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Soil Biology & Biochemistry 38 (2006) 2782–2793

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Nitrogen oxides emission from soils bearing a potato crop as influencedby fertilization with treated pig slurries and composts

Antonio Vallejoa,�, Ute M. Skibab, Lourdes Garcıa-Torresa, Augusto Arcea,Susana Lopez-Fernandeza, Laura Sanchez-Martına

aEscuela Tecnica Superior de Ingenieros Agronomos, Universidad Politecnica de Madrid, Ciudad Universitaria, 28040 Madrid, SpainbCentre for Ecology and Hydrology, Edinburgh, Bush Estate, Penicuik, Midlothian EH26 QB,UK

Received 21 December 2005; received in revised form 13 April 2006; accepted 19 April 2006

Available online 22 May 2006

Abstract

Nitrous oxide, nitric oxide and denitrification losses from an irrigated soil amended with organic fertilizers with different soluble

organic carbon fractions and ammonium contents were studied in a field study covering the growing season of potato (Solanum

tuberosum). Untreated pig slurry (IPS) with and without the nitrification inhibitor dicyandiamide (DCD), digested thin fraction of pig

slurry (DTP), composted solid fraction of pig slurry (CP) and composted municipal solid waste (MSW) mixed with urea were applied at a

rate of 175 kg available Nha�1, and emissions were compared with those from urea (U) and a control treatment without any added N

fertilizer (Control). The cumulative denitrification losses correlated significantly with the soluble carbohydrates, dissolved N and total C

added. Added dissolved organic C (DOC) and dissolved N affected the N2O/N2 ratio, and a lower ratio was observed for organic

fertilizers than from urea or unfertilized controls. The proportion of N2O produced from nitrification was higher from urea than from

organic fertilizers. Accumulated N2O losses during the crop season ranged from 3.69 to 7.31 kg N2O–Nha�1 for control and urea,

respectively, whereas NO losses ranged from 0.005 to 0.24 kg NO–Nha�1, respectively. Digested thin fraction of pig slurry compared to

IPS mitigated the total N2O emission by 48% and the denitrification rate by 33%, but did not influence NO emissions. Composted pig

slurry compared to untreated pig slurry increased the N2O emission by 40% and NO emission by 55%, but reduced the denitrification

losses (34%). DCD partially inhibited nitrification rates and reduced N2O and NO emissions from pig slurry by at least 83% and 77%,

respectively. MSW+U, with a C:N ratio higher than that of the composted pig slurry, produced the largest denitrification losses

(33.3 kgNha�1), although N2O and NO emissions were lower than for the U and CP treatments.

This work has shown that for an irrigated clay loam soil additions of treated organic fertilizers can mitigate the emissions of the

atmospheric pollutants NO and N2O in comparison with urea.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Nitrous oxide; Nitric oxide; Denitrification; Organic fertilizer; Compost; Pig slurry; Dissolved organic C; Dicyandiamide

1. Introduction

Organic and mineral fertilizers are known to bekey variables in the regulation of nitrous oxide (N2O)and nitric oxide (NO) emissions from soils (Mosieret al., 1998). These gases are formed in the soil duringbiological denitrification and nitrification (Firestone andDavidson, 1989). Although the overall impact of mostparameters affecting these processes is largely known,

e front matter r 2006 Elsevier Ltd. All rights reserved.

ilbio.2006.04.040

ing author. Tel.: +34913 365 652; fax: +34 913 365 639.

ess: antonio.vallejo@upm.es (A. Vallejo).

the fine process details, for example how the compositionof organic N fertilizers affects denitrification, nitrifica-tion and emission rates are still insufficiently understood.When an organic N fertilizer is incorporated into thesoil, degradable organic compounds join the ammoniumðNHþ4 Þ and nitrate ðNO�3 Þ pool and the added carboninfluences the proportion of N2O produced in relationto dinitrogen (N2) during denitrification (Scholefieldet al., 1997). However, little information of the impactof increased C input on the generation of N2O, N2

and NO is available (Dendooven et al., 1998; De Weveret al., 2002).

ARTICLE IN PRESSA. Vallejo et al. / Soil Biology & Biochemistry 38 (2006) 2782–2793 2783

Slurries supply easily decomposable organic C that canboth sustain denitrification and induce anaerobiosis bystimulating biological O2 demand (Rochette et al., 2000).Several authors have reported increases in N2O and NOemissions following application of slurry to soils (Gut etal., 1999; Maag and Vinther, 1999). Land application ofuntreated livestock manures increases the health risk forthe animals and people, because of the diffusion ofpathogens to the soil and the air, and also createsunpleasant odours. Anaerobic digestion and compostingcan reduce the number of pathogens in manures (Burtonand Turner, 2003) and control the emissions of odourcompounds by changing the chemical composition, espe-cially of the degradable C fraction. The information on theinfluence of anaerobically digested slurries and manures onthe emissions of N2O, N2 and NO is limited to very fewstudies; for example studies on animal slurries (about 55%cattle and 45% pig slurry) (Petersen, 1999).

Compost from municipal solid waste (MSW) has beenproposed as a useful source of nitrogen for crops (Dıez etal., 2000). This material may increase the amount of readilyavailable C for the microorganisms and stimulate deni-trification, but a high labile C content may also promotethe immobilization-mineralization processes in soil (Azamet al., 1985). Therefore a mineral fertilizer, often urea, isapplied together with the MSW. Limited field data areavailable on N trace gas emissions from aerobicallycomposted materials applied to the soil as fertilizer (DeWever et al., 2002).

The nitrification inhibitor dicyandiamide (DCD), mixedwith NHþ4 or urea-containing fertilizers has been demon-strated to be efficient in mitigating N2O and NO emissionfrom soils (Skiba et al., 1993). DCD has also been usedwith slurries (de Klein et al., 1996; Vallejo et al., 2005), butmore studies determining its efficiency in reducing trace gasemissions from a range of organic fertilizers is stillnecessary.

