Nitrogen oxide emissions affected by organic fertilization in a non-irrigated Mediterranean barley field

Agriculture, Ecosystems and Environment 132 (2009) 106–115

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Nitrogen oxide emissions affected by organic fertilization in a non-irrigated Mediterranean barley field Ana Meijide *, Lourdes Garcı´a-Torres, Augusto Arce, Antonio Vallejo Escuela Te´cnica Superior de Ingenieros Agro´nomos, Polythechnic University of Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain

A R T I C L E I N F O

A B S T R A C T

Article history: Received 26 November 2008 Received in revised form 5 March 2009 Accepted 17 March 2009 Available online 14 April 2009

Contradictory findings can be found in the literature regarding the effects of applying organic instead of mineral fertilizers on the associated emissions of nitrous oxide (N2O) and nitric oxide (NO). The main aim of this experiment was to study the effect on these emissions of applying mineral or organic fertilizers to a non-irrigated crop under Mediterranean conditions. A secondary aim was to determine whether application of the fertilizer had a residual effect on the N2O and NO pulses observed after the first rainfall events in autumn, and the magnitude of these fluxes. A field experiment was carried out with a barley crop (Hordeum vulgare L. cv Bornova). Untreated pig slurry (UPS), digested pig slurry (DPS), municipal solid waste (MSW) and composted crop residues mixed with sewage sludge (CCR + S) were applied to the soil. The resulting emissions were compared with those from a mineral fertilizer, urea (U), and a control treatment (C), in which no nitrogen was applied. Very low NO and N2O fluxes were measured during the entire experimental period in all treatments. The accumulated N2O emissions from the organic and inorganic fertilizers ranged from 266 to 345 g N2O-N ha1 and did not show significant differences. Three of the four organic fertilizers had the positive effect of reducing NO emissions (28.82–44.48 g NON ha1) compared with inorganic fertilizer (61.86 g NO-N ha1). Nitrous oxide pulses were observed in autumn. Negative N2O fluxes were measured on several occasions. The emission factor relating N2O emissions to the N applied as fertilizer, ranged from 0.06 to 0.17% for MSW and DPS, respectively, which is much lower than the default factor proposed by the IPCC. The emission factor which relates N2O emissions to crop production ranged from 241 to 361 mg N2O-N kg1 for DPS and U, respectively, suggesting that DPS should be promoted in order to reduce N2O emissions. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Nitrous oxide Nitric oxide Organic fertilizers Mediterranean agrosystem Sink

1. Introduction Agricultural soils are one of the main anthropogenic sources of nitrous oxide (N2O) (IPCC, 2007), which is a powerful greenhouse gas (GHG), and nitric oxide (NO), which catalyses the photochemical formation of tropospheric ozone (Crutzen, 1979). These nitrogen (N) oxides are predominantly produced by microbial processes, as by-products of nitrification and products of denitrification (Firestone and Davidson, 1989). In agricultural systems, organic and mineral fertilizers are known to be key contributors to N2O and NO emissions from soils (Mosier et al., 1998). Fluxes depend on the amount and chemical composition of fertilizers (Baggs et al., 2002; Vallejo et al., 2006), both of which affect denitrification and nitrification. However, the effect of each fertilizer is also controlled by other conditions of the agrosystem (soil, crop, water-filled pore space (WFPS), temperature, etc.), and it is not possible to establish a general behaviour.

* Corresponding author. Tel.: +34 91 336 56 52; fax: +34 91 336 56 39. E-mail address: [email protected] (A. Meijide). 0167-8809/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2009.03.005

The application of organic amendments to annual crops generally increases denitrification and N2O emissions (Mogge et al., 1999; Jones et al., 2005, 2007; Vallejo et al., 2006; Dambreville et al., 2008; Miller et al., 2008). However, contradictory effects on N oxides have been reported when comparing organic and mineral fertilizers in arable soils. For example, in a maize crop in the Netherlands, Van Groenigen et al. (2004) found a higher emission factor of N2O for manure than for mineral N fertilizer. This effect was attributed to more anoxic conditions produced by the stimulation of denitrification and to the supply of readily available C, a substrate for denitrification (Beauchamp et al., 1989). On the other hand, other studies such as those of Meijide et al. (2007), also in a maize crop, or of Ball et al. (2004), in a silage crop, demonstrated that manure reduced N2O fluxes compared with a source of mineral N. In these latter cases, the authors justified this by saying that conditions were favourable for denitrification, such as under irrigation (Meijide et al., 2007), or in systems with high rainfall (Ball et al., 2004). The application of a labile C source reduced the N2O:N2 ratio of the gases emitted by denitrification processes, thereby decreasing the N2O losses. There is, however, a lack of knowledge concerning the comparative

