Soil Carbon and Nitrogen Dynamics Following Application of Pig

Published July, 2000

Soil Carbon and Nitrogen Dynamics Following Application of Pig Slurry for the 19th Consecutive Year: II. Nitrous Oxide Fluxes and Mineral Nitrogen Philippe Rochette,* Eric van Bochove, Danielle Pre´vost, Denis A. Angers, Denis Coˆte´, and Normand Bertrand ABSTRACT

levels of available C (Tiedje, 1994). The N2O-producing processes are directly controlled by the supply of substrates. Nitrification requires NH4⫹, O2, and CO2, while DRs are favored by adequate levels of available organic C and NO3⫺ under O2 deficiency. When added to soil, animal manure can lead to enhanced N2O emissions by stimulating both nitrification and denitrification (Dendooven et al., 1998). Animal slurries usually contain high concentrations of NH4⫹–N, which is rapidly nitrified when mixed with aerated soils (Morvan et al., 1996). Slurries also supply easily decomposable organic C (Morvan and Leterme, 1999; Rochette et al., 2000) that can both sustain denitrification and induce anaerobiosis by stimulating biological O2 demand. Accordingly, increased N2O emissions were often measured following application of solid (Lessard et al., 1996) and liquid (Christensen, 1983; Hansen et al., 1993; Wagner-Riddle and Thurtell, 1998) cattle (Bos taurus) manures to agricultural soils. The impact of pig slurry on N2O emissions has been mainly studied in the laboratory (Paul et al., 1993; Sommer et al., 1996; Dendooven et al., 1998), and little is known about the quantities of N2O emissions resulting from the application of pig slurry to agricultural soils under field conditions. Land application is currently the most popular method for pig slurry disposal in Que´bec. Commercial hog production in many regions of the province has increased rapidly during the last 20 yr to the point where the amount of slurry produced now exceeds the quantity that can be safely accommodated by the available agricultural land. Under these conditions it is not uncommon that slurry is applied on the same fields every year for long periods. Our objective was to quantify the effects of 19 consecutive years of pig slurry application on N2O emissions. Soil mineral N contents, N2O concentrations in the soil profile, daily rainfall, and denitrification potentials were also determined to explain the variations in N2O emissions.

Agricultural soils often receive annual applications of manure for long periods. Our objective was to quantify the effects of 19 consecutive years of pig (Sus scrofa ) slurry (PS) application to a loamy soil (loamy, mixed, frigid Aeric Haplaquept) on N2O emissions. Soil surface N2O fluxes (FN2O) were measured 36 times in 1 yr. Nitrous oxide concentration profiles, soil NH4⫹- and NO3⫺-N contents, denitrifying enzyme activity (DEA), and denitrification rate (DR) in soil were also determined to explain the variation in FN2O. Long-term (19 yr) treatments on continuous silage maize (Zea mays L.) were 60 (PS60) and 120 Mg ha⫺1 yr⫺1 (PS120) of pig slurry and a control receiving mineral fertilizer at a dose of 150 kg ha⫺1 each of N, P2O5, and K2O. Denitrifying enzyme activity, soil N2O concentrations, and FN2O (⬍25 ng m⫺2 s⫺1) were low in the control plots receiving mineral fertilizer. Annual applications of PS to the soil for 18 yr had positive residual effects on the DEA compared with the long-term fertilized control plots. Following PS application, there was a strong and rapid increase of FN2O (up to 350 ng m⫺2 s⫺1) on manured plots. The PSinduced FN2O increased with increasing quantity of PS, probably as the result of a greater availability of NO3⫺-N for denitrification. The effects of PS on FN2O were mostly limited to the 30 d following application, with low fluxes (⬍10 ng m⫺2 s⫺1) during the rest of the measurement period. Total N2O–N emissions represented 0.62, 1.23, and 1.65% of total N applied in control, PS60, and PS120 plots, respectively. These emission factors for the PS plots agreed with values previously suggested for N-fertilized soils (1.25%).

A

tmospheric models are predicting global warming in response to increases in atmospheric concentration of gases that absorb infrared radiation such as N2O (Intergovernmental Panel on Climate Change, 1990). Emissions of N2O are closely related to human activities. In Canada, it is estimated that agriculture is responsible for ≈70% of anthropogenic emissions of N2O (Janzen et al., 1998). In soils, N2O can be produced during either nitrification or denitrification (Hutchinson and Davidson, 1993) and both processes can take place simultaneously (Kuenen and Robertson, 1994). During nitrification, N2O can be produced by dismutation of HNO or by reduction of NO2⫺ when low O2 levels limit the complete oxidation to NO3⫺ (Poth and Focht, 1985). Nitrous oxide is also a free intermediate product of respiratory denitrification that may escape the soil before being further reduced (Tiedje, 1994). More seldom is the production of N2O by the dissimilatory NO3 reduction to NH4, which is regulated by O2 and can be stimulated in soils by high