In order to understand the influence of mineral andorganic N fertilizers on the nitrification and denitrificationprocesses and consequently the N2O, NO emissions andN2O/N2 ratio, a field experiment was carried out incorpor-ating organic fertilizers (treated and untreated pig slurries,and a MSW compost mixed with urea) into the soil in anirrigated crop, while mineral fertilizer and a control wereincluded for reference. The effect of the DCD nitrificationinhibitor to reduce N2O, NO emission under irrigationconditions was also evaluated.

2. Materials and methods

2.1. Soil characteristics

The field experiment was located at ‘El Encın’ FieldStation, near Alcala de Henares (Madrid) (latitude401320N, longitude 31170W), in the middle of the Henaresriver basin. The soil was a Calcic Haploxerepts with a clayloam texture in the upper (0–28 cm) horizon. Some

physico-chemical properties of the 0–28 cm top soil layermeasured on 10 May 2004 by conventional methods were:total organic C, 8.270.4 g kg�1; total N, 0.7570.12 g kg�1;pHH2O, 7.9; bulk density, 1.4170.03mgm�3; CaCO3,13.170.3 g kg�1; clay, 28173 g kg�1; silt, 16973 g kg�1;sand, 55176 g kg�1. Disolved organic C (DOC) was3573mgCkg�1 and NO�3 was 12.373mg NO3

–N. The10-year mean annual average temperature and rainfall inthis area were 13.2 1C and 430mm.

2.2. Experimental procedure

Twenty one plots (8m� 5m) were selected in theexperimental field (1 ha) and on 16 May, seven treatmentswere applied: (i) untreated pig slurry (IPS); (ii) digestedthin fraction of pig slurry (DTP); (iii) pig slurry mixed withdicyandiamide (IPS+DCD); (iv) composted solid fractionof pig slurry (CP); (v) composted MSW mixed with urea(MSW+U); (vi) urea (U) and (vii) control without any Nfertilizer (Control). The treatments were applied in arandomized block design with three replicates. Thephysico-chemical properties of different organic fertilizersare shown in Table 1.The pig manures (IPS, DTP, and CP) were collected

from a pig slurry treatment plant of Almazan (Soria-Spain). The liquid fraction of pig slurry, obtained byphysical separation of slurry using a rotary sieve drum(0.9mm mesh), was anaerobically digested in a 50m3

continuous digester with a hydraulic retention time of 32days and a fermentation temperature of 35–40 1C. In thisplant, the expected reduction of total organic C from theraw pig slurry was 8–25% with the separation process and25–35% during the anaerobic digestion process. The solidfraction was composted for 3 months and stored for 9months. The MSW was collected from the ValdemingomezMSW-plant (Madrid). The material was composted in thisplant for 3 months and then was stored in the field for 1month. The treatment with DCD was prepared in situmixing the inhibitor with untreated pig slurry in a 100 ltank by continuous agitation for 1 h.The application of organic fertilizers was adjusted to

provide 175kg available Nha�1. The total slurry applied was37.2, 38.9 and 35.2m3ha�1 for IPS, DTP and IPS+DCDtreatments, respectively. To estimate the available N fromCP and MSW a 4 months incubation experiment was carriedout previously (Sanchez et al., 1997). As over this period thepercentage of organic N mineralized was 42–43% from CPand 27–28% from MSW, it was applied at rates of21.2Mgha�1 of CP (300Nkgha�1) and 21.3Mgha�1 asMSW (250Nkgha�1). To the MSW an additional50 kgNha�1 of urea was applied in order to reduceimmobilization N effect in soil.Liquid manures were applied to the soils using a

watering can connected to a 100 l tank with a hosepipe toproduce a uniform distribution to the surface. A slurrypump was used to spread slurry at a constant rate.Solid fertilizers were broadcast onto the surface of plots.

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Table 1

Chemical composition of organic fertilizers and amount of different compounds added with fertilizers

Propertya IPSb DTPb CPb MSWb

Composition

(g kg�1)

Added

(gm�2)

Composition

(g kg�1)

Added

(gm�2)

Composition

(g kg�1)

Added

(gm�2)

Composition

(g kg�1)

Added

(gm�2)

Moisture 947 — 973 — 749 — 565 —

Total N 4.7 17.5 4.5 17.5 14.1 30.0 9.6 25.0c

Non-dissolved

organic N

0.5 1.9 0.7 3.1 10.7 22.8 6.5 17.0

Dissolved

organic N

0.1 0.3 0 0.0 0.0 0.0 0.1 0.4

NHþ4 4.1 15.3 3.7 14.4 3.4 7.2 2.9 7.6

Total C 30.6 114.7 15.8 61.0 145.2 310.0 252.5 658.6

Soluble organic

C

0.272 1.016 0.181 0.706 0.078 0.167 0.332 0.864

Soluble

carbohydrates

0.004 0.015 0.002 0.009 0.001 0.003 0.017 0.044

Soluble

proteins

0.011 0.042 0.000 0.000 0.000 0.000 0.009 0.024

Soluble

phenolic

compounds

0.007 0.025 0.021 0.084 0.003 0.006 0.007 0.017

VFAb 0.046 0.170 0.052 0.202 0.063 0.134 0.050 0.106

C/N 6.5 — 3.5 — 10.3 — 26.3 —

aCalculated in the original fertilizer (wet weight).bIPS—Untreated pig slurry; DTP—digested thin fraction of pig slurry; MSW—composted municipal solid waste; VFA—volatile fatty acids.cAn additional amount of 5 gNm�2 of urea (U) was applied with MSW.

A. Vallejo et al. / Soil Biology & Biochemistry 38 (2006) 2782–27932784

To obtain a homogeneous application of fertilizers(slurries, composts and urea), plots were divided into 8subplots (1m� 5m). The total amount of slurry applied tosoil was regulated by measuring the time of application at aconstant flow rate. For solid fertilizers exact weights wereapplied by hand. Immediately, a rotovator was used toincorporate the slurries and solid fertilizers into the uppersoil layer (0–5 cm). Control and solid fertilizer plots wereirrigated with 39m3 ha�1 of water prior fertilizer applica-tion to maintain the same soil water content on all plots.