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effects of applying organic or mineral fertilizers in non-irrigated semiarid systems. Our hypothesis was that applying a source of C along with N fertilizers would have a small effect in systems where denitrification is not the main process responsible for N2O and NO emissions. Mediterranean climate is characterized warm and dry conditions in summer and moderate precipitation in winter (<300 mm year1). The pattern of N oxides emissions in this system is not well known and probably different to that observed in semiarid grassland by Mosier et al. (1998) or in semiarid agricultural soils by Bronson and Mosier (1993), Malhi et al. (2006) and Malhi and Lemke (2007), because in those studies, most of the rainfall occurred in summer. The upper part of the soil is usually dry during a significant part of the year in non-irrigated Mediterranean systems. When a dry soil is rewetted rapid emission pulses of N2O are generally produced, as observed by various authors in different soils (Davidson et al., 1993; Jørgensen et al., 1998; Kessavalou et al., 1998; Beare et al., 2009). Pulses of NO have also been measured after rewetting, as observed by Dick et al. (2001) in African soils. These pulses are produced because a significant proportion of microorganisms can die during the drying (Van Gestel et al., 1991) and rewetting (Kieft et al., 1987) of soil. The C and N present in dead microbial cells could be released to the soil and cause part of the flush of N (Marumoto et al., 1977). After rewetting, bacteria and fungi resistant to wetting and drying could use these labile compounds, producing pulses of N oxides in the process. The importance of these pulses in Mediterranean agrosystems is, however, not known. Additionally, we hypothesised that the emissions produced as a consequence of autumn rainfall could be influenced by the residual effect of treatments previously applied to the soil during the crop period. This is because C and N from the fertilizer could remain in the soil and affect the nitrification and denitrification processes. It was, therefore, of interest to study N2O and NO emissions during both the period after fertilizer application and after the first rainfall events following the dry period in order to better understand the mechanisms responsible for these emissions and the magnitude of the fluxes during both periods. A field experiment involving the application of different organic fertilizers (compost and liquid manures) and mineral N (urea) to a barley crop (Hordeum vulgare L. cv Bornova) grown in a nonirrigated soil under Mediterranean climatic conditions was carried out. The aims of this experiment were to: (1) evaluate the effect of fertilizer composition (mineral N, organic N, total organic C, soluble organic C) on NO and N2O emissions and the processes responsible of the production of these gases and (2) study the magnitude of N2O and NO pulses after the first rainfall events following the dry period and the processes and fertilizer effects controlling these pulses.

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2. Materials and methods 2.1. Soil characteristics and experimental procedure The field experiment was carried out at ‘El Encı´n’ Field Station (408320 N, 38170 W) in Alcala´ de Henares, Madrid. The site is located in the Henares river basin. The soil is a Calcic Haploxerepts according to the USDA soil taxonomy system (Soil Survey Staff, 1992) and a Calcaric Cambisol according to the FAO taxonomy (FAO, 1998) with a clay loam texture in the upper (0–28 cm) horizon. Some of the physicochemical properties of the top soil layer (measured on 20 January 2006 by conventional methods) are: total organic C, 8.2  0.4 g kg1; total N, 0.75  0.12 g kg1; pHH2 O , 7.9; bulk density, 1.41 Mg m3; CaCO3 13.1  0.3 g kg1; clay, 28%; silt, 17%; sand, 55%, soluble organic C (SOC), 30.6 C mg C kg1; NO3 28 mg NO3-N kg soil1 and NH4+ 5.1 mg NH4+-N kg soil1. The 10-year mean annual averages for temperature and rainfall in this area are 13.2 8C and 430 mm. Eighteen 30 m2 plots were selected and arranged in a randomized block design with three replicates per treatment in a field where no crop had been sown and no fertilizer had been applied in the previous 5 years. The treatments were: untreated pig slurry (UPS), anaerobically digested thin pig slurry fraction (DPS), composted organic waste (MSW), composted crop residues mixed with sludge (CCR + S), urea (U) and a control treatment without any nitrogen fertilizer (C). The physicochemical properties of the different fertilizers and the quantities of the different compounds incorporated into the soil with the different types of waste are summarised in Table 1. Pig slurries were obtained from the treatment plant at Almaza´n (Soria, Spain). The slurry was separated into its liquid and solid fractions using a rotary sieve drum (0.9 mm mesh). The digested pig slurry was obtained by the anaerobic digestion of the liquid fraction in a 50 m3 continuous digester with a hydraulic retention time of 32 days and a fermentation temperature of 35–40 8C. The composted organic waste was a mixture of the organic fraction of municipal solid waste obtained from the Pinto municipal waste treatment plant (Madrid, Spain) and garden waste, combined in a ratio of 1:2. The CCR + S was obtained by composting a mixture of crop residues with sewage sludge obtained, from the waste water ˜ ada, Madrid, in a 3:1 ratio. treatment plant of Villanueva de La Can All the organic treatments were taken to the experimental field between 2 and 4 weeks before the beginning of the experiment and the analysis of the composition of the fertilizers was carried out one week before the treatments were applied. They were applied on 23 January 2006 and were incorporated into the upper 0–5 cm of the soil profile using a rotocultivator. The fertilizers were applied at a rate of 125 kg available N ha1. For the solid treatments, the estimated percentage of organic N mineralised in the soil was around 40%, according to the method described in Sa´nchez et al.

Table 1 Composition of the fertilizers and amounts of the different compounds added with the fertilizers. Property

Moisture pH Total N NH4+ Total C Soluble organic C C/N

UPSa

DPSa

MSWa

CCR + Sa

Composition (g kg1)

Added (g m2)

Composition (g kg1)

Added (g m2)

Composition (g kg1)

Added (g m2)

Composition (g kg1)

Added (g m2)

966 7.1 86 70 186.5 4.95 2.17

– – 12.5 10.2 27.1 0.72 –

942 7.6 82 64 144.0 3.35 1.76

– – 12.5 9.76 22.0 0.51 –

354 7.6 19 3.40 262.0 0.11 14.0

– – 12.5 2.28 172.4 0.07 –

212 6.8 22 4.63 259.0 0.12 11.7

– – 12.5 2.65 146.9 0.07 –

Calculated in dry weight. a UPS—Untreated pig slurry; DPS—digested pig slurry; MSW—municipal solid waste composted with vegetable wastes; CCR + S—composted crop residues with sewage sludge.

