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Agriculture, Ecosystems and Environment 164 (2013) 190– 199

Contents lists available at SciVerse ScienceDirect

Agriculture, Ecosystems and Environment

jo u r n al hom ep age: www.elsev ier .com/ locate /agee

luxes of nitrous oxide in tilled and no-tilled boreal arable soils

atta Sheehya,b,∗, Johan Sixb, Laura Alakukkuc, Kristiina Reginaa

MTT Agrifood Research Finland, Plant Production Research, FI-31600 Jokioinen, FinlandDepartment of Plant Sciences, University of California, Davis, CA, USADepartment of Agricultural Sciences, University of Helsinki, Finland

r t i c l e i n f o

rticle history:eceived 23 December 2011eceived in revised form 11 October 2012ccepted 18 October 2012vailable online 4 December 2012

eywords:o-tillirect drillingonventional tillage

a b s t r a c t

Agricultural management practices can have a significant effect on the emissions of nitrous oxide (N2O)from soils. The aim of this 2-year study was to investigate the effects of no-till (NT) and reduced tillage(RT) practices on annual fluxes of N2O from different soil types typical for the boreal region of northernEurope. We measured the fluxes of N2O in conventional tillage (CT) and NT at four sites of which twoalso had RT treatment. No-till and RT practices had been implemented 8–10 years before our study wasinitiated. Chamber measurements were carried out fortnightly in 2008–2010 on clayey (sites 1–3) andcoarse (site 4) soils. Annual cumulative emissions of N2O varied from 2.4 to 8.3 in CT, 2.5–6.5 in RT and4.9–10.2 kg N2O-N ha−1 in NT. High peaks in measured N2O fluxes occurred during and after thawing ofthe soil in April and after fertilization and high rain events. No-till or RT did not have any significant

educed tillageitrous oxide

effects on soil C or N stocks or potential denitrification of the 0–20 cm soil layer. Dry bulk density andwater-filled pore space (WFPS) were generally higher under NT compared to CT, most probably beingthe main reasons for the increased N2O emissions in the NT systems. Soil temperature varied less in NTby being higher during the colder periods of the year and slightly cooler during hot summer days. Inconclusion, our results indicate that NT induces a risk of increased N2O emissions in clayey soils in smallgrain spring cereal agroecosystems in Northern European boreal climate.

. Introduction

Agricultural lands are a major contributor to global anthro-ogenic emissions of greenhouse gases and accounted for about0% anthropogenic N2O emissions in 2005 thus enhancing globallimatic change (Smith et al., 2007). Most of this increase in N2Omissions is due to enhanced microbial N2O production that is asso-iated with human perturbations to the nitrogen cycle. Most of the2O produced during microbial processes in soils is released to thetmosphere as so called direct emissions from fields but part islso from indirect emissions, e.g. N2O produced from leached orolatilized nitrogen (Nevison, 2000). The main microbial processesontributing to N2O emissions are anaerobic denitrification whichs reduction of nitrate to N2O and N2 and aerobic nitrification thatroduces N2O as a side product when O2 is limited in the soil (Doran,980; Groffman, 1984; Six et al., 2002; Smith et al., 2003). The need

o find ways of reducing greenhouse gas emissions from arable soilss evident. No-till (NT) management is considered a practice thatelps preserve water and carbon in the surface layer of the soil (Lal,

∗ Corresponding author at: MTT Agrifood Research Finland, Plant Productionesearch, FI-31600 Jokioinen, Finland. Tel.: +358 0295300700/+1 7077040998;

ax: +358 20772040.E-mail address: jatta.sheehy@mtt.fi (J. Sheehy).

167-8809/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.agee.2012.10.007

© 2012 Elsevier B.V. All rights reserved.

1997), as well as saving fuel and labor compared to conventionaltillage (CT). In comparison to CT soils, NT soils often have a moredense structure (Schjønning and Rasmussen, 2000; Tebrügge andDüring, 1999) and higher moisture content (Gregorich et al., 2008;Sharratt, 1996), hence favoring the activity of anaerobic denitrify-ing bacteria.

Since N2O is as a greenhouse gas approximately 300 times morepotent than CO2, it has been estimated that its increase in NT couldoffset 75–310% of the advantage gained from carbon sequestrationunder NT (Li et al., 2005). Field measurements have shown bothdecreased and increased emissions of N2O under NT (Aulakh et al.,1984; Ball et al., 1999; Chatskikh and Olesen, 2007; Kaharabataet al., 2003; MacKenzie et al., 1998; Ussiri et al., 2009), demon-strating great variability depending on different soil and climaticconditions. Mathematical models have also tried to capture theeffect of different management practices on N2O emissions withvarying results (Li et al., 1996; Mummey et al., 1998). The differentduration of the experiments may partly explain the contradictingobservations; Six et al. (2004) reported results of a meta-analysisindicating increased N2O emissions during the first years after con-verting to NT, with emissions reducing back to normal levels or less

in humid climate after 20 years of NT. Rochette (2008), on the otherhand, argued that N2O fluxes only increase from poorly aerated soilsunder NT, especially in cool humid climates. Grandy et al. (2006)found that N2O fluxes did not increase under NT in a 10-year study

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J. Sheehy et al. / Agriculture, Ecosyste

nd only offset 56–61% of the carbon sequestered on loamy soils inouth-western Michigan.