On 7 June potato (Solanum tuberosum, cv Desiree) wasplanted in rows with 75 cm spacing and were harvested on15 October. The crop was irrigated using a sprinklersystem. In total, 10 irrigations, each delivering 40–60mmof water, took place between 16 June and 15 September atfrequency of once per week. The exact weekly rate ofirrigation was calculated from soil moisture measurementsusing a Time Domain Reflectometry (TDR) and assumingthat approximately 5% was lost by drainage. The totalamount of water applied over in the study period was460mm.

2.3. Sampling and analysis of N2O and NO

Fluxes of N2O and NO from the soil surface weremeasured using manually operated circular static cham-bers, coated inside with a Teflon film to minimize losses ofNO on the walls of the chambers. Immediately before fluxmeasurements one chamber per plot in each of the three

replicate plots was inserted 3 cm into the soil. Eachchamber had a head space volume of 19.06 l and covereda surface area of 0.096m2. Chambers were located alwayson the ridges between plants, at equal distance from thesprinkler. Chambers were not positioned on the furrows,where fluxes are very likely to be different.At each sampling time the chambers were moved to a

new random location within the plot. For N2O analysis thechambers were closed with flat lids for periods of 30min,when 10ml gas samples were removed from the headspaceatmosphere by syringe and transferred to 10ml evacuatedvacutainers (Venoject) sealed with a gas-tight neopreneseptum. Ambient air samples were taken at time 0 fromthree different chambers. NO fluxes were measuredimmediately after N2O. The lids were opened for a fewminutes and then Ozone-free air (compressed air) waspumped through the chambers at a flowrate of 10 lmin�1

for 3min to remove most of the initial air. After 15min 2 lof gas sampled from the headspace was pumped into aTeflon gas collection bag, which was transported to thechemiluminescence detector (Environment AC31M) placedat the edge of the field (Williams et al., 1998). The NOconcentration was always determined within 5–10min afterthe samples were taken. Due to the heavy clay soil it wasnot possible to directly connect the chambers to theanalysers. On several occasions two bags were filled with1.0 and 0.1 ml NO–N l�1 standards in the field and analysedfor NO in the same manner as described above. Therelative error was always lower than 5%.

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Total denitrification rates and N2O emissions resultingfrom nitrification and denitrification were estimated in fieldincubations using varying concentrations of acetylene(C2H2) (Muller et al., 1998). Incubations of 6 soil coresper plot (2.5� 10 cm deep) with the 3 C2H2 concentrations(0, 5 Pa, and 5% v/v) were performed in 1-l glass jarsinserted into the soil adjacent to the experimental fieldplots. The 5 Pa C2H2 concentration in the jar atmospherewas adjusted by exchanging, an exact calculated headspacevolume with a freshly prepared 1000 Pa C2H2 standard(Muller et al., 1998). After 24 h, a 10ml gas sample wastaken from each jar with a syringe and stored in a 10mlevacuated vial (Venojets).

The N2O content in the vials was analysed by gaschromatography (HP6890), using a 63Ni electron-capturedetector. A capillary column HP-Plot Q was used,incorporating a capillary precolumn of an HP-RetentionGap to remove the water vapour from the sample. Theinjector, oven and detector temperatures were 50, 50 and300 1C, respectively, and the carrier gas flux (N2) was30mlmin�1.

Gas samples from cover boxes (N2O and NO emission)were taken every day during the first 10 days after fertilizerapplication, once or twice a week during July and Augustand fortnightly between September and November. Gassamples from 1 l jars were sampled twice a week during thefirst 2 weeks and on the same dates as N2O emissionmeasurements during the rest of the experiment.

Following the methodology described by Muller et al.(1998), the fraction of N2O produced by denitrification inthe soil incubations was expressed as the fraction I5Pa/I0Pa,where I5Pa and I0Pa were the mean N2O emission fromincubations with 5 and 0Pa C2H2, respectively. The N2Oemission due to denitrification from cover boxes (Fden) wascalculated by multiplying the fraction I5Pa/I0Pa by the dailyN2O flux (Fday), assuming that the fraction determinedfrom the soil incubation was equal to the average dailyfraction in the plots (I5Pa/I0Pa ¼ Fden/Fday). DenitrificationN2 production (IN2) was calculated using the equation:IN2¼ IðN2þN2OÞ � I5Pa; where IðN2 þN2OÞ is N2+N2O

produced from denitrification (incubation with 5% C2H2).Total N2O–N and NO–N emissions and denitrification

losses, per plot were estimated by successive linearinterpolation assuming that emissions (or production)followed a linear trend during the periods when nomeasurements were made. The N2O/N2 emission ratiofrom denitrification was calculated dividing the accumu-lated total I5Pa during the experimental period by total IN2

.

2.4. Analysis of soil

After sampling the headspace, the soil from each jar wasthoroughly mixed and soil NO�3 and NHþ4 were determinedby extracting 10 g of fresh soil with 100ml 0.01M KClsolution; NO�3 and NHþ4 were determined colorimetricallyusing a Technicon AAII Auto-analyser (Technicon Hispa-nia, Spain). To determine DOC, extracts of soil were

obtained and analysed as described by Mulvaney et al.(1997). Water filled pore space (WFPS) was calculated bydividing the volumetric water content by total soil porosity,which was calculated from measured soil bulk density andassuming a particle density of 2.65Mgm�3. Soil tempera-ture was monitored in the field using a temperature probeinserted 10 cm into the soil, average hourly data werestored on a data logger. Rainfall data were obtained fromthe meteorological station located in the field.

2.5. Statistical methods

The statistical analysis was performed using STAT-GRAPHICS Plus 5.1 (Manugistics, 2000). One-wayANOVA was used to establish the influence of fertilizationon the denitrification rate, N2O and NO emissions, DOCand NO�3 �N content in soil. The LSD test was used formultiple comparisons of means. Simple correlation ana-lyses were performed to determine whether the daily N2O,daily NO emissions and denitrification rates were related toWFPS, NO�3 �N content, soil temperature and DOC.Simple regressions were performed to assess relation-

ships between the N2O, NO and denitrification losses anddifferent added compound fractions of organic fertilizersfor 3 different contrasting periods: before, during and afterirrigation. To study the synergic effect of C and Nfractions, multiple linear regressions were performed usingthe best-correlated organic C fractions as a first indepen-dent variable and NHþ4 , dissolved N or non dissolvedorganic N added with fertilizer as a second independentvariable. The quality of all models was evaluated by usingan adjusted r2 coefficient; i.e. r2 adjusted by the number ofindependent variables in the model and the sample size.