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(1997). All plots were additionally fertilized with 75 kg P ha1 and 40 kg K ha1 (Ca(H2PO4)2 and K2SO4, respectively). Liquid manure was applied to the soil using a hosepipe connected to a tank in order to achieve a uniform distribution. To obtain a homogeneous distribution of fertilizer, the plots were divided in 4 sub-plots of 7.5 m2 and the amount of fertilizer required for each subplot was calculated. Barley was sown on the same day that the fertilizer was applied and was harvested on 4 June. This study focused on the two periods when most of the N2O and NO is produced: the first (crop period) extended from 23 January to 4 June, which corresponded to the period from fertilizer application, through the development of the crop, until harvest. The second period (fallow period) covered the first rainfall events of autumn. From June to October, no samples were taken because the soil was dry (WFPS < 24%). The total rainfall during this period was 11 mm, distributed in 5 rainfall events (less than 3 mm per event). After these short rainfalls, only the top layer of soil (0–3 cm) was wetted, which dried within a few hours. During these short periods of wet soil we assumed that the fluxes were negligible, as observed by Dick et al. (2001) in a dry soil. These potential fluxes have not been taken into account for the calculation of total emissions. 2.2. Sampling and analysis of N2O, NO and estimates of denitrification Nitrous oxide and NO fluxes were measured using 19.34 l manual chambers (diameter 35 cm, height 20 cm). All chambers were equipped with inlet and outlet holes which allowed the chambers to be kept closed during N2O sampling and open during NO measurements. The interior surfaces were covered with Teflon to prevent the gases reacting with the plastic of the chambers. Stainless steel rings were inserted in the soil to a depth of 10 cm, into which the chambers were fitted. The rings were inserted at the beginning of the experiment and kept in place throughout the experimental period. Measurements were always made with the barley plants inside the chambers during the crop period. When the plants were higher than 20 cm, plastic intersections of 20 cm covered with Teflon were used between the ring and the chamber. Thermometers were inserted in the chambers to record temperature throughout the sampling period, when the chambers were closed. A syringe was used to remove 10 ml samples from the headspaces of the chambers via a gas-tight neoprene septum. Samples for the N2O analysis were removed 0, 30 and 60 min after chamber closure and stored in previously evacuated 10 ml blood containers (Vacutainers, Venojet; Terumo Europe, Madrid, Spain). N2O concentration was quantified by gas chromatography, using a HP-6890 gas chromatograph (GCs), equipped with a Plot-Q capillary column. 3 ml of sample were manually injected into an automatic loop, which injected 1 ml into the column for analysis. A 63 Ni electron-capture detector (ECD) was used to measure the N2O concentration in the sample. The temperatures of the injector and oven were both 50 8C and the detector was at 300 8C. N2O flux rates were calculated from the rate of change in the concentration in the air inside the chambers during the 1-h sampling period. This was estimated as the slope of the linear regression between concentration and time (after corrections for temperature) and from the ratio between chamber volume and soil surface area (Mackenzie et al., 1998). A gas flow-through system was used to measure NO. During this measurement, the inlet was opened and air (filtered through a charcoal and aluminium/KMnO4 column to remove O3 and NOx) entered and passed through the headspace of the chamber. Gas samples were pumped from the chambers through the outlet to the detection instruments through Teflon tubing. NO was analysed by a chemiluminescence detector (AC31 M-LCD, Environnement S.A., Poissy, France) at a constant flow rate of 0.5 l min1. Flux measurements from the soil were interspersed with 3 measure-

ments from an empty chamber which was closed at the bottom with Teflon, but which had the same inlet and outlet systems as the other chambers. These were taken to take into account reactions with the chamber walls and lids. The NO flux was calculated as the product of the flow rate of the air stream through the chamber and the increase in NO concentration with respect to the control (empty chamber). Gas samples were taken from the chambers 4 times during the first week after fertilizer application, then 2–3 times per week during the first month. Subsequently samples were taken on a weekly basis until the end of the crop period for N2O, and until the emissions were close to zero in all the treatments (day 60) for NO. During the fallow period, when the soil was rewetted after the first rainfall events in autumn, samples were taken every 2–4 days for 3 weeks and then weekly until the end of the period. Denitrification was estimated through incubations using the acetylene (C2H2) inhibition technique (Tiedje et al., 1989) described in Vallejo et al. (2005). The estimates of denitrification losses from these analyses will be called ‘‘N2O production from acetylated soil cores’’ (N2O-ASC). Soil cores (2.5  10 cm depth) were taken with a manual soil corer and incubated with 5% (v/v) acetone-free C2H2 (99.6%) in 320 ml glass jars buried in the soil near the experimental field. After 24 h, a 10 ml gas sample was taken from each jar, using a syringe, and stored in a 10 ml evacuated vial (Vacutainers, Venojet; Terumo Europe, Madrid, Spain). Samples were taken every 1–2 weeks throughout the crop period. There are several associated problems with this technique, such as the poor diffusion of C2H2 within the soil, the use of this compound by microorganisms as a carbon source, the catalytic decomposition of NO in presence of C2H2 (Bollmann and Conrad, 1997) and the inhibition of nitrification at low concentrations (5 Pa) (Muller et al., 1998). In addition, this method, excluding plants, does not adequately evaluate the rhizosphere effects on denitrification and a direct comparison with fluxes of N oxides (measured in large chambers) is not possible. However, taking into account these problems, this system gives a qualitative estimate of denitrification losses in the field and can be used to make comparisons between treatments (Estavillo et al., 2002). 2.3. Soil and plant analyses Soil samples from the 0–10 cm layer were removed for soil analysis every 1–2 weeks during the crop period and 3 times during the fallow period. Approximately 100 g of fresh soil were collected from the field which was mixed in the laboratory. Soil nitrate (NO3) and ammonium (NH4+) contents were determined by extracting 10 g of the mixed fresh soil with 100 ml of 1 M KCl. NO3-N and NH4+-N were colorimetrically quantified using a Technicon AAII Auto-analyser (Technicon Hispania, Spain). For the analysis of soluble organic carbon (SOC) soil samples were extracted with deionized water to obtain a soil:water ratio of 1:1. The mixture was shaken gently with an orbital shaker for 15 min and centrifuged at approximately 7500  g for 5 min. The clear supernatant was filtered under vacuum through a 0.2 mm membrane filter. The extract was analysed for SOC by the method proposed by Mulvaney et al. (1997). Water-filled pore space (WFPS) was estimated by dividing the volumetric water content by total soil porosity. Total soil porosity was calculated by measuring the bulk density of the soil according to the relationship: soil porosity = (1  soil bulk density/ 2.65); assuming a particle density of 2.65 Mg m3. Volumetric water content was determined by oven-drying soil samples at 105 8C. Rainfall and soil temperature data were obtained from a meteorological station located in the field. Barley was harvested using a micro-harvester, which separated grain from straw for each plot. Each part was then weighed in the field.