Reduced tillage (RT, shallow stubble cultivation) is a treatmentetween CT and NT treatments disturbing the soil to a shallowerepth than CT. Effects of RT on N2O emissions vary between differ-nt studies of which some report no change (Abdalla et al., 2010;lmi et al., 2003) and some increased emissions (Beheydt et al.,008) or decreased emissions under RT compared to CT (Chatskikht al., 2008).

At a global scale, the adoption of NT management has increasedore than 230% during the last 10 years, reaching 111 million ha

n 2009 (Derpsch et al., 2010). In Europe, the adoption of NT haseen clearly slower than, for instance, in North and South AmericaSoane et al., 2012). In Finland, the area of agricultural land con-erted to NT has, however, increased rapidly and is estimated toave reached over 150,000 ha in only a little over 10 years after itas introduced to farmers (Tike, 2011), and being relatively one of

he highest in Europe at present. About 13% of the annually sownrea is now under NT and 25% under RT. The reduced cost of fos-il fuel and labor compared to CT has promoted adoption of NTe.g. Derpsch et al., 2010). No-till has been reported to be advan-ageous with respect to erosion control in boreal areas (Børresennd Uhlén, 1991; Puustinen et al., 2007) and to reduce nitrogeneaching from arable fields (Syswerda et al., 2012). As one of the

easures to decrease erosion and nutrient transport from lando watercourses, Finnish Agri-Environmental Programme and theccompanying Support Scheme encourage increasing the crop orrop residue covered area outside the growing season when ero-ion and particulate phosphorus leaching peak (Puustinen et al.,007).

It was estimated that if all the European agricultural soils thatould be converted to NT adopted this management practice, N2Omissions would increase to 20.5 Tg of carbon equivalents emit-ed per year (Smith et al., 2001). However, data on the effects ofifferent management practices on emissions of N2O in boreal cli-ate is scarce. The aim of this 2-year study was to estimate theagnitude of N2O fluxes in CT, NT and RT treatment on soil types

ypical for the boreal region of northern Europe. Measurementsf several environmental and soil parameters were taken to elu-idate the underlying factors controlling N2O emissions under theifferent management practices.

. Materials and methods

.1. Study sites and soil parameters

This study took place on four pairs of CT and NT fields inouthwestern Finland from June 2008 to June 2010. Two fieldssites 1 and 2) were located in Jokioinen (60◦49′N and 23◦30′E).ne field (site 3) was located in Vihti (60◦21′N and 24◦22′E)nd one field pair (site 4) in Säkylä (60◦58′N and 22◦31′E). Sites–3 were field experiments (randomized complete-block designith four replicates) and the fields of site 4 belong to neighbor-

ng farmers (four replicated measurements sites/field, plot size00-250 m2). Soils at sites 1–3 were classified as Vertic Cam-isol and at site 4 as Eutric Regosol (FAO, 2006). Soil propertiesnd other site parameters are shown in Table 1. All sites had CTnd NT treatments. In addition, sites 1 and 2 had plots with RTreatment.

No-till practice, in which the crop was sown without prior soilillage, had been used at the study sites for 8–10 years before our

tudy begun. In autumn (September/October), the soil was mould-oard ploughed to a depth of 20–25 cm on CT plots and soil wastubble cultivated with tined cultivator to 10–15 cm on RT plots.n spring, CT and RT plots were first leveled by a harrow and then

Environment 164 (2013) 190– 199 191

rotary or S-tine harrowed to 4–5 cm depth to prepare the seedbed.Spring barley (Hordeum vulgare) was cultivated at sites 1, 2 and 4.Spring oilseed rape (Brassica rapa subsp. oleifera) was cultivated atsite 3 during the 2008 growing season and spring wheat (Triticumaestivum) during the growing season of 2009. All treatments weresown and fertilized in May. Conventional tillage and RT were sownwith combined drill (shoe coulters) which placed seed (to 4–5 cmdepth) and fertilizer (7–8 cm) at the same time in separate rows.The seed and fertilizer row spacing was 12.5 and 25 cm, respec-tively, and had rolling wheels behind the drill (sites 3 and 4). No-tillwas directly sown to 3–5 cm depth with combined drill havingtriple disc coulters (site 4, row space 15 cm, front single disc coulteris tilling and rear double disc coulter is sowing with roller wheelsbehind the sowing coulters), double disc coulters (sites 1 and 2,row space 14.5 cm, packing wheels behind the drill) or single disccoulters (site 3, row space 12.5 cm, rolling wheels behind the drill).The direct drills placed the seeds and fertilizer in the same row. Thewhole annual fertilizer application (Table 1) was made during sow-ing. Granular ammonium nitrate NPK fertilizer was used, except atsite 4 where liquid fertilizer (UAN 32) was used in the NT plots andthe fertilizing level was lower compared to CT according to farmersexperience on the need of N fertilizer on this plot. At site 4, sam-ples were taken from the fields of private farmers cultivating andfertilizing the field according to their own frameworks. With theexception of tillage and drilling, field operations were carried outfollowing the common farming practice in Finland. The sites wereharvested in August.

