP in

most years studied. Accumulated N2O emissions for the

November to April period were well correlated to the

accumulated degree-hours below 0 1C at 5 cm depth

over the same period, a measure of duration and

intensity of freezing (Fig. 5). The significant treatment

effects on N2O fluxes observed in 2003 and 2004, when

BMP plot(s) had significantly lower emissions, were

directly associated with less freezing degree-hours un-

der BMP. In 2002, plots with soil temperature measure-

ments (one plot per treatment) had similar freezing

conditions, while in 2005 differences between replicates

were similar to treatment effects. Soil temperature data

were not available for BMP in 2001, but snow depth

differences (Table 2) indicate that lower intensity of

freezing may have played a role in the lower N2O flux

observed for BMP during November to April (Table 1).

Mineral N content was higher for CP than BMP and also

may have contributed to higher CP N2O fluxes at spring

thaw in 2001, but this was not the case for the other

years at the time of spring thaw (Fig. 2b).

Larger accumulation of snow under NT and the

resulting lower intensity of freezing when compared

with ploughed soils have been well-studied before

(Gauer et al., 1982; Kay et al., 1985). The duration and

intensity of soil freezing in laboratory conditions have

been shown to affect subsequent N2O flux when soil

thawed (Teepe et al., 2001, 2004), but other field experi-

ments have not observed this effect as distinctively as

observed here, probably due to other confounding

factors, and lack of contrasting weather conditions over

the years studied (Wagner-Riddle & Thurtell, 1998). In

addition, the effect of NT on soil freezing, and subse-

quent N2O fluxes at thawing has not been demonstrated

before in a field study. The soil processes that lead to

increased N2O production and emission due to freeze/

thaw cycles have been related to enhanced microbial

activity due to increased available carbon from freezing

lysis (Christensen & Tiedje, 1990), and disintegrating

aggregates (van Bochove et al., 2000), combined with

higher water content in soil during thawing (Lemke

et al., 1998). Other mechanisms suggested, involve the

release of N2O trapped in unfrozen water films by ice

formation (Teepe et al., 2001; Koponen et al., 2004), and/

or the physical release of N2O produced during winter

and trapped at depth in unfrozen soil as the ice barrier

melts (Bremner et al., 1980; Burton & Beauchamp, 1994).

Recently, Sharma et al. (2006) observed increased CO2

production and expression of denitrifying genes asso-

ciated with N2O production during the thawing of soil

in a microcosm experiment. They suggested that the

N2O flush was a result of increased microbial activity,

Table 2 Air temperature, surface conditions, snow depth on ground and soil temperature at 5 cm depth for plots managed using

CP and BMP for the study period spanning 2000–2005

Nongrowing

season

Air temperature*

( 1C) Treatment

Surface

conditions

Snow

depth (cm)wSoil temperature

at 5 cmz ( 1C)

1999/2000 �0.60 CP Ploughed 9 a NA

BMP Corn stubble 13 b NA

2000/2001 �2.6 CP Chopped corn 22 a �2.0

BMP Corn stubble 29 b NA

2001/2002 0.40 CP Winter-wheat 6 a 0.50 a

BMP Soybean stubble and wheat 5 a 0.50 a

2002/2003 �3.4 CP Ploughed 7 a �3.3 a

BMP Wheat stubble and red clover 15 b 0.10 b

2003/2004 �1.7 CP Ploughed 12 a �1.0 a

BMP Corn stubble 15 b 0.17 b

2004/2005 �2.3 CP Ploughed 8 a �0.65 a

BMP Soybean stubble 8 a �0.26 b

Note that the 1999/2000 period is included here although N2O fluxes for this period are not presented in Table 1. Snow depth and

soil temperature values followed by different letters within a year indicate a significant difference (Po0.05) between treatment

means according to a t-test.

*Averaged over November 1–April 30.wMean depth over the period when snow was present on ground.zAveraged over December 1–February 28.

CP, conventional practice; BMP, best management practices.