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2.4. Calculation and statistical analysis Cumulative N2O-N and NO-N emissions and N2O-ASC per plot were estimated by linear interpolations between sampling dates. Increases in soil NH4+ between sampling dates were assumed to be estimates of ammonification while increase in NO3 and soil mineral N were associated with nitrification and net mineralization, respectively. Other fates for soil mineral N such consumption by the plant or leaching were assumed negligible. We use the term consumption to describe the sum of all process that result in a reduction in NH4+, NO3 or mineral N (NH4+ + NO3) content between 2 sampling events. Differences between treatments at each sampling event and in the cumulative emissions were analysed using analysis of variance (ANOVA, P < 0.05). The Least Significant Difference (LSD) test was used for multiple comparisons between means. When the data were not distributed normally, the Kruskal-Wallis test was used on non-transformed data to evaluate differences at P < 0.05. SchaichHamerle analysis was also carried out as a post hoc test. Linear regression analyses (P < 0.05) were performed to determine relationships between N2O-N, NO-N emissions and N2O-ASC with SOC, NH4+-N, NO3-N and soil and environmental conditions, and also between the cumulative N2O-N, NO-N emissions and N2O-ASC and the composition of the fertilizers. These analyses were also used to test relations between NO and N2O emissions and N2O-ASC with increases and decreases in soil mineral N. 3. Results 3.1. Environmental conditions and the evolution of mineral N and soluble organic carbon Rainfall and soil temperature data are presented in Fig. 1. Average daily soil temperature was around 4.5 8C when fertilizers were applied, with minima below 0 8C, but increased to 27 8C in summer. Average daily soil temperature then decreased to 10– 15 8C in the fallow period. During the experiment, three different periods were observed with respect to soil WFPS (Fig. 1). In the first period, throughout crop growth, WFPS was below 55%. After

Fig. 1. Weekly precipitation, average soil temperature and soil water filled pore space in the 0–10 cm soil layer.

harvest, and throughout the summer, WFPS remained below 24% and, finally, rainfall events at the beginning of the fallow period produced an increase in WFPS, which ranged from 60 to 70% until the end of the season. No significant differences between treatments (P < 0.05) were found in soil WFPS at any sampling time. Fertilizer application significantly increased the NH4+ content in the plots treated with U, DPS and UPS with maximum values occurring 16 days after application (Fig. 2). Two months after the fertilizer application, soil NH4+ concentration was below 11 mg

Fig. 2. NH4+-N and NO3-N concentrations in the 0–10 cm soil layer during the crop period. The vertical arrow indicates fertilizer application. The vertical bars indicate LSD at 0.05 between treatments for each sample time.

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Fig. 3. Soluble organic carbon (SOC) concentrations in the 0–10 cm soil layer during the crop period. The vertical arrow indicates fertilizer application. The vertical bars indicate LSD at 0.05 between treatments for each sample time.

NH4+-N kg1 dry soil for all treatments. At the beginning of the fallow period NH4+ concentrations in the soil ranged from 2.5 mg NH4+-N kg1 dry soil for UPS to 4.3 mg NH4+-N kg1 dry soil for CCR + S. After rainfall events, the concentration decreased and was lower than 0.5 mg NH4+-N kg1 dry soil in all treatments at the final sampling date. During the crop period, significant differences in NO3 concentrations between treatments occurred after 50 days. Maximum values were observed in UPS, DPS and U treatments nearly 3 months after fertilizer application although they decreased later in all treatments. At the end of the crop period, differences with respect to the control were only detected in U. During the fallow period concentrations ranged from 4.7 to 16.8 mg NO3-N kg1 dry soil and no significant differences were found between treatments. Soil soluble organic C ranged from 19.3 to 56.9 mg C kg1 dry soil, with the lowest values observed 50 days after fertilizer application (Fig. 3). Differences between treatments were significant at some sampling times during the crop period, especially for the CCR + S, DPS and MSW treatments, 4 days and 2 and 3 months after fertilizer application, respectively. The behaviour of the U treatment was very similar to that of the control, as no carbon was added in either of these treatments. No significant differences between treatments were found during the fallow period, although a general decrease in the SOC concentration was observed.