Soil samples for the determination of total C and N, soil bulk den-sity and potential denitrification were taken from the 0–20 cm soillayer once in 2009. Samples for mineral N were taken in the springand fall of 2008 and three times in 2009 from the 0–20 cm soil layer.Soil particle size, porosity, total C and N, ammonium and nitrate insoil as well as frost and the amount of earthworm burrows weredetermined as in Regina and Alakukku (2010). Soil porosity wasmeasured from three undisturbed soil core samples per plot. Poresof more than 30 �m in diameter were classified as macropores andpores with a diameter of 0.2 �m or less were considered micro-pores. Samples for total C and N were air dried and analyzed witha CN analyzer (CN-2000, Leco Corp, St Joseph, MI, USA). Mineralnitrogen (NO3

− and NH4+) was analyzed by mixing a subsample

of 100 g with 250 ml of 2 M KCl, then shaken for 2 h on an orbitalshaker after which the extracts were filtered and finally analyzedafter storage of 24 h in 4 ◦C. Depth of frost was measured with PVCtubes inserted in the soil in vertical pipes from all study sites. Thesetubes were filled with dilute methylene blue solution that turnscolorless in temperatures below 0 ◦C thus indicating the locationof the frozen soil layer (Richard and Brown, 1972). Estimation ofthe changes in soil C stocks was based on the equivalent soil massmethod according to Ellert and Bettany (1995) and Lee et al. (2009).Soil temperature was measured using loggers (ElcoLog, ElcoplastOy, Finland, measurement frequency 4 times/day) at a depth of5 cm. The loggers were read twice a year, after tillage in the fall andbefore sowing in the spring. Soil moisture content was determinedat a depth of 15 cm with a TRASE-TDR (Soil Moisture EquipmentCorp., CA, USA), except for site 3 where soil moisture content wasnot measured. The proportion of water filled pore space (WFPS)was calculated by dividing the volumetric soil water content bytotal porosity. Potential denitrification in the soil was determinedas described in Kanerva et al. (2005). Three 10 g subsamples weretaken from each soil sample and amended with 1 mL of KNO3 solu-tion and 1 mL of glucose solution. The flasks were evacuated andflushed three times with helium. Acetylene (10%) was used to block

N2O reduction. The flasks were incubated at 25 ◦C and gas sampleswere taken at 30, 60 and 90 min and analyzed using a HP 6890Series gas chromatograph (Agilent Technologies, Santa Clara, CA,USA).

192 J. Sheehy et al. / Agriculture, Ecosystems and Environment 164 (2013) 190– 199

Table 1Mean climatic parameters and soil properties (±SE when available) at the experimental sites.

Site 1 Site 2 Site 3 Site 4

CT NT CT NT CT NT CT NT

FAO classification Vertic Cambisol Vertic Cambisol Vertic Cambisol Eutric RegosolYears in NTa 8 9 8 10MAP (mm)b 607 607 626 618MAT (◦C)b 4.3 4.3 4.5 4.2Fertilizer 2008 (kg N ha−1 yr−1) 110 110 105 95 80Fertilizer 2009 (kg N ha−1 yr−1) 100 100 105 83 70Fertilizer 2010 (kg N ha−1 yr−1) 100 100 120 96 80Crop 2008 Spring barley Spring barley Spring oilseed rape Spring barleyCrop 2009 Spring barley Spring barley Spring wheat Spring barleyParticle fractionsc

Clay (<2 �m) 46 62 48 19Silt (2–20 �m) 29 19 34 30FS (20–200 �m) 14 11 13 34CS (>200 �m) 11 8 5 17

BD (g cm−3)c

0–5 cm 1.33 1.31 1.14 1.11 1.25 1.20 1.16 1.155–10 cm 1.35 1.33 1.31 1.27 1.30 1.38 1.36 1.3510–20 cm 1.28 1.32 1.29 1.21 1.24 1.40 1.33 1.42

Macroporosity (m3 100 m−3)c,d 11.9 ± 1.1 10.2 ± 1.9 6.4 ± 0.3 9.1* ± 0.9 9.7 ± 0.9 5.9* ± 0.3 13.6 ± 2.5 9.3 ± 0.3Microporosity (m3 100 m−3)c,d 26.3 ± 0.6 25.2 ± 1.3 27.6 ± 0.6 25.9 ± 0.3 24.5 ± 1.0 26.6 ± 0.5 16.9 ± 0.8 20.9* ± 0.5Earthworm burrows (per m2)c,d 37 ± 38 358* ± 50 19 ± 19 170 ± 65 132 ± 82 150 ± 82 113 ± 113 434 ± 161

a At the beginning of the study in year 2008; CT: conventional tillage; NT: no-till.b Reference period 1971–2000 (Drebs et al., 2002); MAP: mean annual precipitation; MAT: mean annual temperature.c At 0–20 cm soil layer; particle fraction shown as percentages; earthworm burrows at 20 cm depth; FS: fine sand, CS: coarse sand, BD: bulk density. Particle fractions as a

mean of four samples from the experimental area.d Macroporosity pores >30 �m; microporosity <0.2 �m; earthworm burrows: cylindrical pores diameter ≥2 mm (Regina and Alakukku, 2010).* Statistically significant difference (p < 0.05) between CT and NT within a study site.

Fig. 1. Daily values of air temperature (on the left axis) and precipitation (on the right axis) at the study sites.