N 2 O E M I S S I O N S F R O M C O N T R A S T I N G M A N A G E M E N T 1731

r 2007 The AuthorsJournal compilation r 2007 Blackwell Publishing Ltd, Global Change Biology, 13, 1722–1736

rather than physical entrapment due to the freeze

barrier. Our data agree with the latter findings, as the

increase in N2O fluxes associated with the thaw event

occurred when most of the profile was still frozen, but

the surface of the soil had started to thaw (Fig. 4).

Factors such as mineral N, carbon and water-filled

pore space also have been shown to have an effect on

N2O flux during soil thawing (Wagner-Riddle & Thur-

tell, 1998; Koponen et al., 2004; Sehy et al., 2004). When

plotting N2O emissions from a previous study by

Wagner-Riddle & Thurtell (1998) at the same site on

Fig. 5, it can be noted that some of these additional

factors may be at play. While emissions from barley,

corn and canola plots fit the general trend of increasing

N2O fluxes with increasing freezing degrees observed

in this study, emissions from unfertilized grass are well

below the trend line. N2O emissions from ploughed-

down alfalfa and fallow plots were higher than ex-

pected based on freezing degrees. Low mineral N

content in grass plots, and abundant substrate (N, C)

in alfalfa and fallow plots probably were factors deter-

mining these responses. In the study by Wagner-Riddle

& Thurtell (1998) a correlation between N2O fluxes and

NO3� concentration was observed (r 5 0.70), but some of

the plots studied had much higher NO3� concentrations

at the beginning of winter than observed here (20–30 vs.

o10 mg N kg�1), a result of manure application or

alfalfa incorporation in the fall. For similarly low levels

of NO3� (o10 mg N kg�1), Rover et al. (1998) did not find

a relationship between nitrate and N2O emissions dur-

ing winter. This suggests that as long as N applications

do not occur at the end of the cropping season, freezing

degrees are the most important factor explaining year-

to-year variability and between treatment differences

in winter and spring thaw N2O fluxes for the conditions

of our study.

Dorsch et al. (2004) compared overwinter N2O emis-

sions from a cultivated field and a grass–legume–mal-

low field under permanent fallow, and concluded that

microclimatic conditions controlled N2O emissions,

regardless of ecosystem-level differences in nutrient

cycling. While high ridges in the cultivated field were

subjected to freeze–thawing cycles, soil conditions were

approximately isothermal under the insulating snow

cover in the fallow field, resulting in much lower N2O

emissions for the latter. Here, we suggest that decreas-

ing the number of freezing-degrees through NT could

be used as a mitigation practice in cold climates to

decrease N2O emissions from agricultural soils.

Overall N2O emissions

Frequency histograms of daily N2O fluxes measured

from 2000 to 2005 were similar for GS and NGS (Fig. 6).

Both showed a relatively higher number of fluxes in the

�5 to 1 5 g N ha�1 day�1 range for BMP (77% vs. 68%

for CP), but larger frequency of days with fluxes in 5 to

425 g N ha�1 day�1 classes for CP (32% vs. 23% for

BMP) (Fig. 6). Although the shift due to BMP affected

only 10% of days, the impact on N2O emissions was

large due to the effect of extreme flux values. This effect

is illustrated by the contribution of each flux class to

total N2O emission over the 5 study years, where fluxes

425 g N ha�1 day�1 contributed 1.8–2.6 kg N ha�1 for

CP (or 35–44% of total emissions) and 0.8 kg N ha�1

for BMP (25% of total emissions), depending on season,

despite occurring o5% of days (Fig. 6). BMP reduced

the occurrence of these extreme events from 3.5% to

1.8% during GS, and 4.7% to 2.3% of days during NGS.