3.2. N2O emissions Nitrous oxide emissions were influenced by adding fertilizers to the soil (Fig. 4). Several N2O emission peaks occurred during the experimental period. The highest emission peak was observed 4 days after fertilizer application in the case of the U treatment. In the fallow period, pulses occurred after rainfall events for all the treatments except CCR + S. After the rainfall events during the fallow period, maximums were observed for the U, DPS and C treatments. In the pulses that occurred in the first few days after a rainfall event, no differences between treatments were observed although several days after, the fertilizer treatments emitted more than the control. In this experiment, the soil also acted as a sink for N2O at some sampling times and significant negative fluxes were detected both during the crop (January–February, especially for the C treatment) and fallow periods (November–December, mainly for U and DPS). There is a linear correlation between the increase in NH4+ and N2O emissions (r = 0.85, P < 0.001). No significant differences between treatments were detected in cumulative emissions for the fallow period. Considering the whole period (the crop and fallow periods), the DPS, CCR + S and U treatments produced higher cumulative N2O emissions than the control (Table 2). Between 74 and 87% of total N2O emissions for

Fig. 4. N2O emissions from the soil during the crop and fallow periods. The vertical arrow indicates fertilizer application. The vertical bars indicate LSD at 0.05 between treatments for each sample time. Note that the time scale of the two parts of the graph is different.

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Table 2 Cumulative N2O-N and NO-N emissions and denitrification losses over the crop and fallow periods and N2O and NO emission indexes. N2O and NO fluxes and denitrification rate data are the averages of means from three repetitions  standard deviation. Treatment

Control (C) Untreated pig slurry (UPS) Digested pig slurry (DPS) Municipal solid waste (MSW) Composted crop residues + sludge (CCR + S) Urea (U)

Crop period

Fallow period

Total

N2O (g N2O-N ha1)

NO (g NO-N ha1)

Denitrification rate (g N2O-N ha1)

N2O (kg N2O-N ha1)

NO (g NO-N ha1)

N2O (g N2O-N ha1)

NO (g NO-N ha1)

150  56a 264  52b 275  72b 219  82ab 290  84b

2.18  2.55a 37.62  3.38c 65.27  6.36e 29.55  10.20bc 23.01  3.69b

3340  1629b 7507  2894c 8350  2813c 2114  1116b 1946  1106a

52  12a 51  98a 76  109a 48  116a 82  20a

11.60  2.07b 6.86  4.22ab 3.65  1.65a 7.31  1.46ab 5.51  1.68a

202  47a 316  137ab 351  25b 266  19ab 373  99b

13.78  3.95a 44.48  6.18c 68.92  5.41d 36.86  9.31bc 28.82  2.37b

302  14b

58.43  2.63d

425  159a

43  30a

3.42  3.42a

345  109b

61.86  5.25d

Different letters within columns indicate significant differences applying Fisher’s Unprotected Least Significant Difference (LSD) test at P < 0.05. Table 3 Crop yields and emission indexes. Data from crop yield are the averages of means from three repetitions  standard deviation. Yield

Control (C) Untreated pig slurry (UPS) Digested pig slurry (DPS) Municipal solid waste (MSW) Composted crop residues + sludge (CCR + S) Urea (U)

Emission factors

kg grain ha1

kg grain + straw ha1

EFN2O, Napplied (%)

EFNO, N applied (%)

EFN2O, crop yield

972  145a 1125  111ab 1508  64b 1105  145ab 1032  126a 1061  220ab

1825  160a 2117  150ab 2381  72b 1879  130a 2004  202ab 2079  189ab

– 0.09 0.12 0.05 0.14 0.11

– 0.025 0.044 0.018 0.012 0.038

208 281 233 241 361 323

Different letters within columns indicate significant differences applying Fisher’s Unprotected Least Significant Difference (LSD) test at P < 0.05.

the whole season were emitted during the crop period. Percentages of N lost as N2O compared with total N applied in the different treatments, discounting N2O losses in the control, are included in Table 3. Cumulative N2O emissions were positively correlated with the NH4+ content of the fertilizers (r = 0.96, P < 0.05). 3.3. NO emissions Nitric oxide fluxes increased after the NH4+ peak in the soil (Fig. 5). The largest NO fluxes were observed for the inorganic fertilizer on 13 February. Thereafter, emissions decreased for all

treatments except DPS, whose emissions remained high for a longer period. No significant differences between treatments were found in terms of NO emissions during most of the fallow period. However, a larger emission was observed in the control treatment in the first sampling event after rainfall, compared with the other treatments. Correlations were found between NO emissions and soil WFPS (r = 0.74, P < 0.001), and between NO flux and NH4+ concentration in the upper soil layer (0–10 cm) (r = 0.46, P < 0.01). NO fluxes were also correlated with NH4+ consumption (r = 0.48, P < 0.01), NO3 consumption (r = 0.70, P < 0.01) and mineral N consumption (r = 0.75, P < 0.001). No significant correlations (P < 0.05)

Fig. 5. NO emissions from the soil during the crop and fallow periods. The vertical arrow indicates fertilizers application. The vertical bars indicate LSD at 0.05 between treatments for each sample time. Note that the time scale of the two parts of the graph is different.