J. Sheehy et al. / Agriculture, Ecosystems and Environment 164 (2013) 190– 199 193

Table 2Total C and N (t ha−1) and potential denitrification (�g N g−1 h−1) ±SE of study sites.

Site 1 Site 2 Site 3 Site 4

Total carbona

CT 61.7 ± 2.8a 67.1 ± 1.6a 92.8 ± 3.9a 58.5 ± 0.5aNT 65.4 ± 3.1a 67.4 ± 1.0a 82.1 ± 6.1a 56.0 ± 1.5aRT 61.6 ± 3.8aAverage 62.9 ± 3.1 67.2 ± 1.2 87.4 ± 5.5 57.3 ± 1.3

Total nitrogena

CT 4.7 ± 0.2a 4.2 ± 0.1a 6.3 ± 0.3a 2.9 ± 0.1aNT 5.0 ± 0.1a 4.3 ± 0.1a 5.7 ± 0.5a 3.0 ± 0.1aRT 4.7 ± 0.3aAverage 4.8 ± 0.2 4.3 ± 0.1 6.0 ± 0.4 2.9 ± 0.1

Potential denitrificationa

CT 0.38 ± 0.03a 0.47 ± 0.06a 0.32 ± 0.01a 0.21 ± 0.02aNT 0.42 ± 0.03a 0.46 ± 0.02a 0.33 ± 0.02a 0.19 ± 0.01aAverage 0.40 ± 0.03 0.47 ± 0.02 0.32 ± 0.01 0.20 ± 0.01

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T: conventional tillage; NT: no-till; RT: reduced tillage. Statistically significant diffa At 0–20 cm soil layer.

Precipitation and air temperature data were obtained fromearby meteorological stations (Finnish Meteorological Institute)Fig. 1). The stations were located 1, 7 and 30 km away from sites–2, 3 and 4, respectively. Annual precipitation during the firstxperimental year was higher at sites 1, 2 and 4 but lower at site

compared to the 30-year average (Drebs et al., 2002). During theecond year, the precipitation was higher at site 3 and lower at site

compared to the means while precipitation at sites 1 and 2 waslose to average. However, the precipitation during the summeronths was higher in the first year at all experimental sites com-

ared to the second year of measurements. Mean temperature wassually higher at site 3 compared to average years while similar toverage years at all other sites.

.2. Flux measurements

A closed chamber technique was used to measure N2O fluxes athe different study sites. The method was adapted from Regina andlakukku (2010). One base frame made of steel (60 cm × 60 cm) for

he gas collection chambers was installed in each of the four repli-ate plots. The measurements were done with aluminum chambers,ach closed with a water seal that was formed when a groove onhe upper end of the frame was filled with water to ensure the gas-ightness of the chamber. In winter, NaCl was added to the water tovoid ice formation. At site 4 the sampling was made along a tran-ect in each field with 30 m between each replicate. The base framesere removed only during field operations, like tillage or harvest-

ng, and installed back at the same locations afterwards. Ventingubes were installed on the chambers to minimize pressure changesHutchinson and Livingston, 2001). The emissions were measurediweekly, one field during 1 day between 10 a.m. and 2 p.m. andll fields during the same week. By the end of October 2009, weade more frequent measurements to study the short-term effects

f tillage operations on N2O fluxes at sites 1 and 2. The first mea-urements were done between 6 and 10 h after tillage operations,ollowed by measurements 24 h, 48 h, 4 and 7 days after soil tillage.

To measure N2O flux, three gas samples were taken during a0-min chamber enclosure at 0, 15 and 30 min. At each time point,

20 ml sample was taken with a plastic syringe (Becton, Dickinsonnd Company, Franklin Lakes, NJ, USA) and transferred immedi-tely into a pre-evacuated 12 ml glass vial (Exetainer, Labco Ltd.,igh Wycombe, UK). A vacuum pump (Speedivac 2, Edwards) with

pressure meter was used to evacuate 12 vials at a time. The

ials were evacuated to the pressure of <2 kPa. Gas samples werenalyzed within 72 h of sampling using a HP 6890 Series gas chro-atograph. For more details of the GC system see Regina et al.

2004). Standard gas mixture (AGA Gas AB, Lidingö, Sweden) of

es (p < 0.05) are represented with a letter (a, b) within each site.

known concentrations of N2O was used for diluting a calibrationcurve with seven points. The gas production rate was calculatedusing the equation:

F = �C

�t× V

A(1)

where C is the concentration of N2O, t is time, V is chamber vol-ume, and A is the chamber area. Calculation of the amount of N inthe sample was based on the ideal gas equation. The chamber tem-perature was measured during each gas sampling. The cumulativeannual fluxes for each management practice were calculated bylinearly interpolating the emissions between consecutive samplingdays. The fluxes were averaged by calculating the cumulative fluxvalue for each chamber separately and then averaging the treat-ments in each field. The emission rates of N2O were calculated asthe mass of N in N2O (N2O-N).

