Biol Fertil Soils (2004) 40: 93–100DOI 10.1007/s00374-004-0742-6

ORIGINAL PAPER

Antonio Vallejo . José A. Díez .Luís M. López-Valdivia . María C. Cartagena .Ana Tarquis . Pedro Hernáiz

Denitrification from an irrigated soil fertilized with pig slurryunder Mediterranean conditions

Received: 3 September 2003 / Revised: 26 February 2004 / Accepted: 26 February 2004 / Published online: 16 April 2004# Springer-Verlag 2004

Abstract Under a Mediterranean climate, denitrificationlosses were quantified for 2 years on a sandy loam soilwith an irrigated maize crop. The effect of pig slurryapplication at two different rates (165 and 495 kg N ha−1,respectively, for PS1 and PS3) was compared with that ofurea (U) applied at 165 kg N ha−1 and with a controltreatment (P0) without fertilizer. After application, thedenitrification rate (DR) increased in PS1 and PS3 respectto P0 and decreased to the levels of the control treatmentafter 5 days. In July and August (the irrigation period) theDR increased considerably in all treatments with maxi-mum values for the PS3 treatment (0.134 g N m−2 day−1 inthe first year and 0.147 g N m−2 day−1 in the second year).The differences in DRs between each treatment could beexplained by the pattern of water filled pore space, NO3

−

concentration of the soil solution and the soil temperatureduring the maize growing season. In the first yeardenitrification losses in the 0–10 cm layer were 1.90,2.49, 2.87 and 4.00 g N m−2 for P0, U, PS1 and PS3,respectively, while in the second year the losses were 1.21,2.28, 2.47 and 3.42 g N m−2. Finally, a simple predictivemodel (SOILN) was evaluated and found to giveacceptable results.

Keywords Denitrification . Irrigated maize .Mediterranean climate . Pig slurry . SOILN model

Introduction

Denitrification can be an important cause of low N useefficiency (Liang and McKenzie 1997; Simek et al. 2000)and if N2O is the end-product of the process, it cancontribute to ozone depletion and global warning (Wang etal. 1976; Crutzen and Ehhalt 1977).

O2 is considered to be the principal regulator ofdenitrification in arable soils (Mosier et al. 1986). Theincrease in soil moisture as a result of rainfall or irrigationoften restricts soil aeration in the field. Denitrificationlosses measured in irrigated crops showed great variabilityranging from 4 (Mahmood et al. 1998) to 200 kg N ha−1

(Ryden and Lund 1980). However, denitrification losseshave been rarely measured in irrigated crops of southernEuropean countries (Teira-Esmatges et al. 1998; Sánchezet al. 2001) despite the extension of these cropped surfaceareas. Conditions of these soils favour denitrification ashigh moisture contents due to irrigation coincide with highsoil temperatures, another factor affecting the denitrifica-tion (Maag and Vinther 1999).

The rates of N fertilizers applied to such irrigated soilsare generally very high due to their low organic mattercontent, usually <2%. The use of fertilizers with additionsof organic matter residues is getting more popular. As thesecond pig producer in the EU, Spain generates 2×1010 kg year−1 pig slurry of which over half is directlyused as fertilizer (Chávez and Babot 2001). The applica-tion of slurry to soil stimulates the activity of denitrifyingbacteria due to the addition of easily degradable organic C(Christensen 1985; Rochette et al. 2000). In addition, thecontent of available N is higher than a crop’s Nrequirement due to these applications. Therefore, theconditions of pig slurry-treated soils may be highlyfavourable to denitrification losses.

The aims of this study were: (1) to quantify thedenitrification rate (DR) and the denitrification lossesoccurring in irrigated crops under a Mediterranean climate;(2) to compare the effect of the pig slurry application onthe denitrification losses with that of urea fertilization; (3)

A. Vallejo (*) . L. M. López-Valdivia . M. C. Cartagena .A. TarquisEscuela Técnica Superior de Ingenieros Agrónomos,Universidad Politécnica de Madrid, Ciudad Universitaria,28040 Madrid, Spaine-mail: avallejo@qaa.etsia.upm.esTel.: +34-91-3365650Fax: +34-91-3365639

J. A. Díez . P. HernáizCentro de Ciencias Medioambientales CSIC,Serrano 115,28006 Madrid, Spain

to simulate denitrification by a simple prediction model(SOILN).