Total annual N2O emissions for CP varied between

1.10 to 3.32 kg N ha�1 yr�1 with mean of 2.19 kg N ha�1

yr�1, and for BMP between 0.96 and 2.18 kg N ha�1 yr�1

0 2000 4000 6000 8000 10 000

Accumulated degree-hours < 0°C at 5 cm depth

0

1

2

3N

itrou

s O

xide

Em

issi

on –

Nov

. to

Apr

. (kg

N h

a–1)

1

2

3

4

45

5

2

3

4

45

5

CP

BMP

W-R&T

F

A

G

B

Ca

G

Co

Fig. 5. Accumulated nitrous oxide (N2O) emissions over the

November–April period as a function of accumulated freezing

degree hours (o0 1C) calculated using soil temperature mea-

sured at 5 cm depth for plots managed according to conventional

practices (CP) and best management practices (BMP). Labels in

symbols indicate year of thaw: 1 5 2001; 2 5 2002, etc.; with

repeated values representing the two plots for each treatment

when soil temperatures were available. Regression equation for

line shown is Y 5 0.216� 10�3X 1 0.426, n 5 13, r2 5 0.902. For

comparison purposes we show November to April values by

Wagner-Riddle & Thurtell (1998) (W-R&T in legend) for unferti-

lized grass (G), barley (B), canola (Ca), corn (C), fallow (F) and

alfalfa (A). W-R&T used methodology described in Wagner-

Riddle et al. (1996) which employed a correction factor of 1.3,

which was not used in this study. Hence, values originally

published were divided by 1.3 to be comparable with the values

obtained here.

1732 C . WA G N E R - R I D D L E et al.

r 2007 The AuthorsJournal compilation r 2007 Blackwell Publishing Ltd, Global Change Biology, 13, 1722–1736

with mean of 1.41 kg N ha�1 yr�1 for BMP treatment

(Table 3). Emissions during the GS totalled between

0.31 and 1.76 kg N ha�1 for CP, and 0.19 and

1.65 kg N ha�1 for BMP, while NGS emissions ranged

from 0.42 to 2.91 kg N ha�1 for CP, and 0.49 and

0.90 kg N ha�1 for BMP. These latter amounts repre-

sented between 38% and 88% of the total annual N2O

emissions for CP, and from 28% to 79% for BMP.

Highest GS emission for both management practices

was associated with the two ‘corn’ years, while crop

type was not associated with any trends in NGS emis-

sions. As discussed above, intensity of soil freezing, as

determined by weather conditions and management,

affected NGS emissions. For CP, the total NGS N2O

emission increased exponentially with decreasing mean

air temperature (r2 5 0.95; data not shown), explaining

the large variation in NGS emissions observed for the 5

years studied. But, the insulation provided by residue

plus snow cover in BMP meant that the intensity of

freezing was less than for CP, and extreme values such

as in 2002/2003 for CP were not observed. Conse-

quently, the year with highest overall emissions was

not coincident for both treatments, with ‘corn’ years

showing highest emissions for BMP, while emissions

from CP were highest in the 2002/2003 ‘wheat’ year.

However, as discussed, this effect was not related to

crop type per se but rather to soil conditions established

by the interaction of weather and soil management.

The reduction in total N2O emissions over the GS and

NGS due to BMP ranged from 0.11 to 2.21 kg N ha�1, or

8% to 76% of CP emissions, and decreases during the

NGS were generally larger than in the GS (Table 3).

Average reductions in annual emission under BMP

varied from 0.136 kg N ha�1 yr�1 (12%) during the soy-

bean year of 2001/2002 to 2.43 kg N ha�1 yr�1 (73%)

during the wheat year of 2002/2003, with nonsignifi-

cant reductions during the soybean year of 2004/2005.

Mean overall reduction due to BMP was

0.786 kg N ha�1 yr�1, which amounted to 36% of CP

emissions, with the largest reduction occurring during

the NGS. This reduction corresponds to 383 kg CO2

equivalent ha�1 yr�1 (from 1020 to 655 kg CO2 equiva-

lent ha�1 yr�1). This value is approximately one-third of

the carbon sequestration observed for a similar crop

rotation under no-till in a long-term study in Michigan,

although N2O emissions were not significantly different

from conventional plots (Robertson et al., 2000). This

emphasizes the importance of N2O mitigation practices

in overall greenhouse gas emission reduction.

Li et al. (2005) used a biogeochemical model that

simulates the carbon and N cycle to conclude that NT

could result in increased N2O emissions (expressed in

CO2-equivalent) from a maize/soybean rotation in

Iowa, USA, offsetting 75% of the carbon sequestered.