3. Results

3.1. Environmental conditions, evolution of mineral N,

soluble organic carbon

Immediately after fertilizer application the WFPS wasgreater than 70% (Fig. 1), but rapidly decreased to 52%because irrigation was not applied for another 30 days.Due to a high evapotranspiration of 6–7mm H2Od�1

between June and September, irrigation was necessary tosustain crop growth. During the irrigation period a WFPSabove 60% was generally maintained. After the irrigationperiod (September–November), the WFPS ranged from50% to 75%. The average daily soil temperature in the0–10 cm soil layer (Fig. 1) varied between 18 and 24 1Cfrom June to September and between 3 and 17 1C fromOctober to November.The concentration of NHþ4 decreased quickly after the

application of fertilizers (Fig. 2). The IPS+DCD treatmentmaintained larger concentration than the IPS treatmentbetween 7 and 20 days after the application, althoughdifferences were not significant (po0.05). The soil NO�3concentrations in the 10 cm upper layer (Fig. 2) generally

ARTICLE IN PRESSA. Vallejo et al. / Soil Biology & Biochemistry 38 (2006) 2782–27932786

increased during the first 10 days after fertilizer applica-tion. A significantly (po0.05) larger NO�3 concentrationwas measured in soils fertilized with DTP than with IPSand IPS+DCD treatments 20–30 days after application. Apositive net mineralization of N was also observed in the

Fig. 1. Soil temperature and water-filled pore space (WFPS) during the

experimental period in the upper soil layer (0–10 cm).

Fig. 2. NHþ4 (a,b) and NO�3 (c,d) concentrations in the 0–10 cm soil layer durin

treatments for each sample time. The single arrow indicates the date of fertili

first 30 days after application when MSW was mixed withU. The mineral N concentration was 2.4 times larger thanfrom control plots. With the first irrigation, NO�3concentrations decreased rapidly in all treatments. Ingeneral, the application of organic N fertilizers increasedthe DOC, differences were significant (po0.05) on someoccasions during the first 40 days after irrigation (Fig. 3).An interesting result is that the DOC was lower in the DTPthan IPS during the period before irrigation. A peak of42mg DOC–Ckg�1 was also observed for the U treatmentimmediately after fertilizer application.

3.2. N2O+N2 production from denitrification

The application of organic and mineral N fertilizersstimulated denitrification rates during the first 5–20 dayswhen the soil WFPS was 70% (Fig. 1). Largest denitrifica-tion rates were measured from plots fertilized withMSW+U, IPS and IPS+DCD (44.2, 34.3, and 23.0mgN2O–Nm�2 d�1, respectively) (Fig. 4). The first irrigationon 16 June increased the WFPS to 72% and improved theenvironmental conditions that favour denitrification. As aresult much larger denitrification rates were measured fromall treatments, including the unfertilized control. Largestdenitrification rates were measured from plots fertilizedwith IPS and IPS+DCD (74.8 and 78.7mgN2O–Nm�2 d�1). The nitrification inhibitor did notinfluence the denitrification rate; the N2+N2O productionfrom IPS with and without DCD was not significantlydifferent (po0.05) at any sampling time (Table 2). For thesolid N fertilizers, denitrification peaked after the secondirrigation (23 June) obtaining values of 100 and 70mg

g the experimental period. The vertical bars indicate LSD at 0.05 between

zer application; double-headed arrow, first irrigation.

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Fig. 3. Dissolved organic C (DOC) in the 0–10 cm soil layer during the

experimental period. The vertical bars indicate LSD at 0.05 between

treatments for each sample time. The single arrow indicates the date of

fertilizer application and the double-headed arrow indicates first irriga-

tion.Fig. 4. Denitrification rate (DR) in the 0–10 cm soil layer during the

potato-growing season. The vertical bars indicate LSD at 0.05 between

treatments for each sample time. The single arrow indicates the date of

fertilizer application and the double-headed arrow indicates first irriga-

tion.

A. Vallejo et al. / Soil Biology & Biochemistry 38 (2006) 2782–2793 2787

N2O–Nm�2 d�1 for U and CP, respectively. Denitrificationrates decreased markedly when irrigation stopped on 15September. For the period (15 September–15 October)differences between treatments were not significant(po0.05).

The cumulative denitrification losses of the upper soillayer (0–10 cm) varied greatly between treatments (Table2). The maximum losses were measured from theMSW+U treatment (33.3 kgNha�1). The IPS andIPS+DCD treatments, that added to the soil similarorganic C, had the same denitrification losses throughoutthe experimental period. The DTP and CP reduced thedenitrification losses (21.7 and 21.6 kgNha�1, respectively)in comparison with IPS or MSW+U. Depending oftreatment, losses during the irrigation period (between 16June and 7 September) varied from 64% to 83% of thetotal denitrification losses. The rate of denitrificationduring the experimental period for each treatment wassignificantly related (po0.05) to the environmental proper-ties (WFPS, temperature) and NO�3 �N concentration inthe upper soil layer. The DOC concentration of the soil wasnot correlated with the cumulative denitrification rate forthe entire study period, but for individual sampling dates,

DOC correlated significantly (po0.01) with the denitrifica-tion rate for 2nd (24 June) and 3rd (3 August) denitrifica-tion peaks in Fig. 4.Denitrification losses for the 25 days following applica-

tion of fertilizer before irrigation were highly correlatedwith added soluble carbohydrates (r2 ¼ 0:94, po0.001,n ¼ 7) and after the irrigation period added total C andnon dissolved organic N provided the best correlations(r2 ¼ 0:77, po0.01, n ¼ 7; and r2 ¼ 0:62, po0.05, n ¼ 7,respectively). Added dissolved N was the best indicator forthe period during irrigation (r2 ¼ 0:74, po0.05, n ¼ 7) andtotal study period (r2 ¼ 0:52, po0.05, n ¼ 7). Added DOCalso provided a high correlation for the period duringirrigation for organic fertilizers (r2 ¼ 0:66, po0.05, n ¼ 7).