2.3. Statistical analysis

Statistical differences between the different management prac-tices were determined with linear analysis of variance (ANOVA).Tukey’s test was used for separation of means to explore differencesbetween treatments when there were more than two treatments inone site. Paired-samples T-test was used to compare soil and envi-ronmental parameters in different treatments. Pearson correlationcoefficients were determined for the cumulative N2O emissions ofeach chamber, total N and C content, bulk density, microporosity,macroporosity, soil mineral N content, potential denitrification andthe amount of earthworm burrows. Pearson correlation coefficientswere also determined for the daily gas fluxes, air temperature,precipitation, WFPS at 15 cm and soil temperature at 5 cm. Log-transformed values of the variables were used when necessary tonormalize the distribution. PASW Statistics 18.3 (IBM Corporation,Somers, NY, United States) was used for the statistical analyses ofthe data.

3. Results

3.1. Soil and environmental parameters

Differences in soil C stocks between the treatments were notconsistent (Table 2). In NT, either an increase, decrease or no changecompared to CT occurred after 9–11 years of NT but none of the dif-

ferences were statistically significant. No difference in soil C stockwas observed between RT and CT at site 1. Soil C stocks were greaterat site 3 compared to the other sites. There were no significant dif-ferences in the potential denitrification between the treatments

194 J. Sheehy et al. / Agriculture, Ecosystems and Environment 164 (2013) 190– 199

Table 3NO3

− and NH4+ content (mg N kg dry soil−1 ± SE) in the 0–20 cm layer of the soil.

May 2008 October 2008 June/July 2009 August 2009 October 2009

NitrateSite 1 CT 31.9 ± 12.3a 4.8 ± 0.6a 32.5 ± 9.0a 4.2 ± 1.5a 6.4 ± 0.7a

NT 24.7 ± 7.1a 5.4 ± 0.7a 17.8 ± 1.9a 1.0 ± 0.2a 5.3 ± 0.3aAverage 30.8 ± 9.2 4.6 ± 0.5 25.1 ± 7.2 2.6 ± 1.3 5.8 ± 0.5

Site 2 CT 26.0 ± 2.7a 5.3 ± 0.9a 11.7 ± 3.7a 1.0 ± 0.1a 5.2 ± 0.8aNT 40.3 ± 3.2a 2.8 ± 0.8a 11.9 ± 3.6a 1.0 ± 0.2a 4.4 ± 0.6aAverage 33.2 ± 4.7 4.0 ± 1.0 11.8 ± 3.3 1.0 ± 0.1 4.8 ± 0.7

Site 3 CT 35.8 ± 9.8a 1.9 ± 0.7a 11.8 ± 0.9a 0.8 ± 0.1a 2.6 ± 0.3aNT 43.3 ± 0.8a 1.2 ± 0.0a 12.1 ± 0.2a 0.7 ± 0.1a 2.6 ± 0.1aAverage 39.5 ± 5.2 1.5 ± 0.4 12.0 ± 0.5 0.7 ± 0.1 2.6 ± 0.1

Site 4 CT 26.6 ± 2.7a 2.0 ± 0.3a 3.0 ± 1.8a 0.7 ± 0.1a 3.8 ± 0.3aNT 21.8 ± 3.9a 3.0 ± 0.6a 5.0 ± 1.9a 0.7 ± 0.2a 2.5 ± 0.1bAverage 24.2 ± 3.1 2.5 ± 0.5 4.0 ± 1.7 0.7 ± 0.2 3.2 ± 0.4

AmmoniumSite 1 CT 25.7 ± 20.9a 1.9 ± 0.1a 7.6 ± 2.4a 0.8 ± 0.1a 1.2 ± 0.2a

NT 13.5 ± 6.2a 1.6 ± 0.1a 2.1 ± 0.1a 1.2 ± 0.1a 1.8 ± 0.1bAverage 16.5 ± 14.6 1.8 ± 0.2 4.8 ± 2.2 1.0 ± 0.1 1.5 ± 0.2

Site 2 CT 5.8 ± 2.0a 1.2 ± 0.2a 1.7 ± 0.4a 0.9 ± 0.1a 1.2 ± 0.2aNT 7.8 ± 4.9a 1.6 ± 0.1a 1.4 ± 0.2a 1.1 ± 0.1a 1.3 ± 0.1aAverage 6.8 ± 3.5 1.4 ± 0.1 1.6 ± 0.3 1.0 ± 0.1 1.2 ± 0.2

Site 3 CT 13.2 ± 1.1a 1.0 ± 0.0a 1.4 ± 0.2a 1.1 ± 0.0a 1.3 ± 0.0aNT 12.1 ± 2.3a 1.3 ± 0.4a 1.5 ± 0.1a 1.2 ± 0.0a 1.8 ± 0.2bAverage 12.6 ± 1.3 1.2 ± 0.2 1.4 ± 0.1 1.2 ± 0.0 1.5 ± 0.2

Site 4 CT 13.5 ± 3.0a 1.9 ± 0.3a 2.1 ± 0.2a 1.8 ± 0.1a 1.5 ± 0.1aNT 11.7 ± 4.8a 1.8 ± 0.1a 2.7 ± 0.1b 2.2 ± 0.2a 2.0 ± 0.1b