The increase in N2O emissions was related to the

simulated increase in soil organic carbon content under

NT over the long term. This and other simulations using

process-based models for regions where freezing occurs

(Kaharabata et al., 2003; Grant et al., 2004) have not

considered different snow depth and residue cover

effects. But, as this study demonstrates, physical effects

during winter conditions associated with residue place-

ment on the soil surface can have a significant impact

on N2O emissions and should be considered in process-

based models.

Conclusion

Micrometeorological measurements of N2O fluxes from

a corn–soybean–winter wheat rotation managed with

two contrasting systems over 5 years, indicated that NT

combined with N fertilizing strategies that involved

applying N according to soil N reserves and matching

the time of application to crop uptake, significantly

reduced N2O emissions. As crop yields were not

significantly different in the two management systems,

0

20

40

60

0

1

2

3

N O

em

issi

on (

kg N

ha

)

<–10

–10

to –

5–5

to 0

0 to

5

5 to

10

10 to

15

15 to

20

20 to

25

>25

0

20

40

60

Fre

quen

cy (

%)

0

1

2

3

CP(a)

(b)

BMP

N O flux (g N ha day )

Fig. 6. Frequency distribution of measured daily N2O flux

(bars) over 5 years (2000–2005) for plots managed according to

conventional practices (CP) and best management practices

(BMP) measured during (a). The growing season (GS) (May–

October), and (b). Nongrowing season (NGS) (November

to April). The contribution of each frequency class to the total

N2O emissions over those periods is shown with lines (solid

for CP, dashed for BMP). Note that frequencies for class

425 g N ha�1 day�1 comprise values up to 255 g N ha�1 day�1.

Sum of N2O emission contribution from all classes corresponds

to total emission over 5 years.

N 2 O E M I S S I O N S F R O M C O N T R A S T I N G M A N A G E M E N T 1733

r 2007 The AuthorsJournal compilation r 2007 Blackwell Publishing Ltd, Global Change Biology, 13, 1722–1736

this means the BMP studied here had a lower N2O

output per dry weight of grain produced, when com-

pared with CP (365, 117 and 304 g N2O–N Mg�1 for

BMP vs. 430, 487 and 423 g N2O–N Mg�1 for corn,

winter-wheat and soybeans for CP, respectively).

The reduction in N2O emissions under BMP occurred

due to two main effects: (1) reduction in soil mineral N

content related to reduced fertilizer application in corn,

which offset any potential increases in emissions due to

higher moisture content under NT; and (2) lower degree

and intensity of freezing due to the insulating effects of

the snow cover plus corn and wheat residue in NT plots

during winter. The fertilization effect occurred mostly

during the GS (May–October) and accounted for an

average reduction of 0.16 kg N ha�1 (20% of the mean

annual reduction of 0.79 kg N ha�1). The freezing effect

was more significant accounting for 80% of total reduc-

tion, or 0.63 kg N ha�1 averaged over 5 years. While the

magnitude of reduction can not be extrapolated easily

to a large geographical region, we can conclude that

significant reductions in net greenhouse gas emissions

can be obtained when NT is combined with a strategy

that matches N application rate and timing to crop

needs. This study highlights the need for year-round

measurements over an extended period of time to cap-

ture the interaction of crop management and weather,

and the subsequent effect on N2O emissions. In addi-

tion, the change in winter soil freezing in the BMP

compared with CP treatment has important implica-

tions for modeling the effect of NT on N2O emissions in

cold climates. Our study demonstrates that the physical

effects associated with residue placement on the soil

surface should be considered in modeling efforts to

assess the long-term effects of NT on greenhouse gas

emissions from soils in cold climates.

Acknowledgements

Primary funding for the research was provided by the CanadianFoundation for Climate and Atmospheric Sciences (CFCAS), andthe Ontario Ministry of Agriculture and Food (OMAF). Addi-tional funding was provided by BIOCAP Canada and theClimate Change Funding Initiative in Agriculture (CCFIA) fromAgriculture Canada administered by Canadian Agri-Food Re-search Council. Robert Sweetman provided technical assistancefor this project. Sean Shaw, J. P. Bezeau, Alison Veale, KateTaillon and Karen Clark provided field assistance in variousaspects of the project.