Materials and methods

Experimental site

The field was located at La Poveda Field Station in Arganda del Rey(Madrid) (40°19′N, 3°19′W) in the middle of the Jarama river basinand the experiment was carried out in 1998 and 1999. The TypicXerofluvent soil is a sandy loam in the uppermost 0.5 m. Somephysico-chemical properties of the (0–20 cm) soil were: 14 g totalorganic matter kg−1; 1.1 g total Kjeldahl N kg−1; pHH2O8.1;1.47 Mg m−3 bulk density; 34 g CaCO3 kg−1; 20.2% (w/w) fieldcapacity; 44.5% porosity; 37% sand; 45% silt; and 13% clay. Watersoluble organic C was 37 mg C kg−1 and the NO3

−–N content was15.5 mg NO3

−–N kg−1, on 1 April 1998. The mean temperature inthis area (in the last 10 years) was 13.5°C. In the maize growingseason, the daily mean temperatures varied between 13°C and 30°C.The mean annual rainfall was 460 mm (in the last 10 years).Soil water measurement devices, time domain refractometry

(TDR) probes and vertical tensiometers, capable of measuring waterpressures of 0 to −80 kPa at depths of 0.2, 0.4, 0.6 and 0.8 m wereplaced in an experimental area of 0.2 ha containing 12 plots.

Pig slurry

A fattening farm with 680 animals was selected to provide the pigslurry. The slurry sewage, kept in storage tanks, was collected fromthe discharge points. Before slurry application, the N content was

estimated to calculate the application doses. The main characteristicsof the slurry applied in the first and second year were: pH 7.0 and7.0; 85 and 76 g dry matter kg−1; 740 and 704 g organic matter kg−1

(dry weight); 3.13 and 2.70 g total N kg−1 ; 2.26 and 1.84 g NH4+–

N kg−1 ; 1.0 and 1.6 g total P kg−1; and 2.10 and 1.98 g total K kg−1,respectively. Following the commonly used local techniques, awaterspout was connected to a tanker and slurry was applied to thesoil. A rotocultivator was used 5 days after application, so the pigslurry was incorporated into the upper soil layer (0–5 cm). Thistechnique has the disadvantage of causing a considerable emissionof NH3 into the environment (Thompson et al. 1987).

Experimental design and crop

Two doses of pig slurry were applied to plots (9.9×11.1 m) on 14April; an optimal N dose (165 kg N ha−1) (PS1) and a triple dose(495 kg N ha−1) (PS3). Based on the electroultrafiltration soilanalysis and according to criteria established by Sánchez et al.(1998), the optimal N dose established for the maize crop was closeto 165 kg N ha−1 in both years. Thus, 52.7 and 61 m3 pig slurry ha−1

was applied to PS1 plots in the first and the second year,respectively, and the triple dose to PS3. Another treatment involvedan urea (U) application on 3 June at the optimal rate of 165 kgN ha−1. Untreated plots, used as controls (P0), were not treated withN fertilizer for the last 7 years. Each treatment was replicated 3times. In order to maintain the same soil water content on all plots,U and control plots were irrigated with 18 mm and PS1 plots with12 mm of water on 14 April.On 24 April maize (Zea mays L. cv Juanita) was sown in rows

with 75 cm spacing at a density of 75,000 plants ha−1. Maize washarvested at the end of October in both years when the grain wasmature. An overhead mobile-line sprinkler irrigation system wasused during the maize growth period. Field data provided by the

Fig. 1 a,b Rainfall and irriga-tion, c,d water filled pore spaceand e,f soil temperature duringthe maize growing season. AprApril, Jul July, Sep September

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instruments located at the measuring sites were used to adjustirrigation depth; for more details see Román et al. (1996). Over thestudy period, July–August, ten irrigation sessions at a frequency ofonce per week were conducted. Each irrigation ranged from 32 to60 mm and was calculated by considering the soil water balanceduring each session so that the final drainage was around 15% ofapplied water. Figure 1 includes the irrigation doses and rainfall. Thetotal amount of water applied was 532 and 554 mm in the first andsecond year, respectively.