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Fig. 6. Molar NO/N2O ratio of gases emitted from the soil during the crop and fallow periods. Note that the time scale of the two parts of the graph is different.

were found between the cumulative NO emission and the composition of the fertilizers. Cumulative NO emissions are included in Table 2. During the crop period, the highest NO emissions were observed in the DPS treatment. For the fallow period, cumulative NO fluxes from the control were significantly higher than those from U, DPS and CCR + S. The molar NO/N2O ratio was calculated from the sampling times when both NO and N2O emissions were positive. The highest values of the ratio (>1) were observed in February, while in March the values ranged between 0.01 and 0.9 (Fig. 6). During the fallow period, the highest values for the ratio were observed during the first 2 sampling events. Thereafter, they were always below 0.3. Several correlations were found between the NO/N2O ratio and the variations in soil mineral N. The ratio was correlated with NH4+ consumption (r = 0.56, P < 0.01), increase of soil NO3 (r = 0.56, P < 0.01), NO3 consumption (r = 0.82, P < 0.01) and mineral N consumption (r = 0.84, P < 0.001).

Two different emission factors were calculated relating N2O and NO emissions to N applied, the ‘‘EFN2O, Napplied’’ and the ‘‘EFNO, Napplied’’, respectively, as well as other emission factor relating N2O emissions to the crop yield, the ‘‘EFN2O, crop yield’’ (Table 3). These factors are defined as follows: EFN2 O; Napplied ¼ N2 O-N emitted per N appliedð%Þ EFNO; Napplied ¼ NO-N emitted per N appliedð%Þ EFN2 O; crop yield ¼ mg N2 O-N emitted per kg grain of barley The ‘‘EFN2O, Napplied’’ and ‘‘EFNO, Napplied’’ were calculated after discounting N2O and NO losses associated with the control treatment, respectively. The ‘‘EFN2O, Napplied’’ represented less than 0.17% for all treatments, with the lowest value for MSW, while ‘‘EFNO, Napplied’’ represented less than 0.044%, with the lowest value for CCR + S. ‘‘EFN2O, crop yield’’ was highest for the CCR + S and lowest for the control and DPS treatments (Table 3).

3.4. N2O production of acetylated soil cores (N2O-ASC) 4. Discussion In the acetylated soil cores, several N2O emission peaks occurred for all the treatments except U (Fig. 7), for which no significant peaks were observed. The highest N2O-ASC peaks were observed for DPS and UPS, 4 days and 1.5 months after fertilizer application, respectively. Only DPS and UPS exhibited higher cumulative N2O-ASC than the control (Table 2) and the inorganic treatment produced the smallest N2O-ASC. 3.5. Crop production yield and emission factors Biomass production associated with each treatment is presented in Table 3. The highest yields (grain) were observed in the DPS treatment and the lowest in the control.

Fig. 7. N2O-ASC during the crop period. The vertical arrow indicates fertilizers application. The vertical bars indicate LSD at 0.05 between treatments for each sample time.

4.1. Effect of fertilizer composition on fluxes and processes involved in N2O and NO emissions The NO/N2O ratio provides an indirect index for evaluating the source of N oxides; nitrification or denitrification (Anderson and Levine, 1986; FAO, 2001). These authors suggested that the molar NO/N2O ratio was >1 for cultures of nitrifiers, while it was less than 0.01 for denitrifiers. Under field conditions, where nitrification and denitrification simultaneously take place, this index often shows intermediate values, between 0.01 and 1 and for example, Meijide et al. (2007) established a ratio of >0.11 for the predominance of nitrification (>50%) measured under field conditions in a sandy loam soil. This value was based on comparing this index with the proportion of N2O produced by nitrification measured using the acetylene technique (Muller et al., 1998). In our experiment, nitrification was supposedly the most important source of these gases immediately after fertilizer application (February) because the NO/N2O ratio was close to 1 during this initial period. This coincided with the consumption of NH4+, the increase of soil NO3 and a low denitrification rate (Fig. 6). The highest NO peak observed for all fertilizer treatments occurred on 13–14 February, indicating that nitrification was taking place in spite of the low soil temperatures (<10 8C). In March, the NO/N2O ratio was <0.1, coinciding with an increase of soil temperature (>15 8C) and WFPS. In this period, N2O-ASC also increased, confirming that denitrification was an important source of N2O production (Fig. 6). However, during this period the index did not adequately reflect that nitrification simultaneously took place, as confirmed by the increase of NO3 soil content in some sampling events. Therefore