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T: conventional tillage; NT: no-till. Statistically significant differences (p < 0.05) ar

ut statistically significant differences were found between differ-nt study sites (p = 0.000–0.019), the values being greatest at site 2nd lowest at site 4 (Table 2). The biggest difference in dry bulk den-ity between NT and CT was found at site 3 where bulk density was% higher in NT compared to CT, but no statistically significant dif-erences were found between management practices (Table 1). Themount of earthworm burrows was higher in NT in all experimentalelds, with a statistically significant difference at site 1 (p = 0.042).ignificant difference in soil macroporosity was only found at site

where macroporosity was significantly greater (p = 0.040) in NTompared to CT. No-tillage increased microporosity significantlyt site 4 (p = 0.014). There were no consistent differences betweenhe treatments in NO3

− and NH4+ content in the top 0–20 cm soil

Table 3). Highest values of NO3− after harvest were discovered at

ite 2 in 2008 and site 1 in 2009 and the lowest values at site 3 inoth studied years. Significant differences in NO3

− between differ-nt treatments were only found in October 2009 at site 4. In 2009,ll fields had more NH4

+ left after harvest in NT compared to CTreatments. Significant differences in the NH4

+ levels were foundt sites 1, 3 and 4.

No-tillage increased WFPS compared to CT during the growingeasons of 2008 and 2009 and was significantly different betweenanagement practices even though temporal variation was rela-

ively high (Fig. 2). The highest peak of 87% WFPS was found at site after a heavy rainfall event. The average WFPS was between 45nd 59% in CT while the average values in NT were between 53 and1% (Table 4). The frost went deeper and developed faster in theall in CT compared to NT. Depth of frost in RT was between thatf CT and NT. Soil temperature varied less in NT by being higheruring the colder periods of the year and slightly cooler during hotummer days (Fig. 3).

.2. N2O emissions

The lowest hourly flux was observed at site 2 where the flux wasypically below 0.05 mg N2O-N m−2 h−1 (Fig. 4). In general, averageuxes ranged from 0.05 to 0.14 mg N2O-N m−2 h−1 for the clayeyoils and from 0.06 to 0.09 mg N2O-N m−2 h−1 for the coarse soil.

2.4 ± 0.2 2.0 ± 0.2 1.7 ± 0.2

esented with a letter (a, b) within each site and measurement time.

Largest fluxes were observed during the months of April to July.High peaks in April are due to snow melt and thawing of frozensoil. Larger peaks in May and June occurred after sowing and fer-tilization and rain. Smaller peaks in N2O fluxes were observed alsoafter individual high rain events. Tillage in the autumn of 2009 didnot have any statistically significant effects on N2O emissions.

Annual cumulative emissions of N2O for years 2008–2009 and2009–2010 ranged from 2.4 kg N2O-N ha−1 to 10.2 kg N2O-N ha−1

(Table 5). The lowest annual N2O flux was found in the first year inCT at site 2 and the greatest in the second year in NT at site 3. Sta-tistically significant differences between treatments were found atall sites across both studied years. The largest difference in annualN2O fluxes between management practices was observed in thefirst year at site 1 where N2O emissions were 150% and 90% greaterin NT soils compared to their CT (p = 0.000) and RT (p = 0.001) coun-terparts, respectively. Emissions of N2O were significantly higherin NT compared to CT at site 3 (p = 0.001). At site 4, N2O emissionswere lower in NT compared to CT soils (p = 0.037). Cumulative N2Oemissions in RT were significantly lower compared to NT at site 1(p = 0.001) but no significant differences were found at site 2 acrossboth studied years.

Daily values of N2O fluxes correlated strongly with WFPS(r = 0.554, p < 0.001). Statistically significant correlations did notexist with daily air or soil temperature or precipitation. Soilbulk density and earthworm burrows had statistically significantpositive correlations with the cumulative N2O fluxes (r = 0.406,p < 0.001; r = 0.346, p < 0.05) while NO3

− had a negative and NH4+ a

positive correlation (r = −0.315, p < 0.05; r = 0.325, p < 0.05). Poten-tial denitrification had a significant negative correlation with N2Ofluxes (r = −0.302, p < 0.05).

4. Discussion

Emissions of N2O were clearly increased under NT on clayey

soils compared to CT whereas the emissions from RT were similarto those from CT. Thus, for the clayey soils our results are con-sistent with the results from Canada compiled by Gregorich et al.(2005) indicating that NT increases N2O emissions from clayey soils

J. Sheehy et al. / Agriculture, Ecosystems and Environment 164 (2013) 190– 199 195

Fig. 2. Water-filled pore space (WFPS; %) and frost (cm) at sites 1–2 and 4 in conventional tillage (CT), no-till (NT) and reduced tillage (RT). WFPS was measured at 15 cm.Frost is presented at an average depth considering that during the winter frosting starts from the top and during the spring frost stays deeper while melting from the toplayer.

Table 4Average water-filled pore space (WFPS, %) ±SE for CT and NT and the whole study site in the layer of 0–15 cm in the growing seasons of 2008 and 2009.