3.3. N2O emission

The N2O emissions were influenced by additions oforganic and mineral fertilizers (Fig. 5). For the slurries,higher fluxes of N2O were generally observed from plots

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Table 2

Denitrification losses from the upper soil layer (0–10 cm) integrated over different periods during the potato growing season

Treatment Denitrification losses (kgNha�1)

Before IPa

(16 May–16 June)

During IPa

(17 June–15 Sept)

After IPa

(16 Sept–15 Oct)

Total season

Control 0.3770.17ba 6.7474.0 a 0.9370.3 a 8.073.1 a

Untreated pig slurry (IPS) 3.2871.56 b 22.775.8 b 2.4771.0 a 28.574.9 cd

Digested thin fraction of pig slurry (DTP) 2.8870.85 b 16.972.4 b 1.9270.5 a 21.771.0 b

Pig slurry+ DCD (IPS+DCD) 2.8571.7 b 24.175.7 b 1.4370.8 a 28.473.8 cd

Composting pig slurry fraction (CP) 2.2270.74 ab 16.176.0 b 3.2371.6 ab 21.673.1 b

Municipal solid waste+Urea (MSW +U) 6.5571.45 c 21.674.8 b 5.2072.1 b 33.375.1 d

Urea (U) 1.3270.73 ab 19.871.2 b 2.7171.9 a 23.871.7 bc

aIP is the irrigation periodbMean value of accumulated denitrification losses from three plots7standard deviation. Different letters within each column indicate significant

differences between fertilizer treatment (po0.05) according to LSD test.

Fig. 5. Emission of N2O from soil during the potato-growing season. The

vertical bars indicate LSD at 0.05 between treatments for each sample

time. The single arrow indicates the date of fertilizer application and the

double-headed arrow indicates first irrigation.

A. Vallejo et al. / Soil Biology & Biochemistry 38 (2006) 2782–27932788

treated with IPS than DTP or IPS+DCD treatments,especially in the period following to the application offertilizer before irrigation. For solid fertilizers, largest N2Ofluxes were measured from the U and MSW+U treat-

ments, which peaked at 15.3 and 13.7mg N2O–Nm�2 d�1

during the irrigation period, respectively.Cumulative N2O emissions are shown in Table 3. Before

irrigation, plots fertilized with organic fertilizers, especiallyIPS and MSW+U, emitted more N2O than control plotsor those fertilized with U. However, during the irrigationperiod largest cumulative N2O emissions were measuredfrom plots fertilized with U. DTP emitted less N2O thanIPS, although this difference only was significant at po0.05before irrigation. For the entire period DTP reduced N2Oemissions by 48% in relation to the IPS treatment. On theother hand, the CP increased the emission (40%) incomparison with the IPS treatment because of increasedemissions after the irrigation period. The nitrificationinhibitor was very effective and reduced total N2O fluxesby 83% compared with those emitted from the IPStreatment. Above the N2O loss from the unfertilizedcontrol plots, the percentage of N2O lost in relation tothe available N during the experimental period was 1.0%(IPS), 0.57% (DTP), 0.18% (IPS+DCD), 1.55%(CP), 1.12(MSW+U) and 2.07% (U).Regression analyses showed that the daily N2O emission

was significantly correlated with soil temperature (r2 ¼ 19:6po0.01, n ¼ 308), but not with WFPS or the other soilparameters (NHþ4 , NO�3 , DOC). Moreover, the cumulativeN2O emission during the first month was positivelycorrelated with added DOC (r2 ¼ 57:8, po0.05, n ¼ 7) andadded soluble carbohydrates (r2 ¼ 47:1, po0.05, n ¼ 7). Forthe entire study period no significant correlation betweenN2O emission and variable measured was found.Denitrification was the main process responsible for N2O

emission from plots treated with organic fertilizers,whereas nitrification was the most important from plotstreated with mineral fertilizer. Following the acetyleneinhibition method proposed by Muller et al. (1998), thepercentage of total N2O losses via the denitrificationprocess from fertilizers was 75% (IPS), 52% (DTP), 73%(IPS+DCD), 67%(CP), 75% (MSW+U), 35% (U) and70% (Control).

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Table 3

Cumulative N2O emitted integrated over different periods during the potato growing season

Period N2O emission (kgNha�1)

Before IPa

(16 May.–16 June)

During IP

(17 June–15 Sept)

After the IP

(15 Sept–15 Oct.)

Total season

Control 0.4470.09ba 1.9670.11 a 1.1870.39 a 3.6970.36 a

Untreated pig slurry (IPS) 0.7870.10 c 2.9370.22 bc 1.7570.19 ab 5.6270.23 c

Digested thin fraction of pig slurry (DTP) 0.6070.05 b 2.6570.38 bc 1.2870.88 a 4.6970.60 b

Pig slurry+ DCD (IPS+DCD) 0.6070.07 b 2.4370.43 ab 0.6970.92 a 4.0170.42 ab

Composting pig slurry fraction (CP) 0.6370.08 bc 2.8870.76 bc 2.7570.54 b 6.4170.57 cd

Municipal solid waste+Urea (MSW +U) 0.7770.02 c 3.3370.42 c 1.4070.47 a 5.6570.81 c

Urea (U) 0.4370.02 a 4.6070.26 d 1.8270.75 ab 7.3170.39 d

aIP is the irrigation period.bMean value of accumulated N2O emission from three plots7standard deviation. Different letters within each column indicate significant differences

between fertilizer treatment (po0.05) according to the least significant significance (LSD) test.

Fig. 6. Emission of NO from soil during the potato-growing season. The

vertical bars indicate LSD at 0.05 between treatments for each sample

time. The single arrow indicates the date of fertilizer application and the

double-headed arrow indicates first irrigation.