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we consider that the ratio is a useful tool when only nitrification or denitrification takes place, or when one process clearly dominates, but it does not give a clear picture when the two processes take place simultaneously. During the rest of the crop period (April–May), environmental conditions favoured nitrification, although NO and N2O fluxes were low because the soil NH4+ content was also very low for all treatments. In summer, the soil is dry and emissions of N oxides have been found to be insignificant, as we observed in a previous experiment in an arable soil (non-published data). This is characteristic of dryland crops cultivated in Mediterranean areas. As a consequence, most of the emissions of N oxides during the crop period occurred within 2 months of fertilizer application. The negative correlation of NO fluxes and WFPS was consistent with results of Skiba et al. (1993) who showed that NO emissions were mainly associated with nitrification and therefore reduce with increasing soil wetness. Another potential indicator of NO losses is the net N-cycling rates (net rates of mineralization, ammonification or nitrification) (Stark et al., 2002). However in this study, the NO flux was better explained by NH4+ consumption than by the increase of NO3. This was probably because this last parameter took into account the uptake of NO3 by the crop and losses due to leaching, whereas the NH4+ consumption was probably mainly due to nitrification. The emissions of N2O and NO are the result of the production, consumption and diffusion of these gases within the soil. Due to the fact that nitrification and denitrification also occur deep in the soil (deeper than 10 cm), the correlation of the fluxes with measurements of controlling factors of these fluxes (NH4+, NO3, SOC, etc.) carried out in the upper soil layer (0–10 cm), could give a biased view. However, Jime´nez et al. (1989) and Jones et al. (2007) observed that following fertilizer application, most of the NH4+ was retained in the upper 10 cm layer and only a small proportion of NO3 leached to the lower soil layers. Considering this, and taking into account that the fluxes of N oxides were highly dependent on nitrification in this study, we consider that studying the controlling factors in this upper soil layer gives enough information to evaluate the effects of fertilizer application on the fluxes. The effects of different treatments on N2O, NO and N2O-ASC could have been attributed to differences in the composition of the fertilizers. As conditions favoured nitrification after fertilizer application, the amount of NH4+ had an important influence on N oxide emissions and this explains the high correlation observed between total fluxes of N2O and the amount of NH4+ added to the soil by the fertilizers. The addition of organic C, which included labile C compounds, influenced these emissions, and a higher N2OASC was observed for three of the four organic treatments compared with urea (Table 2). These larger values of N2O-ASC can be explained by denitrifiers using these compounds as a source of energy and also by the addition of a C source which would have simultaneously favoured the creation of anaerobic microsites as consequence of a higher demand for O2 (Skiba and Smith, 2000). The higher N2O-ASC for organic fertilizer treatments was not related to a significant effect on N2O emissions but it decreased NO emissions (Table 2), probably because the addition of a source of C favoured denitrification and promoted the consumption of NO (Sa´nchez-Martı´n et al., 2008). The decrease of NO emission with increasing N2O-ASC may reflect the shift from nitrification to denitrification. However, the observed reductions were smaller than those reported for irrigated crops by Vallejo et al. (2006) and Meijide et al. (2007), who also observed lower emissions with organic compared with inorganic fertilizers. Therefore, it was not possible to establish a general effect of fertilizers on N2O or NO emissions that was solely dependent on their composition, because fluxes are highly dependent on environmental conditions. Under high values of WFPS, as occurs in irrigated soils in semiarid

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areas, the organic C compounds in fertilizers have a considerable influence on N2O and NO fluxes, whereas under dryland conditions (as in this experiment) C mainly affects NO fluxes. The application of a source of mineral N without C reduced N2OASC emissions compared with the control. Although no simple explanation can be found for this effect, the short period when denitrification was favoured (March), together with a significant decrease in SOC in the U treatment during that period (probably as result of N inmobilization), reduced the N2O-ASC. The pre-treatment of liquid manure had no significant effect on N2O emissions because the NH4+ contents were very similar in both digested and untreated pig slurry. One possible explanation for the lower NO fluxes from the untreated pig slurry compared with the digested pig slurry is that the higher amount of soluble organic C compounds in this treatment favoured the consumption of NO, which in turn resulted in lower emissions compared with digested slurry, as previously observed in irrigated crops by Vallejo et al. (2006) and Meijide et al. (2007). In the case of the composted residues, an appropriate composting period produced stabilized materials with C/N ratios of 14.0 and 11.7 for MSW and CCR + S, respectively. Since the fertilizer was applied taking into account the amount of N available, less C was added in the CCR + S than in the MSW treatment. However, soluble organic C applied was low in both treatments, as a consequence of the composting process (Table 1), and there was not enough to produce significant differences in N2O and NO emissions between these two treatments. 4.2. Pulses of N oxides after rainfall One important characteristic of soils from arid and semiarid regions is that the upper part of the soil remains dry for several months of the year, as a consequence of high temperatures and low soil water availability. During the dry period, N2O production is markedly reduced (Beare et al., 2009) because microbiological activity is low. Many bacteria, fungi and plants die, thereby increasing the concentrations of labile N and C available in the soil. The first rainfall on dry soil produces pulses of N2O and NO (Davidson et al., 1993; Jørgensen et al., 1998; Dick et al., 2001), as occurred in this experiment, both at the end of the crop period and during the fallow period. These initial fluxes are thought to be caused by the accumulation of inorganic N in dry soils and the reactivation of water-stressed bacteria upon wetting, which then metabolize the pool of available inorganic N (Cabrera, 1993; Davidson et al., 1993). In this experiment, the first NO and N2O emissions in autumn (lasting for 4 days) were mainly produced by nitrification because the NO/N2O ratio had values close to 1. During this period, no differences between treatments were observed with respect to mineral N, NH4+ or NO3 in dry soil (before rewetting). Consequently, we did not observe any differences in the N2O fluxes that occurred during the first 4 days after rewetting. The small amount of NH4+ (< 5 mg N kg1) observed in all treatments explained the small magnitude of the fluxes observed during the first days following the beginning of the rainfall events. Larger N2O pulses were measured when soil WFPS was maintained above 65% for a period longer than 3–4 days, as occurred in November. This coincided with a low NO/N2O ratio in the produced fluxes. Therefore, during this period, denitrification was probably the primary source of N2O following the rewetting of dry soil, as proposed by Groffmann and Tiedje (1988) and Davidson (1992). Under these conditions, the residual effect of the fertilizer could have affected the emission of N oxides, producing the observed differences in the measured N2O fluxes between treatments. Only one pulse was measured during the entire fallow period in the control and U treatments, while several pulses were measured in soils treated with organic fertilizers. A possible