2008 2009

CT NT Average CT NT Average

Site 1 47.6 ± 3.4a 60.9 ± 2.9b 54.2 ± 2.4 45.5 ± 1.7a 58.1 ± 2.1b 51.8 ± 1.8Site 2 59.5 ± 2.7a 71.3 ± 3.1b 65.4 ± 2.3 56.8 ± 2.1a 62.4 ± 2.9b 59.6 ± 1.9Site 4 47.7 ± 5.1a 59.3 ± 2.9b 53.5 ± 3.1 47.2 ± 2.1a 53.3 ± 2.8a 50.2 ± 1.8

C tweens

iaadfc

TC

Ct

T: conventional tillage; NT: no-till. Statistically significant differences (p < 0.05) betudy years separately.

n a humid climate. Six et al. (2004) came to the same conclusionnd added that the increase is most distinct during the first decadefter converting to NT, but the emissions from NT systems might

ecrease after that. Our sites have been under NT approximatelyor 10 years and may have still been experiencing the changes asso-iated with conversion. However, in an experiment in France the

able 5umulative N2O fluxes (kg ha−1 yr−1 ± SE).

Year 1

CT NT RT

Site 1 3.9 ± 0.7a 9.8 ± 1.0b 5.2 ± 1.0a

Site 2 2.4 ± 0.6a 6.0 ± 0.7b 2.5 ± 0.6ab

Site 3 5.3 ± 0.2a 8.4 ± 1.1b

Site 4 7.5 ± 0.9a 6.6 ± 1.3b

T: conventional tillage; NT: no-till; RT: reduced tillage. Statistically significant differenche compiled statistics across the whole 2-year study period.

CT and NT treatments are represented with a letter (a, b) within each site and both

N2O emissions did not decrease and were consistently higher in NTcompared to CT 32 years after the conversion (Oorts et al., 2007).N O flux data from site 4 with coarse soil texture, however, gave

2contrasting results, i.e. the emissions in NT were lower both yearscompared to CT. This has also been found in an experiment estab-lished in Denmark on loamy sand soil in 2002 where N2O fluxes

Year 2

CT NT RT

4.8 ± 0.5a 8.1 ± 0.8b 5.5 ± 1.0a3.7 ± 0.5a 5.8 ± 0.7b 6.5 ± 1.0ab6.4 ± 0.4a 10.2 ± 0.9b8.3 ± 0.9a 4.9 ± 0.3b

es (p < 0.05) are represented with a letter (a, b) for each study site separately with

196 J. Sheehy et al. / Agriculture, Ecosystems and Environment 164 (2013) 190– 199

Site 1

So

il te

mp

era

ture

(ºC

)

-10

0

10

20

30

CT

NT

Site 2

So

il te

mp

era

ture

(ºC

)

-10

0

10

20

Site 3

So

il te

mp

era

ture

(ºC

)

-10

0

10

20

30

Site 4

(mo

5/1/2008 9/1/2008 1/1/2009 5/1/2009 9/1/2009 1/1/2010 5/1/2010

So

il te

mp

era

ture

(ºC

)

-10

0

10

20

ventio

m(

wnase2smattttdc

eltb

Date

Fig. 3. Daily soil temperature at 5 cm depth in con

easured from 2003 to 2005 were lower from NT compared to CTChatskikh et al., 2008).

No statistically significant differences in cumulative N2O fluxesere found between different soil types in the CT treatment (resultsot shown) which is consistent with the results from a pilot studyt the same sites in 2005 (Regina and Alakukku, 2010), even thoughand or clay content of the soil often correlates with measured N2Omissions (Bouwman et al., 2002; Chadwick et al., 1999; Freibauer,003). However, the N2O emissions in NT were higher for the clayeyoils of sites 1–3 than for the coarse textured soil of site 4. Thisay be explained by better water holding capacity and thus more

naerobic conditions in clayey soils under NT. Also the denitrifica-ion potential of the top soil layer of site 4 was lower comparedo the clayey sites. On the other hand, the farmer at site 4 reducedhe N fertilizer rate (about 15%) in NT, which might partly explainhe lower emissions for this field. At this site, however, there wasifferent fertilizer types used in CT versus NT which reduces theomparability of the treatments.

We observed a clear increase in N2O emissions after fertilization

very year. Sometimes there was a delayed effect probably due toack of moisture in the soil but the first rain events after fertilizationriggered denitrification activity (Fig. 4). Elevated levels of N2O haveeen reported several weeks after fertilization events (Gregorich

nth/day/year)

nal tillage (CT) and no-till (NT) at the study sites.

et al., 2008; Zebarth et al., 2008). In support of findings in otherstudies (Maljanen et al., 2003; Regina and Alakukku, 2010; Reginaet al., 2004), our data also shows large peaks in N2O fluxes duringand after snow and frost melt in April. It has been shown that thaw-ing accelerates microbial activity (Regina et al., 2004) and increasesamounts of NH4

+, NO3− and dissolved organic carbon in the soil

(Herrmann and Witter, 2002; Jacinthe et al., 2002). However, thereis always some residual NO3

− for denitrification available in agri-cultural soils, thus even the increase in water content during springthaw may be enough to create optimum conditions for denitrifica-tion. There is a possibility that in Northern European conditionsthe long period with no plant uptake of N increases the annualN2O emissions, especially if freeze–thaw cycles occur during win-ter (Syväsalo et al., 2004). Jungkunst et al. (2006) reported thatagricultural lands that are characterized by regular freeze–thawevents are more likely to have higher N2O-to-N-input ratios com-pared to similar soils characterized by less regular frosting. In thisstudy, the winters were cold with very few freeze–thaw events andwinter-time emissions were relatively low, about 40% of the annual

emissions, yet forming a significant part of the total.