Table 3 N2O emission from plots managed using conventional and best management practices, and difference between emissions

for the GS (May–October), NGS (November–April) of each year, and mean values for the study period spanning 2000–2005

Year* Period/crop

N2O emissionw (kg N ha�1)

Conventional Best management Differencez

2000/2001 May–October (corn) 1.216 (0.077) 0.999 (0.065) 0.217 (18%)

November–April 1.214 (0.057) 0.823 (0.008) 0.391 (32%)

All year 2.429 (0.021) 1.822 (0.058) 0.608 (25%)

2001/2002 May–October (soybean) 0.681 (0.057) 0.473 (0.013) 0.209 (31%)

November–April 0.419 (0.002) 0.491 (0.090) NS

All year 1.100 (0.059) 0.964 (0.103) 0.136 (12%)

2002/2003 May–October (wheat) 0.408 (0.034) 0.188 (0.127) 0.220 (54%)

November–April 2.910 (0.386) 0.700 (0.111) 2.21 (76%)

All year 3.318 (0.352) 0.888 (0.016) 2.43 (73%)

2003/2004 May–October (corn) 1.761 (0.204) 1.646 (0.161) 0.115 (6%)

November–April 0.916 (0.089) 0.530 (0.109) 0.386 (42%)

All year 2.677 (0.115) 2.176 (0.051) 0.501 (19%)

2004/2005 May–October (soybean) 0.310 (0.077) 0.282 (0.013) NS

November–April 1.132 (0.141) 0.904 (0.149) NS

All year 1.442 (0.218) 1.187 (0.136) NS

Mean May–October 0.875 (0.059) 0.718 (0.045) 0.158 (18%)

November–April 1.318 (0.134) 0.690 (0.090) 0.628 (48%)

All year 2.193 (0.075) 1.407 (0.046) 0.786 (36%)

Totals based on measured and interpolated data.

*Start of year was considered May 1 and end of year April 30 of following year.wMean from two monitored plots and SE in brackets.zEmissions from conventional minus best management plots, and amount expressed as percentage change in relation to

conventional management practices (in brackets), shown only for periods when median fluxes were significantly different

(Po0.05, Table 1).

NGS, nongrowing season; GS, growing season; N2O, nitrous oxide.

1734 C . WA G N E R - R I D D L E et al.

r 2007 The AuthorsJournal compilation r 2007 Blackwell Publishing Ltd, Global Change Biology, 13, 1722–1736

References

Arya SP (1988) Introduction to Micrometeorology. Academic Press,

London.

Aulakh MS, Doran JW, Mosier AR (1992) Soil denitrification –

significance, measurements and effects of management. Ad-

vances in Soil Science, 18, 1–57.

Aulakh MS, Rennie DA, Paul EA (1984) Gaseous nitrogen losses

from cropped and summer-fallowed soils. Canadian Journal of

Soil Science, 62, 187–196.

Ball BC, Horgan GW, Clayton H, Parker JP (1997) Spatial

variability of nitrous oxide fluxes and controlling soil and

topographic properties. Journal of Environmental Quality, 26,

1399–1409.

Bouwman AF (1990) Exchange of greenhouse gases between

terrestrial ecosystems and the atmosphere. In: Soils and the

Greenhouse Effect (ed. Bouwman AF), pp. 61–127. Wiley and

Sons, New York.

Bremner JM, Robbins SG, Blackmer AM (1980) Seasonal varia-

bility in emission of nitrous oxide from soil. Geophysical

Research Letters, 7, 641–644.

Burton DL, Beauchamp EG (1994) Profile of nitrous oxide and

carbon dioxide concentrations in a soil subject to freezing. Soil

Science Society America Journal, 58, 115–122.

Christensen S, Tiedje JM (1990) Brief and vigorous N2O produc-

tion by soil at spring thaw. Journal of Soil Science, 41, 1–4.

Cicerone R (1989) Analysis of sources and sinks of atmospheric

nitrous oxide. Journal of Geophysical Research, 94, 18265–18271.