Sampling and determining denitrification in soil

Denitrification was estimated in the slurry treated plots until themaize harvest in each of the 2 years. The measurements were takenat different intervals, depending on the DR recorded in the lastsampling and on the value of soil moisture. Therefore, during thefirst 15 days after the slurry application three samples were takenand thereafter every 2 weeks due to the low water filled pore space(WFPS) level. During July and August one sample a week wastaken and the sampling was carried out between 12 and 48 h afterthe end of irrigation. In the period after irrigation (September andOctober), samples were taken every 15 days. During the wholegrowth period samples were also taken after every rainfall event ifthe WFPS was >60%; in this case the sampling was also carried out12–48 h after the end of rainfall. Denitrification was estimated in thefield by the C2H2 inhibition technique (Ryden et al. 1987). Six intactsoil cores (3 cm diameter×10 cm in depth contained in perforatedPVC sleeves) were placed in 1-l hermetically sealed glass bottleswith an atmosphere of 5% C2H2 created by replacing 50 ml of airwith 50 ml C2H2. In the field, they were incubated in holes madewithin the experimental field and gas was sampled after 24 h; theN2O content was analysed via gas chromatography (HP6890) with a63Ni electron-capture detector. A HP-Plot Q capillary column wasused, incorporating a HP-Retention Gap capillary pre-column toremove the water vapour from the sample. The injector, oven anddetector temperatures were 50, 50 and 300°C, respectively, and thecarrier gas (N2) flux was 30 ml min−1.Total denitrification over the experimental period was estimated

by integrating daily losses over time per plot and then the averagevalue was calculated per treatment. Some authors (Aulakh andRennie 1984; Parkin et al. 1985), assume a constant rate betweenmeasurements based on the use of 15N, whereas others (Parkin 1987;Christensen and Tiedje 1990) do not agree with this. In our case, theintegration between two consecutive measures (during irrigationperiod) was carried out only if the WFPS value was >65%, duringthe considered period; usually this occurred 4 or 5 days after eachirrigation session. The 65% value was the threshold value of WFPSafter which a substantial increase in denitrification occurred (Linnand Doran 1984).

Soil analysis

After sampling the headspace, the soil from each jar was thoroughlymixed and soil NO3

− and NH4+ were determined by extracting 10 g

fresh soil with 100 ml 0.01 M CaCl2; NO3− and NH4

+ weredetermined colorimetrically using a Technicon AAII Auto-analyser(Technicon Hispania, Spain) according to the procedure described inISO 14255 (1998). WFPS was calculated by dividing the volumetricwater content by the total soil porosity. Total soil porosity wascalculated by measuring the bulk density of soil, according to thefollowing relationship: soil porosity = (1−soil bulk density/2.65);and assuming a particle density of 2.65 Mg m−3 (Linn and Doran1984). Twice a week the volumetric water content was determinedin the field by means of TDR. The moisture content in the cores wasdetermined gravimetrically, although the result was also expressedin the form of WFPS (Fig. 1). Soil temperature was monitored in thefield using a temperature probe inserted 20 cm into the soil, andconnected to a data logger.

Model application

Numerous attempts have been made to simulate the N cycle inagricultural soils using different mathematical models. The SOILNmodel (Johnsson et al. 1987), developed by Johnsson et al. (1991),describes the most important processes affecting N dynamics inagricultural soils with simulations of the denitrification process.Denitrification in a specific soil horizon z is calculated as

Da ¼ kd zð Þet zð Þemd zð ÞeNO3 zð Þ (1)

where Da is the actual DR, kd(z) is the potential DR, et(z) is thetemperature factor, emd(z) is the soil water/aeration status and eNO3

zð Þ is the NO3− reduction function (Johnsson et al. 1987). The

temperature factor et(z) is given by the following relationship

et zð Þ ¼ Q0:1 T zð Þ�Trefð Þ10 (2)

where T(z) is the temperature for the z layer, Tref is the temperature atwhich et(z) is equal to 1 and Q10 is increase factor of et(z) when the Tvalue increase by 10°C. Function emd(z) depends on soil volumetricwater content (θ) by the following equation

emd zð Þ ¼ � � �dð Þ��s � �dð Þ

� �d(3)

where θs is soil volumetric water content at saturation, θd is thethreshold water content and d is a dimensionless empirical curve

Fig. 2 Denitrification rate (DR)during the maize growth periodfor 2 consecutive years. Longarrow Date of pig slurry appli-cation, short arrow urea appli-cation, double-headed arrowfirst irrigation. Vertical barsindicate LSD at 0.05 betweentreatments for each sample time.P0 Control without fertilizer, Uurea applied at 165 kg N ha−1,PS1 165 kg N ha−1, PS3 495 kgN ha−1, d-1 day-1; for otherabbreviations, see Fig. 1

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shape parameter. The dimensionless reduction function for NO3−

concentration in soil solution is expressed by the Michaelis-Mentenequation type

eNO3 zð Þ ¼ NO�3

NO�3 þ km

(4)

where NO3− is NO3

−–N concentration in soil solution and km, is theMichaelis-Menten half-saturation constant.