The annual cumulative N2O fluxes were within the reportedvalues for mineral agricultural soils that range from 0.8 kg N2O-N ha−1 to 24 kg N2O-N ha−1 in NT and 1.8 kg N2O-N ha−1 to 13.2 kg

J. Sheehy et al. / Agriculture, Ecosystems and Environment 164 (2013) 190– 199 197

Fig. 4. Fluxes of N2O in 06/2008 to 06/2010 in conventional tillage (CT), no-till (NT) and reduced tillage (RT). Fertilization events in spring during sowing are marked witha

NaMsNaNdosaNtthho

n F and tillage events with a T. Vertical lines indicate the ±SE.

2O-N ha−1 in CT (Aulakh et al., 1984; Ball et al., 1999; Chatskikhnd Olesen, 2007; Kaharabata et al., 2003; MacKenzie et al., 1998).easured fluxes were also in the same range as similar mineral

oils in Finland with values reported between 1.5 and 7.8 kg N2O- ha−1 (Syväsalo et al., 2004, 2006). The average difference innnual N2O emissions between CT and NT was 2.9 and 1.4 kg N2O-

ha−1 in the first and second year, respectively, and reflectedifferences in WFPS. This was very close to the average differencef 2 kg N2O-N ha−1 reported by Rochette (2008) on poorly aeratedoils. However, it should be noted that the differences between CTnd NT at the different study sites ranged from −2.2 to 4.6 kg N2O-

ha−1 on the coarse loamy soil and clayey soils, respectively. Sincehe global warming potential of N2O is about 300 times greater than

hat of CO2 and there was no increase in the total soil C in NT, theigher emissions of N2O in the clayey soils under NT are effectivelyaving a negative effect on the net greenhouse gas balance. On thether hand, the situation is reversed at site 4 where the positive net

impact of NT is twice as high as the negative impact of NT on theclayey soils.

Poor aeration is the most likely driving factor of greater N2Oemissions of the clayey soils under NT. The higher soil bulk densityat sites 1 and 3 with lower micro- and macroporosity (Regina andAlakukku, 2010) indicates that soil was less aerated in NT comparedto CT. WFPS was rarely above 60% in CT but in NT it was more oftenabove this threshold, which is thought to induce denitrification inthe soil (Linn and Doran, 1984). While WFPS was occasionally aslow as 30% in CT during the growing season, it stayed above 50% inthe NT systems at sites 1 and 2, while the NT system on the coarsetextured soil at site 4 had lower levels of WFPS. These results arein accordance with the results of Rochette (2008) who concluded

that N2O emissions are generally increased in NT in poorly aeratedsoils but not necessarily in medium or well-aerated soils with bet-ter drainage. High N2O emissions in NT are often closely linked toincreasing soil moisture content and fine soil texture (Doran, 1980;

1 ms and

Le

pNNebefiscer1s

5

sabmadcs(ccspt

A

MvMMfittcs

R

A

A

B

B

B

B

C

C

98 J. Sheehy et al. / Agriculture, Ecosyste

inn and Doran, 1984) as well as humid cool climate (Gregoricht al., 2005; Six et al., 2004).

Soil WFPS explained the variability of N2O emissions better thanrecipitation. Soil and air temperature had a poor correlation with2O. This may be explained by optimal soil moisture content for2O production not coinciding with changing soil temperature forxample during the spring when snow and frozen soil are meltingut soil temperatures still remain relatively low. The presence ofarthworms in the soil has been reported to accelerate the denitri-cation by soil-derived bacteria (Drake and Horn, 2006). This wasupported by the results of this study with a significant positiveorrelation between the amount of earthworm burrows and N2Omissions. Also significantly higher amounts of earthworm bur-ows and N2O emissions were found in NT compared to CT at site, which might partly explain the higher emissions in NT at thisite.

. Conclusions

Our results show that there is a risk of increased N2O emis-ions on clayey soils under NT practice in small grain spring cerealgroecosystems in Northern European boreal climate. At compara-le N levels increased emissions on clayey soils are mainly due toore dense soil structure causing increased soil moisture and poor

eration in NT compared to CT under humid and cool climatic con-itions. Reduced tillage did not show any significant differencesompared to CT in cumulative nitrous oxide emissions in eithertudied year, and as 25% of arable lands in Finland are under RTtwice as much compared to NT), it makes RT a notable option forlayey soils. Since the increase of N2O emissions in NT appears to belosely related to soil aeration, mitigating these emissions requirespecial attention to soil structure and drainage. A long continuouseriod of NT practices may gradually improve the soil structure ashe biological activity enhances and adapts to the new conditions.

cknowledgements

This study was funded by Maj and Tor Nessling Foundation andTT Agrifood Research Finland and done in cooperation with Uni-

ersity of Helsinki, Finland and University of California, Davis, USA.any thanks to Ari Seppänen, Pekka Kivistö, Ilkka Sarikka andarja-Liisa Westerlund for doing a wonderful job helping in the

eld and Leena Seppänen and Mirva Ceder for valuable advice inhe laboratory. The staff of the research center of Yara Finland, andhe farmers Timo Rouhiainen and Ilmari Seppälä are greatly appre-iated for giving us the chance to use their fields as a part of thistudy.

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