Conrad R, Seiler W, Bunse G (1983) Factors influencing the loss

of fertilizer nitrogen into the atmosphere as N2O. Journal of

Geophysical Research, 88, 6709–6718.

Dorsch PA, Palojarvi A, Mommertz S (2004) Overwinter green-

house gas fluxes in two contrasting agricultural habitats.

Nutrient Cycling in Agroecosystems, 70, 117–133.

Drury CF, Tan CS, Welacky TW, Oloya TO, Hamill AS, Weaver

SE (1999) Red clover and tillage influence on soil temperature,

water content, and corn emergence. Agronomy Journal, 91,

101–108.

Dyer AJ, Hicks BB (1970) Flux-gradient relationships in the

constant flux layer. Quarterly Journal of Royal Meteorology

Society, 96, 715–721.

Firestone MK, Davidson EA (1989) Microbiological basis of NO

and N2O production and consumption in the soil. In: Exchange

of Trace Gases Between Terrestrial Ecosystems and the Atmosphere

(eds Andreae MO, Schimel DS), pp. 7–21. Wiley and Sons,

Chichester.

Follett RF, Shafer SR, Jawson MD, Franzluebbers AJ (2005)

Research and implementation needs to mitigate greenhouse

gas emissions from agriculture in the USA. Soil and Tillage

Research, 83, 159–166.

Gauer E, Shaykewich CF, Stobbe EH (1982) Temperature and

soil water under zero tillage in Manitoba. Canadian Journal

of Soil Science, 62, 311–325.

Grandy S, Loecke TD, Parr S, Robertson GP (2006) Long-term

trends in nitrous oxide emissions, soil nitrogen, and crop

yields of till and no-till cropping systems. Journal of Environ-

mental Quality, 35, 1487–1495.

Granli T, B�ckman OC (1994) Nitrous oxide from agriculture.

Norwegian Journal of Agricultural Sciences, 12, 1–127.

Grant B, Smith WN, Desjardins R, Lemke R, Li C (2004) Esti-

mated N2O and CO2 emissions as influenced by agricultural

practices in Canada. Climatic Change, 65, 315–332.

Helgason BL, Janzen HH, Chantigny MH et al. (2005) Toward

improved coefficients for predicting direct N2O emissions

from soil in Canadian agroecosytems. Nutrient Cycling in

Agroecosystems, 72, 87–99.

Holland JM (2004) The environmental consequences of adopting

conservation tillage in Europe: reviewing the evidence. Agri-

culture, Ecosystems and Environment, 103, 1–25.

IPCC. (2001) Climate Change 2001: The Scientific Basis. Cambridge

University Press, New York.

Jayasundara S, Wagner-Riddle C, Parkin G et al. (2006) Mineral

nitrogen dynamics and enhanced fertilizer nitrogen efficiency

in a corn-soybean-winter wheat rotation with best manage-

ment practices. Nutrient Cycling in Agroecosystems, doi:

10.1007/s10705-007-9103-9.

Jin HJ, Li SX, Cheng GD, Wang SL, Li X (2000) Permafrost and

climatic change in China. Global and Planetary Change, 26,

387–404.

Jorgensen RN, Jorgensen BJ, Nielsen NE (1998) N2O emission

immediately after rainfall in a dry stubble field. Soil Biology and

Biochemistry, 30, 545–546.

Kaharabata SK, Drury CF, Priesack E, Desjardins RL, McKenney

DJ, Tan CS, Reynolds D (2003) Comparing measured and

Expert-N predicted N2O emissions from conventional till

and no till corn treatments. Nutrient Cycling in Agroecosystems,

66, 107–118.

Kay BD, Grant CD, Groenevelt PH (1985) Significance of ground

freezing on soil bulk density under zero tillage. Soil Science

Society of America Journal, 49, 973–978.

Kay BD, VandenBygaart AJ (2002) Conservation tillage and

depth stratification of porosity and soil organic matter. Soil

and Tillage Research, 66, 107–118.

Kessavalou A, Mosier AR, Doran JW, Drijber RA, Lyon D,

Heinemeyer O (1998) Fluxes of carbon dioxide, nitrous oxide,

and methane in grass sod and winter wheat-fallow tillage

management. Journal of Environmental Quality, 27, 1094–1104.