Statistical analysis

The statistical analysis was performed using the STATGRAPHICSPlus 5.1 (Manugistics 2000). One-way ANOVA also served toestablish the effect of fertilizer treatment on the DR and NO3

−–Ncontent. The LSD test was used for multiple comparisons of means.Simple correlation analyses were performed to determine therelationship between DR and WFPS, NO3

−–N, and soil temperature.

Results

Denitrification rate

In all treatments three periods with very different DRswere clearly distinguished and they were related todifferent soil moisture (Fig. 2). The first period fromApril to June the soil was not irrigated and the denitri-fication was low, the second was the irrigation period(from July to August) with an important denitrificationactivity and the third period after irrigation (fromSeptember to October), was characterized by a decreasein denitrification.

Three or 4 days after slurry application a peak in DRactivity was observed. In the first year the peaks were0.022 and 0.027 g N m−2 day−1 for PS1 and PS3,respectively, and 0.015 and 0.023 g N m−2 day−1 in thesecond year (Fig. 2). As soon as the slurry was buried, 5days after its application, the DRs decreased veryconsiderably. Significant differences (P<0.05) betweentreatments were not found from 19 April to the beginningof the irrigation period.

The first irrigation increased DR activity, and the levelwas kept up until the end of irrigation period. In the firstyear, maximum emission took place 10 days after thebeginning of irrigation for P0 and U (0.060 and 0.083 gN m−2 day−1, respectively) treatments; higher DR peaks(0.072 g N m−2 d−1 on 11 August for PS1 and 0.134 gN m−2 d−1 on 22 July for PS3) were observed in the pigslurry treatments. In the second year, maximum emissionin all the treatments took place 18 days after the beginningof irrigation and the DR reached 0.077, 0.117, 0.133 and0.147 g N m−2 day−1 for P0, U, PS1 and PS3, respectively.During the whole irrigation period (July–August), the DRwas highest in the PS3 treatment; the DR was higher in thePS1 and U treatments than in the control (P0) in thisperiod. As mentioned above, denitrification decreasedmarkedly in September and October, and there were nosignificant differences (P<0.05) between the treatments.

The DR of each treatment was significantly (P<0.001)correlated with WFPS (Table 1). In the first phase, WFPSwas <65% in both years, even after each rain eventbecause the rainfall was not heavy. The threshold value ofWFPS, after which a substantial increase in denitrificationoccurred, was 65%. WFPS >70% was obtained afterirrigating twice in the first year and after irrigating 3 timesin the second year. The WFPS reported in Fig. 1 peaked12 h after each irrigation. The values decreased markedly4 or 5 days after irrigation, as a consequence of the intenseevapotranspiration of the crop.

The mean temperature of the 0–10 cm soil (Fig. 1)varied between 17°C and 20°C from April to June,between 20°C and 25°C from July to August and between17°C and 20°C from September to October. Thus, thehighest temperatures coincided with the highest moisturelevels, which also contributed to the DR activation. Therewas a significant exponential correlation (P<0.05) be-tween DR and the temperature in each treatment (r=0.68–0.80, n=36).

Variations in soil NO3− concentrations

The NO3− concentration in soil (Fig. 3) was significantly

higher in the slurry treatments than in the plots withoutfertilization before irrigation. The NO3

− concentrationpeak was observed 15–40 days after urea application tosoil. At the beginning of the irrigation period, NO3

− wasleached to lower horizons and therefore its concentrationdecreased and it was <20 mg N kg−1 in the surface layer ofall treated soils. Small concentration peaks were observedin the PS1 and PS3 treatments during the irrigation periodas a consequence of the organic N mineralization and thesuccessive nitrification. In neither years was there asignificant correlation (P<0.05) between DR and the NO3

−

concentration in the soil without fertilizer (P0) in theperiod between the first irrigation and the end of theexperiment (Table 1). There was not a significant corre-lation between DR and NO3

− concentrations for any of thefertilizer treatments.