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hypothesis, which could justify this effect, was that insoluble organic molecules from fertilizers (compost or slurry) could have been mineralized, providing labile N and C for denitrifiers during this period and contributing to the successive pulses. However, considering the whole fallow period, total emissions of N2O were low and no significant differences were observed between treatments, probably due to a high standard deviation for some of the organic treatments, which affected the statistical analysis. In the case of NO, significantly lower total flux was observed for organic fertilizer compared with the control. The effect of soluble C mentioned previously would partly explain these results. 4.3. Sink for NO and N2O Net N2O consumption has previously been measured in agricultural systems (Yamulki et al., 1995; Merino et al., 2004), especially at low levels of available mineral N (Chapuis-Lardy et al., 2007). In our experiment, negative fluxes of N2O and NO were always observed in conditions of low mineral N and low temperature (below 12 8C), both during the crop period (January–February) in the case of the control and in the fallow period (November–December), especially for slurries. Although, like Donoso et al. (1993), we only observed the N2O sink at low temperatures, other researchers have reported a clear, positive relationship between soil temperature and net N2O consumption (Yamulki et al., 1995). One difference between the 2 periods in which a sink effect was observed was soil WFPS, which was lower in the crop than in the fallow period. Theoretically, the main N2O consumption pathway could be denitrification, in which N2O could be reduced to N2. This could explain the sink effect observed during the fallow period, when N2O was mainly consumed by denitrification. However, during January–February, conditions favoured nitrification and the negative fluxes disappeared when the soil was rewetted (March). Therefore, we speculated that the sink effect observed in the control treatment could have been mainly produced by a different pathway. Chapuis-Lardy et al. (2007) pointed out that the process behind N2O uptake in dry soils was still not very clear and suggested that other N2O-reducing processes could include nitrifier denitrification and aerobic denitrification. However, it is also possible that the rainfall events at the beginning of the year created anaerobic conditions at some soil microsites (in soil or even in the subsoil). In this situation diffusion of N2O and NO to these microsites could have favoured the consumption of these gases by denitrification. The measurements taken in this experiment do not give enough information to clarify which pathway was responsible for this consumption and, therefore, it is recommended that further studies should be carried out in order to address this issue. 4.4. N2O emission factor The N2O emissions were generally low compared with previous studies carried out in similar systems but under irrigated conditions (Vallejo et al., 2006; Meijide et al., 2007). Bronson and Mosier (1993) measured lower fluxes in non-irrigated systems when compared with irrigated crops. The emission rates were, however, within the range reported in other studies (Rochette et al., 2000; Dambreville et al., 2008), suggesting the strong influence of environmental conditions. Our results suggests that in Mediterranean non-irrigated systems most of the N2O emissions are produced in winter (> 70% in the 3 months following fertilizer application) Fluxes in other semiarid non-irrigated soils, such as those measured by Bronson and Mosier (1993) in Colorado, were mainly produced in spring or summer because in those cases rainfall was concentrated in this period.

The effect of N fertilizer application in national inventories is normally calculated by a factor that relates annual N2O emissions to the total amount of N applied as fertilizer (IPCC, 2006). The type of fertilizer applied has a clear influence on fluxes. As a result, in this experiment the emission factors obtained differed from treatment to treatment. The ‘‘EFN2O, Napplied’’ was very low for all treatments, ranging from 0.06% for MSW to 0.17% for the DPS treatment and much lower than the 1% default emission factor proposed by the IPCC (2006). Environmental conditions favoured low total N2O emissions, because denitrification dominated for only a few days of the year. The factors were almost 10 times smaller than those obtained for the same soil but for irrigated crops (Vallejo et al., 2006). The utility of the emission factor which relates N2O emissions with crop production ‘‘EFN2O, crop yield’’ has already been shown in previous studies (Fleesa et al., 2002), when comparing organic and conventional farming. It is not useful for treatments where no N fertilizer is applied, as low emission values could be associated with low yields. According to this emission factor, CCR + S and U should be avoided if the aim is to reduce N2O emissions without affecting crop yield, while the use of digested slurry should be encouraged. However, this emission factor has an associated problem, which is the annual variability in yield as a consequence of external factors, non-inherent to fertilization, such as climatic conditions (storms, frosts, etc.), plagues, etc. In this experiment, the yield obtained in the fertilized plots was below that obtained in the same area other years (2000–2500 kg grain ha1) and this affected the emission factor. Dryer conditions (as a consequence of higher temperatures) when the grain fill took place, reduced the yield (Samarah, 2005) and consequently the efficiency of the fertilizer. However, the higher temperatures during grain fill period did not influence the emissions of NO and N2O, as there was little NH4+ in the soil at that time and WFPS was never high enough to denitrify NO3. This has also been observed in a previous experiment (Meijide et al., 2007), where the emissions at this time of the year were low until irrigation was applied. The variability in the onset of high temperatures, inherent to Mediterranean climate, means that these lower yields can be encountered often. 5. Conclusions Application of different organic fertilizers and urea to a nonirrigated barley crop in a Mediterranean climate resulted in small N2O emissions. This was probably because the low WFPS of the soil during the period in question was not favourable for denitrification. In contrast, three of the four organic fertilizers produced smaller NO emissions compared with urea. The ammonium content of fertilizer could be the most important fertilizer parameter for the regulation of N2O fluxes in Mediterranean non-irrigated systems, although more studies should be carried out to confirm this hypothesis. In this experiment, the residual effect of the fertilizer applied also had a small effect on the emission pulses, resulting in lower NO emissions compared with the control. A nitrous oxide sink was observed at several points in time, during both the crop and fallow periods and was especially notable when soil temperatures were low. Further studies should be carried out in order to understand the processes responsible for this uptake and to help develop management strategies to increase consumption of N2O. Finally, different N2O-emission emission factors were evaluated to determine which fertilizer strategies are most efficient by taking into account N2O emissions and crop yield. From this analysis it is evident that the application of digested slurry should be recommended for these types of agrosystems.

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