A. Vallejo et al. / Soil Biology & Biochemistry 38 (2006) 2782–2793 2789

3.4. N2O/N2 ratio

The cumulative N2 production in the upper soil layer(0–10 cm) was larger than the cumulative N2O productionfrom denitrification in all treatments and the application offertilizer and fertilizer type influenced the N2O/N2 emissionratio relative to the control treatment (ratio of 0.19). The Ntreatments U and CP had the highest ratios (0.11 and 0.15,respectively), followed by 0.09 for the IPS and 0.07 forDTP and MSW+U. Although the nitrification inhibitordid not reduce the total denitrification rate relative to theIPS treatment, the N2O/N2 ratio was reduced to 0.06. Ingeneral, the N2O/N2 ratio was negatively correlated withadded DOC (r2 ¼ 0:67, po0.05, n ¼ 7) and with dissolvedN (r2 ¼ 0:65, po0.05, n ¼ 7). Others fractions were notsignificantly correlated.

3.5. NO fluxes

A significant increase in NO emission rate was observedduring the 20 days following fertilizer application beforeirrigation. The largest rates were measured from the Utreated plots with a maximum flux of 3.8mg NO–N -m�2 d�1 12 days after fertilization (Fig. 6). On this date,NO emissions from the pig slurry application (DTP andIPS) also peaked at 1.5mg NO–Nm�2 d�1. After the first20–25 days NO fluxes remained below 0.5mg NO–N -m�2 d�1 in all treatments and during the irrigation periodoccasionally negatives fluxes of NO were measured. TheDCD inhibited NO emission and fluxes close to those fromthe control plots were measured throughout the experiment(Table 4). Above the NO lost from the unfertilized controlplots, the percentage of N lost as NO relative to N appliedwas 0.06, 0.06, 0.01, 0.10, 0.04 and 0.13% for IPS, DTP,IPS+DCD, CP, MSW+U and U, respectively.

Regression analyses showed that the daily NO emissionwas positively correlated with NHþ4 , (po0.001), NO�3(po0.05) and negatively with soil temperature (po0.001)(n ¼ 272 for all). A combination of added dissolved N and

DOC was the best indicator for cumulative emissions forthe total study period (r2 ¼ 0:87, po0.01, n ¼ 7), theperiod before irrigation (r2 ¼ 0:84, po0:01, n ¼ 7) andduring irrigation (r2 ¼ 0:89, po0.01, n ¼ 7).The molar NO/N2O ratio was less than 1 at all sampling

times and for all treatments, and during the irrigationperiod was less than 0.01, 0.003, 0.0037, 0.043, 0.013, 0.053,

ARTICLE IN PRESS

Table 4

Cumulative NO emitted integrated over different periods during the potatoe growing season

Period NO emission (gNha�1)

Before IPa,b

(16 May–16 June)

During IP

(17 June–15 Sept)

After the IP

(16 Sept–15 Oct)

Total season

Control 4.172.2 a 1.170.8 a 0.270.3 a 5.573.6a

Untreated pig slurry (IPS) 96.3719.3 cd �2.670.8 a 9.672.9 a 103.2722.8 b

Digested thin fraction of pig slurry (DTP) 91.2730.5 cd 3.174.0a 6.773.6a 101.0724.6 b

Pig slurry+ DCD (IPS+DCD) 29.778.7 ab �4.873.6 a 2.175.0 a 27.0716.0 a

Composting pig slurry fraction (C P) 169.1771.7 d �0.771.2 a 2.575 a 170.9776.7 c

Municipal solid waste+Urea (MSW +U) 60.1714.9 abc 2.372.7%a 4.873.2 a 67.2710.1 ab

Urea (U) 212.2790.0 d 26.5727.0 b 1.874.8 b 240.5765.5 c

aIP is the irrigation period.bMean value of accumulated NO emission from three plots7standard deviation. Different letters within each column indicate significant differences

between fertilizer treatment (po0.05) according to LSD test.

A. Vallejo et al. / Soil Biology & Biochemistry 38 (2006) 2782–27932790

0.024 and 0.066 for Control, IPS, DTP, IPS+DCD, CP,MSW+U and U treatments, respectively.

4. Discussion

4.1. Effect of organic fertilizer type on processes

Organic residues, MSW and manures, are considered aresource, not a waste, and their application to soil isrecommended (Dıez et al., 2000). Farmers are interested insupplying a readily available source of N to be utilizedimmediately by the crop. The separation of solid fractionfrom pig slurry with mechanical separators reduces theoverall organic load in terms of total organic C andachieves reduction in non-soluble components in the slurry(Burton and Turner, 2003). Anaerobic digestion andcomposting reduces the amount of organic C, especiallythe soluble carbohydrate and protein fractions (Burton andTurner, 2003). By fixing the rate of available N applicationto the same value of 175 kgNha–1 for all fertilizers applied,the composition of total carbon and nitrogen compoundsdiffered largely and had a significant effect on denitrifica-tion and N2O, NO emission rates, the soluble carbohydrateand protein fractions. All comparisons in this study wereperformed after field application. Losses of gasses duringthe digestion and composting processes and storage werenot measured.