Table 1 Correlation coefficients between denitrification rate andwater-filled pore space (WFPS), soil temperature and NO3

−

concentration in each treatment (n=36). P0 Control treatment, Uurea treatment, PS1 pig slurry at 165 kg N ha−1, PS3 pig slurry at495 kg N ha−1

Treatment WFPSa Soil temperaturea NO3− concentrationb

P0 0.76*** 0.80*** 0.51**U 0.79*** 0.77*** 0.21NSPS1 0.82*** 0.72*** 0.35 NSPS3 0.82*** 0.68*** 0.09 NS

**P<0.01, ***P<0.001, NS not significant at P<0.05aExponential correlationbLinear correlation

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Modelling

The potential DR (Kd) of the top 10 cm of the soil was0.121 g N m−2 day−1 for U and control treatments, basedon the maximum field value measured from these plotsduring the experimental period (Johnsson et al. 1991). Dueto the influence of organic C on Kd (Hénault and Germon2000) different values of Kd were selected for each pig

slurry treatment, i.e. 0.133 and 0.147 g N m−2 day−1 forPS1 and PS3, respectively.

The threshold volumetric moisture content of soil fordenitrification (θd=0.285 m3 m−3), as determined from theDR values measured in the field, corresponded to a WFPSvalue of 65%. Soil volumetric moisture at saturation (θs)was 0.445 m3 m−3. The constant d (Eq. 1) and theMichaelis-Menten half-saturation constant (Km) werecalibrated using the root-mean-square (RMS) of residuals

Fig. 3 NO3− concentration in

the 0–10 cm soil layer duringthe maize growth period and for2 consecutive years. Long arrowDate of pig slurry application,short arrow urea application,double-headed arrow first irri-gation. The vertical bars indi-cate LSD at 0.05 between treat-ments for each sample time. Forother abbreviations, see Figs. 1and 2

Fig. 4 Comparison between si-mulated (lines) and measuredfield DRs (point without lines)in different treatments. Longarrow Date of pig slurry appli-cation, short arrow urea appli-cation, double-headed arrowfirst irrigation. For abbrevia-tions, see Figs. 1 and 2

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which gave d=2 and Km=14 mg N l−1. Under the climaticconditions of the field experiment, the Tref value was24°C. The Q10 coefficient of soil was calculated bySánchez et al. (2001) and was equal to 3.

Figure 4 shows simulated and measured values for thesurface soil (0–10 cm) of each of treatments. In all casesthe simulated and measured values were significantlycorrelated (P<0.05).

Cumulative denitrification

The accumulated denitrification losses of the upper soillayer (0–10 cm) were rather high (Table 2); the maximumvalues were reached in the PS3 treatment (4.00±0.87 and3.42±0.22 g N m−2 for the first and second year,respectively) and the lowest in the control (P0) (1.90±0.22 and 1.21±0.24 g N m−2, respectively). The PS1 andU treatments did not show significant differences in Nlosses. Between 83% and 92% of the losses occurred inthe irrigation period (between 1 July and 25 August). Withthe increase in the slurry dose, there was a 39% and 38%increase in the denitrification losses in the first and secondyear, respectively. The average coefficient of variation fortotal denitrification losses was 15.9%, and the range was6.4–30.9%.

The cumulative denitrification losses during the cropperiod (200 days) in the 2 years were also calculated bythe model using daily temperatures, water content andNO3

− concentration of the soil solution. Soil NO3−

concentrations measured on the sampling days, wereused whereas for the remaining days NO3

− concentrationswere estimated assuming a linear variation between twomeasures. The simulated values using the SOILN modelare included in Table 2 and were close to the measuredvalues.

Discussion

Rainfalls in the period with the highest water demand ofthe crop (June–August) were very low, i.e. 21 and 62 mmin the first and second year, respectively. Therefore, underthese climatic conditions, it was necessary to frequentlyirrigate the maize crop. As a consequence of the amount ofwater added with each irrigation (60 mm) in a very shorttime, the soil was temporarily flooded for 3 or 5 days afterirrigation, and moisture levels were higher than the WFPSvalue of 65%. According to Smith and Patrick (1983),frequent changes from anaerobiosis to aerobiosis and viceversa increase the emission of N2 and N2O; indeed DRpeaks occurred during irrigation.