Koponen HT, Flojt L, Martikainen TJ (2004) Nitrous oxide emis-

sions from agricultural soils at low temperature: a laboratory

microcosm study. Soil Biology and Biochemistry, 36, 757–766.

Lal R, Kimble JM, Follett RF (1998) Land use and soil C pools in

the terrestrial ecosystems. In: Management of Carbon Sequestra-

tion in Soil (eds Lal R, Kimble JM, Follett RF, Stewart BA),

pp. 1–10. CRC Press, Florida.

Leclerc MY, Thurtell GW (1990) Footprint prediction of scalar

fluxes using a Markovian analysis. Boundary-Layer Meteorology,

52, 247–258.

Lemke RL, Izaurralde RC, Nyborg M (1998) Seasonal distribu-

tion of nitrous oxide emissions from soil in the Parkland

region. Soil Science Society America Journal, 62, 1320–1326.

Lemke RL, Izaurralde RC, Nyborg M, Solberg ED (1999) Tillage

and N source influence soil-emitted nitrous oxide in the Alberta

Parkland region. Canadian Journal Soil Science, 79, 15–24.

Li CS, Frolking S, Butterbach-Bahl K (2005) Carbon sequestration

in arable soils is likely to increase nitrous oxide emissions,

offsetting reductions in climate radiative forcing. Climatic

Change, 72, 321–338.

N 2 O E M I S S I O N S F R O M C O N T R A S T I N G M A N A G E M E N T 1735

r 2007 The AuthorsJournal compilation r 2007 Blackwell Publishing Ltd, Global Change Biology, 13, 1722–1736

Liebig MA, Morgan JA, Reeder JD, Ellert BH, Gollany HT,

Schuman GE (2005) Greenhouse gas contributions and mitiga-

tion potential of agricultural practices in northwestern USA

and Western Canada. Soil and Tillage Research, 83, 25–52.

MacKenzie AF, Fan MX, Cadrin F (1997) Nitrous oxide emission

as affected by tillage, corn–soybean–alfalfa rotation and nitro-

gen fertilization. Canadian Journal of Soil Science, 77, 145–152.

Maggiotto SR, Webb J, Wagner-Riddle C, Thurtell GW (2000)

Nitrous and nitrogen oxide emissions from turfgrass receiving

different forms of nitrogen fertilizer. Journal of Environmental

Quality, 29, 621–630.

McCoy AJ, Parkin G, Wagner-Riddle C, Warland JS, Lauzon J,

von Bertoldi P, Jayasundara S (2006) Using automated soil

water content and temperature measurement to estimate soil

water budgets. Canadian Journal of Soil Science, 86, 47–56.

McKenney DJ, Wang SW, Drury CF, Findlay WI (1993) Denitri-

fication and mineralization in soil amended with legume,

grass and corn residues. Soil Science Society of America Journal,

57, 1013–1020.

McSwiney CP, Robertson GP (2005) Nonlinear response of N2O

flux to incremental fertilizer addition in a continuous maize (Zea

mays L.) cropping system. Global Change Biology, 11, 1712–1719.

Mosier AR (1994) Nitrous oxide summary. In: CH4 and N2O:

Global Emissions and Controls from Rice Fields and other Agricul-

tural and Industrial Sources (eds Minami K, Mosier A, Sass R),

pp. 135–139. NIAES, Tsukuba.

Mulvaney RL, Khan SA, Ellsworth TR (2006) Need for a soil-

based approach in managing nitrogen fertilizers for profitable

corn production. Soil Science Society Of America Journal, 70,

172–182.

Oke TR (1990) Boundary Layer Climates. Routledge, New York,

NY, USA.

Ontario Ministry of Agriculture Food and Rural Affairs (2002)

Agronomy Guide to Field Crops, Publication 811. Ministry of

Agriculture, Food and Rural Affairs, Toronto.

Parkin TB, Kaspar TC (2006) Nitrous oxide emissions from corn–

soybean systems in the Midwest. Journal of Environmental

Quality, 35, 1496–1506.