The soil had a low organic C content (8.1 gCkg�1) andthe addition of organic N fertilizer increased C mineraliza-tion and thereby the DOC concentration in the upperhorizon. Several studies have shown that the amountof DOC is a readily available C source for microbialgrowth, (Liang et al., 1996) and therefore correlates highlywith microbial biomass C (Zack et al., 1990). In ourexperiment evolution of DOC was also affected byirrigation, and probably an important part of solubleorganic compounds were leached from the upper horizon.The input of carbon stimulated the denitrification rate,but for different time periods in this study differentfractions of carbon provided the best correlation. The

accumulated denitrification losses during the first monthafter fertilizer application (before irrigation), was bettercorrelated with added soluble carbohydrates than addedDOC because the carbohydrates fraction is an easilydegradable fraction, whereas DOC includes less easilydegradable carbon compounds An explanation to thestrong correlation between soluble carbohydrates anddenitrification could not be reached. Nevertheless, it isvery likely that the easily degradable compounds (includingsoluble carbohydrates) have been the first used bydenitrifiers. For the intermediate period (1–4 months,during irrigation), a good indicator of denitrification ratewas DOC and for periods44 months (after irrigation)total organic C was the best indicator. Denitrification wasnot correlated with added NHþ4 , but was correlated withadded dissolved N during the irrigation period and addednon dissolved organic N for the period after irrigation,probably because these fractions are mineralized slowlyand increased the pool of NO�3 , which also promoteddenitrification.The digestion of thin fraction of pig slurry and the

composting of the solid fraction of pig slurry reduced thesoluble organic fractions, and so this explained a reductionof a 33% denitrification losses compared to untreated pigslurry. In laboratory experiments, where denitrificationfrom anaerobically digested slurry was compared withundigested slurry, losses of N by denitrification from thedigested slurry were reduced by 49% (Stevens et al., 1995)and 90% (Petersen et al., 1996).The addition of organic C reduced the N2O/N2 ratio and

the best indicator for predicting this reduction was theDOC added with the fertilizer. Other soluble organic Cfractions, as well as NHþ4 , protein N, non-dissolved organicN did not correlate significantly with this ratio. Theaddition of organic fertilizers increased organic C avail-ability stimulating biological O2 demand (Rochette et al.,2000). These conditions will have favoured the consump-tion of N2O obtaining N2. Dittert et al. (2005) also foundlower N2O/N2 emitted from slurry (1:14) than frommineral fertilizer (1:1).

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Several authors have demonstrated that the addition ofNO�3 to soil increased the N2O/N2 ratio (Swerts et al.,1996; Scholefield et al., 1997), because denitrifiers use thisoxidative compound as electron acceptor (De Wever et al.,2002) and NO�3 seems to inhibit the N2O reductase enzyme(Blackmer and Bremner, 1978). The NHþ4 added to this soilwas rapidly nitrified, increasing the soil NO�3 content.However, the N uptake by the crop and NO�3 leachedduring the period of weekly irrigations from June toSeptember, contributed to reduced NO�3 concentrationsand consequently a reduced N2O/N2 ratio. The fine textureof this soil together with the irrigation provided conditionsof reduced diffusivity of N2O produced in this soil andthereby favoured the consumption of N2O by denitrifiers.

The origin of the N2O emitted was affected by thefertilizer type. In spite of the WFPS being the same in alltreatments, denitrification was the most important sourcefor N2O in organic treatments, whereas nitrificationdominated in the mineral treatment (U). The addition oforganic compounds enhanced the N2O production viadenitrification by providing the carbon substrate fordenitrification and by stimulating general heterotrophicmicrobial growth, which promoted oxygen consumptionthat created temporary anaerobic microsites (Cannavo etal., 2003). This effect could also explain the higher N2Oproduction via nitrification from digested pig slurry thanfrom untreated pig slurry, because the DOC added in thefirst treatment was lower than in the second (Table 2).Similarly, this effect could explain the higher nitrificationrate from U, than U mixed with MSW. DCD reduced N2Ofrom nitrification during the first month, although afterthat time DCD became less effective and differences inrelation to the untreated pig slurry without DCD treatmentwere not significant at po0.05. Dicyandiamide wasinefficient in preventing nitrification entirely, because ofthe uneven distribution of the inhibitor, resulting in somenitrification of NHþ4 derived from organic matter inmicrosites not penetrated by the DCD (Skiba et al., 1993).

The methodology used to distinguish N2O emission fromnitrification and denitrification and to evaluate the N2O/N2

ratio was based on the inhibition of nitrification in thepresence of 5–10 Pa C2H2 without blocking the N2Oreductase required in the denitrification pathway (Mulleret al., 1998). N2 and N2O production from nitrificationwere not measured directly and values were calculatedfrom differences between measured denitrification rates,total N2O production rates and N2O production fromdenitrification. This method is useful for comparativepurposes between treatments, as also implied by Estavilloet al. (2002), but the absolute values of N2 and N2O fromnitrification should be interpreted with caution.

4.2. Emissions of nitrous oxide and nitric oxide

The observed emissions of N2O and NO are the result oftwo counteracting phenomena: the emission of nitrogenoxides from the soil and their consumption by the soil. For

that reason, the emission from the soil depends on (i) theformation of N2O and NO during denitrification andnitrification and its diffusion to the atmosphere, (ii) theconsumption of NO and N2O by denitrifying microorgan-isms and the diffusion of these gases into the soil.The addition of N fertilizer increased the N2O and NO

emission, but the type of fertilizer had an important effecton the total nitrogen oxides emission. Urea enhanced N2Oand NO emissions more than the organic fertilizers studied,which is the opposite compared to the effect on totaldenitrification, as discussed earlier. The degradable solubleC added with the organic fertilizers, but not with urea,reduced the N2O/N2 and nitrification-N2O/denitrification-N2O ratio favouring the consumption of N2O by deni-trifiers.In the days following slurry application, large and rapid

emissions of N2O from slurries have been reported byothers authors (Chadwick et al., 2000; Vallejo et al., 2005)when soil conditions (moisture and temperature) favourednitrification. In this clay loam soil studied here, N2O fluxesfrom untreated pig slurry were smaller than observed bythe above listed authors and the most important emissionswere produced during the irrigation period.The direct contribution from the applied fertilizer was