The WFPS threshold value of this soil (65%) is similarto the value reported by Rolston et al. (1984), Grundmannand Rolston (1987), Hénault and Germon (2000) andVallejo et al. (2001). According to Barton et al. (1999), thethreshold value can vary depending on the soil texture.These authors found higher values (74% and 83%) thanour value in soils with a coarse texture (sandy and sandyloam), while in loam soils the value ranged from 62% to83%. Below the threshold value the O2 content issufficiently high to inhibit denitrifying enzymes (Smithand Tiedje 1979). The influence of soil water content onthe DR was studied using the SOILN model equation(Eq. 3). This equation is similar to the soil-waterrelationship published by Grundmann and Rolston(1987) and included in the NEMIS model (Hénault andGermon 2000). The d parameter value obtained was verysimilar to that found by the authors mentioned above.

In each treatment DR was affected by soil temperature.Other authors (Mahmood et al. 1998; Sánchez et al. 2001)have found positive correlations between the DR and thesoil temperature in irrigated maize crops. Most functionsdescribing the effect of temperature on microbial processesare based on the Arrhenius or Van’t Hoff laws. The SOILNmodel uses the et(z) equation to estimate the effect of

Table 2 Denitrification losses from the upper soil layer (0–10 cm) integrated over different periods during the maize growing season. IPIrrigation period; for other abbreviations, see Table 1

Period Denitrification losses (g N m−2)

P0 Ua PS1 PS3

Maize season first yearBefore IP (15April–1 July) 0.11±0.08b (0.18)c 0.12±0.12 (0.18) 0.16±0.08 (0.21) 0.18±0.10 (0.23)IP (1 July–28 August) 1.65±0.23 (1.26) 2.23±0.51 (2.02) 2.57±0.30 (2.47) 3.68±0.89 (2.88)After the IP (29 August–15 October) 0.14±0.08 (0.09) 0.14±0.12 (0.16) 0.14±0.09 (0.35) 0.14±0.06 (0.41)Total season 1.90±0.22 (1.53) 2.49±0.77 (2.36) 2.87±0.30 (3.03) 4.00±0.87 (3.52)Maize season second yearBefore IP (15 April–1 July) 0.17±0.07 (0.01) 0.14±0.07 (0.01) 0.27±0.07 (0.08) 0.34±0.11 (0.10)IP (1 July–28 August) 1.01±0.17 (1.61) 2.02±0.34 (2.14) 2.06±0.31 (2.52) 2.89±0.18 (2.91)After IP (29 August–15 Oct) 0.03±0.05 (0.07) 0.12±0.06 (0.11) 0.16±0.05 (0.21) 0.19±0.05 (0.24)Total season 1.21±0.24 (1.69) 2.28±0.37 (2.25) 2.47±0.25 (2.81) 3.42±0.22 (3.25)

aApplied 3 JunebMean value of accumulated denitrification losses from three plots ±SDcSimulated denitrification losses in parentheses

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temperature, and this equation depends on Q10. For ourfield the soil Q10 was nearly 3 (Sánchez et al. 2001) andwas calculated indirectly through the Arrhenius equation.Johnsson et al. (1991) found a similar value. The Tref ofEq. 2 was 24°C, which was also the temperature of theirrigation period. A good agreement between Tref and themeasured soil temperature is necessary, in order to reduceerrors of the estimate (Rodrigo et al. 1997). Johnsson et al.(1991) used a Tref of 20°C and by using this value theRMS was higher.

The low DRs found 10 days after the slurry applicationwere due to the low WFPS values, and low temperatures(16–18°C) of April, and the burying of the slurry 5 daysafter its application by ploughing; this managementpractice increases the soil aeration. It is well establishedthat tillage decreases denitrification losses (Aulakh et al.1984, 1992).