Paulson CA (1970) The mathematical representation of wind

speed and temperature profiles in the unstable atmospheric

surface layer. Journal of Applied Meteorology, 9, 857–861.

Robertson GP, Paul EA, Harwood RR (2000) Greenhouse gases in

intensive agriculture: contributions of individual gases to the

radiative forcing of the atmosphere. Science, 289, 1922–1925.

Rover M, Heinemeyer O, Kaiser EA (1998) Microbial induced

nitrous oxide emissions from an arable soil during winter. Soil

Biology and Biochemistry, 14, 1859–1865.

Sehy UJ, Dyckmans J, Ruser R, Munch JC (2004) Adding dis-

solved organic carbon to simulate freeze–thaw related N2O

emissions from soils. Journal of Plant Nutrition and Soil Science,

167, 471–478.

Sharma S, Szele Z, Schilling R, Munch JC, Schloter M (2006)

Influence of freeze–thaw stress on the structure and func-

tion of microbial communities and denitrifying popula-

tions in soil. Applied and Environmental Microbiology, 72,

2148–2154.

Six J, Ogle SM, Breidt FJ, Conant RT, Mosier AR, Paustian K

(2004) The potential to mitigate global warming with no-tillage

management is only realized when practised in the long term.

Global Change Biology, 10, 155–160.

Staley TE, Caskey WH, Boyer DG (1990) Soil denitrification and

nitrification potentials during the growing season relative to

tillage. Soil Science Society of America Journal, 54, 1602–1608.

Steel RGD, Torrie JH, Dickey DA (1997) Principles and Procedures

of Statistics: A Biometrical Approach, 3rd edn. McGraw-Hill,

New York.

Teepe R, Brumme R, Beese F (2000) Nitrous oxide emissions from

frozen soils under agricultural, fallow and forest land. Soil

Biology and Biochemistry, 32, 1807–1810.

Teepe R, Brumme R, Beese F (2001) Nitrous oxide emissions from

soil during freezing and thawing periods. Soil Biology and

Biochemistry, 33, 1269–1275.

Teepe R, Vor A, Beese F, Ludwig B (2004) Emissions of N2O from

soils during cycles of freezing and thawing and duration of

freezing. European Journal of Soil Science, 55, 357–365.

van Bochove E, Jones HG, Bertrand N, Prevost D (2000) Winter

fluxes of greenhouse gases from snow-covered agricultural

soil: intra-annual and interannual variations. Global Biogeo-

chemical Cycles, 14, 113–125.

Wagner-Riddle C, Thurtell GW (1998) Nitrous oxide emissions

from agricultural fields during winter and spring thaw

as affected by management practices. Nutrient Cycling in

Agroecosystems, 52, 151–163.

Wagner-Riddle C, Thurtell GW, Edwards GC (2005) Trace gas

concentration measurements for micrometeorological flux

quantification. In: Micrometeorology in Agricultural Systems

(eds Hatfield JL, Baker JM), pp. 321–344. Agronomy Mono-

graph no. 47 ASA-CSSA-SSSA, Madison, WI, USA.

Wagner-Riddle C, Thurtell GW, Kidd GK, Beauchamp EG,

Sweetman R (1997) Estimates of nitrous oxide emissions from

agricultural fields over 28 months. Canadian Journal Soil

Science, 77, 135–144.

Wagner-Riddle C, Thurtell GW, King KM, Kidd GE, Beauchamp

EG (1996) Nitrous oxide and carbon dioxide fluxes from a bare

soil using a micrometeorological approach. Journal of Environ-

mental Quality, 25, 898–907.

Williams PJ, Smith MW (1989) The Frozen Earth: Fundamentals

of Geocryology. Cambridge University Press, Cambridge.

Yates TT, Si BC, Farrell RE, Pennock DJ (2006) Probability

distribution and spatial dependence of nitrous oxide emission:

temporal change in hummocky terrain. Soil Science Society

of America Journal, 70, 753–762.

1736 C . WA G N E R - R I D D L E et al.

r 2007 The AuthorsJournal compilation r 2007 Blackwell Publishing Ltd, Global Change Biology, 13, 1722–1736