evaluated discounting the N2O emitted from the bulk soilwithout fertilizer. Applying digested thin fraction of pigslurry as N fertilizer reduced N2O emissions by 48% incomparison with IPS. This reduction was slightly higherthan that measured by Petersen (1999), who reportedreductions of 20–40% in a Danish soil, with otherwisesimilar N2O flux rates. The difference between the twostudies was that the organic C of the Danish soil wasgreater (26 gCkg�1) than of the Spanish soil (8 gCkg�1).Equally larger N2O emissions were observed from thecomposted solid fraction of pig slurry compared to theuntreated slurry because of a smaller amount of soluble Cfractions applied. Laboratory incubations by Scholefield etal. (1997) and Weier et al. (1993) found that soluble Cadditions reduced the N2O/N2 emission ratio. Theseobservations suggest that in this low C soil additions oforganic C reduced the proportion of N2O emitted, andtherefore organic fertilizers can mitigate the emissions ofthe atmospheric pollutants NO and N2O in comparisonwith urea. Huang et al. (2004) found that cumulativeemissions of N2O were negatively correlated with the C:Nratio in plant residues. Khalil et al. (2002) also reportedthat N2O production was increased by decreasing the C:Nratio of different organic matter. This same effect was onlyobserved in our experiment, when comparing the com-posted materials (CP and MSW). CP with a C:N ratio of 10emitted 6.41 kg N2O–Nha�1, while MSW+U with a C:Nratio of 22 emitted 5.64 kg N2O–Nha�1, although differ-ences were not significant at po0.05.The NO fluxes correlated with the NHþ4 content in soil,

indicating that nitrification is the main source of the NO(Skiba et al., 1993). For these reasons, high fluxes wereobserved in the 1st month after fertilizer addition, but

ARTICLE IN PRESSA. Vallejo et al. / Soil Biology & Biochemistry 38 (2006) 2782–27932792

before irrigation. The very wet conditions provided byirrigation very likely inhibited nitrification and reduced thediffusivity of NO to the atmosphere from this clay loamsoil (Cardenas et al., 1993). This higher retention time ofNO produced by nitrification or denitrification under thoseconditions favours the consumption by denitrifiers, result-ing in little NO emission from soil, but provided a sink forNO, as observed on some occasions.

Differences between treatments were best explained bythe combined effect of the variables dissolved NðNHþ4 þdissolved organic NÞ and DOC added with fertilizers. DOCexplained an additional 19% and 48% of the total variancefor the total study period and the irrigation periodrespectively. NHþ4 enhanced the fluxes of NO because itaffected nitrification, whereas the addition of DOCdiminished these fluxes because by increasing soil respira-tion and thereby providing the anaerobic conditions thatfavoured denitrification and the consumption of NO.Therefore, the mineral fertilizer (U) and CP with a lowDOC, emitted more NO than the other organic fertilizers.An exception was the digested thin fraction of pig slurry,which in spite of smaller DOC additions emitted similaramounts of NO as the untreated pig slurry.

Dicyandiamide reduced NO emissions from plotsfertilized with untreated pig slurry by 77%. Skiba et al.(1993) also found a high reduction of NO in (NH4)2SO4

treated with DCD in a greenhouse experiment. OverallDCD reduced 77% of the NO and N2O (83%), suggestingthat DCD additions can mitigate nitrogen emissions fromagricultural soils fertilizer with pig slurries.

Most soils under xeric and arid climates have low levels oforganic matter and additions of organic matter are necessaryto improve the physical, chemical and biological soilproperties. When organic N fertilizers were applied to suchsoils denitrification rates in relation to the application ofmineral fertilizer increased (Vallejo et al., 2004, and Vallejo etal., 2005). This loss of valuable N fertilizer could beconsidered negative, but as shown in this study, the additionof organic fertilizer under irrigation conditions to a clay loamsoil with low organic C content reduced the N2O and NO.Application of fertilizers with a high soluble organic Ccontent instead of urea could therefore be a good agriculturalpractice to reduce N emissions from such irrigated soils.However, further investigations on a range of soils differingin texture, carbon content and moisture conditions should becarried out. In this experiment, significant differences in yieldat po0.05 were only observed between fertilized plots andControl (27,500kgha�1). Treated organic wastes wereeffective in maintaining potato yields (ranged from 32,000to 37,000kgha�1), as well as urea (37,800kgha�1) oruntreated pig slurries (35,300kgha�1). Therefore, themitigation of the emissions using treated organic wastes isa suitable option for farmers. The application of untreatedorganic residues creates odours and health risks and can beprevented by composting and anaerobic digestion. Theseprocesses furthermore stabilize and homogenize the product,and the separation of solid before anaerobic digestion reduce

its viscosity and therefore facilitates a better distribution andinfiltration into the soil. This effect also decreases NH3

volatilization and the potential for the anaerobic processes insoil (Petersen, 1999). In our study treated organic materialsmitigated denitrification losses and under certain conditionsalso NO and N2O emissions. These positives characteristicsimprove the quality of these materials and for that reason itis proposed that organic residues incorporated to the soilshould be previously treated.

5. Conclusions

This study underlines the key role of degradable C addedwith the organic fertilizer in the emissions of N2O and NOfrom soils with a low organic C content under irrigationconditions. Soluble organic C compounds favoured thedenitrification process, increased the proportion of N2Oarising from denitrification, decreased the N2O/N2 emis-sion ratio and reduced the emissions of NO and N2Ocompared to those from soils fertilized with mineral N. Forthat reason, in this type of soils organic fertilizers should berecommended instead of urea, because they mitigate N2Oand NO emissions.Anaerobic digestion and separation improved the

quality of pig slurry as fertilizer and is an option tomitigate denitrification losses and N2O emissions, althoughno effect was observed on NO fluxes. The use ofdicyandiamide mixed with untreated pig slurry mitigatedN2O and NO emissions, although denitrification was notaffected. Changes in the composition of the organic Cfraction and C:N ratio as caused by composting organicresidues affected gaseous emissions. When this materialhad a low soluble organic C denitrification was reduced butN2O and NO emission were increased.This study has shown that in order to provide effective

mitigation of N trace gas emissions it is necessary tounderstand the effect of all commonly used organic andinorganic fertilizers in different agroecosystems and underdifferent management practices.

Acknowledgements

The authors are grateful to the Spanish Commission ofScience and Technology (CICYT) for financing thisresearch and IMIA (Instituto Madrileno InvestigacionAgraria) for lending the experimental field. It is a pleasureto acknowledge Roberto Sainz, Ana Ros, Raquel Mantecaand Maria C. Tejeiro for their technical assistance. Wewould like to thank Javier Litago for his assistance onstatistical analysis.

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