In May and June, there were no significant DRdifferences between the various treatments, including thecontrol treatment, because the NO3

−–N level was highenough to maintain denitrification. With the first irrigationa remarkable decrease occurred in the NO3

− concentrationof the upper soil due to leaching and the plant uptake. Forexample, the NO3

− concentration in the control treatmentdecreased to 1 mg NO3

−–N kg−1 soil, thus confirming thatNO3

− can be a limiting factor in non-fertilized agriculturalsoils (Groffman et al. 1993; Tenuta and Beauchamp 1996).This also explains the significant correlation between theDR and the NO3

− concentration in the control treatment,but not in the others treatments. The NO3

− concentrationssupposed to limit denitrification range from 1 to 10 mgNO3

−–N kg−1 (Ryden 1983; Jordan 1989; Estavillo et al.1994). The DR decreases during the second month ofirrigation of the fertilized plots could have been due to thelow NO3

− values (1–3 mg NO3−–N kg−1) in the soil after

irrigation even though the WFPS values were not <65.In the SOILN model the NO3

− reduction functioneNO3ð Þ is supposed to follow the Michaelis-Mentenkinetic. The Km value obtained in this study (14 mgN l−1) was slightly higher than that used by Johnsson et al.(1991) in a Swedish soil (10 mg N l−1). Strong and Fillery(2002) reported Km values ranging from 1 to 100 mg N l−1

in sandy soil, depending on the soil C concentration andmatric potential.

The highest DR observed in the slurry-treated soils wasdue to the addition of easily decomposable organic C(Rochette et al. 2000) because in these soils the solubleorganic C level was low (37 mg C kg−1). Sánchez et al.(2001) have shown that the DR can increase when organicC is added to these soils.

The potential DR (Kd) in the SOILN model is lowerthan that of other models (Hénault and Germon 2000),probably because the latter have been calculated fromlaboratory experiments, where NO3

− was added underanaerobic conditions. As the organic C evolution of soilscan influence Kd (Hénault and Germon 2000), somemodels, such as DAISY (Hansen et al. 1991) andSONICG (Bril et al. 1994), calculate this parameter byincluding the respiration rate or C degradation rate. Asmentioned earlier, Kd is the highest DR measured in thefield during the experimental period and accounts foreffects on denitrification which can be ascribed to

parameters such as available C content, soil type, plant/roots, etc. (Johnsson et al. 1991). In this study, different Kd

values (0.134 g N ha−1day−1 for PS1, and 0.147 gN m−2day−1 for PS3) were selected in order to account forthe different available organic C contents, resulting fromthe addition of pig slurry. The RMS of simulated values,using the Kd for each treatment, was lower than thoseobtained using an unique Kd value for all of them.

In this study, both simulated and measured denitrifica-tion values refer to the top 10 cm layer. It can be assumedthat the denitrification losses (Table 2) from the soil wereunderestimated because denitrification can also occurbelow the 0–10 cm surface layer. Although denitrificationlosses measured for the 10–20 cm layer were close tothose from the 0–10 cm soil layer (Mahmood et al. 1998),generally the major contribution to denitrification is fromthe upper 10 cm soil layer (Svensson et al. 1991; Estavilloet al. 2002). Generally simulated denitrification lossesagreed with those obtained by integrating the measuredDR. However, the model underestimated losses when themoisture content was lower than θd, because measureddenitrification losses were higher than the 0 valuepredicted by the SOILN model.

Studies on denitrification in Mediterranean countries arescarce. In Italy, denitrification losses of 0.397 g N m−2

were measured by Arcara et al. (1999) from non-irrigatedmaize, while in north-east Spain losses ranged between0.19 and 1.17 g N m−2 from irrigated maize (Teira-Esmatges et al. 1998). In this study, carried out in centralSpain, denitrification losses ranged from 1.21 to 4.00 gN m−2 and were of the same order of magnitude asleaching losses measured in the same soil by Díez et al.(2001) in 1998 (1.20 g N m−2 for P0; 2.88 g N m−2 for U;2.83 g N m−2 for PS1 and 4.24 g N m−2 for PS3 in thewhole growth period).

In conclusion, the study shows that N losses bydenitrification are important in an irrigated soil under aMediterranean climate, especially during the irrigationperiod (July, August), when the soil is fertilized with ureaor pig slurry. Fertilization of the soil with pig slurry shouldbe carefully carried out so as to avoid high applicationrates, which stimulate denitrification losses. The denitri-fication process can be effectively simulated by consider-ing the moisture, NO3

− content and temperature of soil.The SOILN model could be a very useful tool forsimulating and predicting the effect of a N fertilizer rateand irrigation on denitrification losses.

Acknowledgements The authors are grateful to the INIA (SpanishMinistry of Agriculture, Fisheries and Food Resources) (projectSC98-C2.2) and the Spanish Commission of Science and Technol-ogy (project AGL2000 1554-C02) for financing this research. It is apleasure to acknowledge M.D. Atienza for his technical assistance.

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