MATERIALS AND METHODS The field experiment was conducted in a Le Bras loam (frigid Aeric Haplaquept) near Que´bec city, Canada (Rochette et al., 2000). The treatments consisted of annual applications since 1979 of either mineral fertilizer (150 kg NH4⫹ NO3⫺-N ha⫺1, 150 kg P2O5 ha⫺1, 150 kg K2O ha⫺1) (control) broadcasted prior to planting (27 May 1997) or PS at rates of 60 (PS60) or 120 Mg ha⫺1 (PS120) banded (0.6 m) at the sixto eight-leaf stage (30 June 1997) repeated three times in randomized blocks. In 1997, the slurry was from a commercial hog and sow operation and contained 2.1 kg m⫺3 total N, 0.99

Philippe Rochette, Eric van Bochove, Danielle Pre´vost, Denis A. Angers, and Normand Bertrand, Soils and Crops Research and Development Centre, Agriculture and Agri-Food Canada, 2560 Hochelaga Blvd., Sainte-Foy, QC, Canada, G1V 2J3; Denis Coˆte´, Institut de recherche et de de´veloppement en agroenvironnement, Complexe Scientifique, 2700 Einstein St., Sainte-Foy, QC, Canada, G1P 3W8. Received 1 July 1999. *Corresponding author ([email protected]).

Abbreviations: DEA, denitrifying enzyme activity; DR, denitrification rate; PS, pig slurry; PS60, application of pig slurry at 60 Mg ha⫺1 yr⫺1; PS120, application of pig slurry at 120 Mg ha⫺1 yr⫺1; SWE, snow water equivalent.

Published in Soil Sci. Soc. Am. J. 64:1396–1403 (2000).

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kg m⫺3 NH4⫹–N, 1.15 kg m⫺3 P, and 1.29 kg m⫺3 Ca. Average maize aboveground dry matter yields between 1990 and 1995 were 7.1 Mg ha⫺1 for the control, 7.9 Mg ha⫺1 for PS60, and 9.3 Mg ha⫺1 for PS120.

Soil Surface Nitrous Oxide Fluxes During the Snow-Free Season In situ N2O fluxes (FN2O) were measured 36 times from 30 May 1997 to 27 May 1998. When snow was absent (31 samplings), fluxes were measured by the static chamber method detailed by Lessard et al. (1994) briefly described as follows. Three acrylic frames (0.60 by 0.60 m; 0.14-m height; 6.35-mm wall thickness) were inserted to a depth of 0.10 m in each plot on 29 May 1997. The frames completely covered the width of the slurry application band between maize rows and were left at the same locations for the duration of the experiment. The height of the frames was measured (48 measuring points per frame) at regular intervals during the experiment to account for variations in headspace due to soil settling. At sampling time, the frames were covered with a lid and air samples were taken through a rubber septum at regular intervals (0, 10, 20, and 30 min) with 10-mL Plastipack syringes (Becton Dickinson and Co., Rutherford, NJ). Flux measurements were always made between 0900 and 1200 h (Eastern Standard Time). Soil surface N2O fluxes (FN2O) were calculated using the following equation (Hutchinson and Livingston, 1993):

FN2O

冢

冣

⳵C VMmol ⫽ ⳵t AVmol

[1]

where ⳵C/⳵t is the rate of change of N2O concentration (nmol mol⫺1 s⫺1), V is the chamber headspace volume, Mmol is the molecular weight of N2O (44 g mol⫺1), A is the surface area covered by the chamber (0.36 m2), and Vmol is the volume of a mole of gas at 20⬚C (0.024 m3 mol⫺1). Nitrous oxide concentrations in closed chambers varied linearly with time in 77% of cases. For these observations, the slope (⳵C/⳵t ) obtained from simple linear regression between concentration and time was used to calculate the fluxes. The relationship between concentration and time was tested with the Student’s t test for n ⫽ 4 and ␣ ⫽ 0.10 (Hutchinson and Livingston, 1993). Second-order polynomial equations were fitted to concentrations when the rate of change decreased with time, and the Student’s t test was applied to the initial slope of the curve (time ⫽ 0). In both cases, if the calculated t was greater than the critical value of t, the rate of change in concentrations was considered significantly different from zero. During the snow-free season, concentrations of N2O in the soil profile were monitored within 2 m of the chamber frames. Air samples were taken simultaneously with flux measurements in each plot using soil gas probes. The probes were made of plastic mesh cylinders (10-cm length; 3.5-cm diameter) containing glass beads (3-mm diameter) (Rochette and Flanagan, 1997). The cylinders were inserted horizontally at depths of 5, 10, 20, and 40 cm on 29 May 1997 and connected to the surface using plastic tubes (Bev-a-line IV, Ryan Herco Industrial Plastics, Seattle, WA) (0.60-m length; 6.35-mm o.d.; 3.18-mm i.d.) with two-way Luer-type stopcock valves (Cole Parmer, Vernon Hills, IL) at the surface ends. Air samples were collected through sleeve stoppers with a 10-mL Plastipak syringe after expelling 20 mL of air from the tubes to account for the dead volume of the tubes.

Soil Surface Nitrous Oxide Fluxes During Winter Winter N2O fluxes were calculated using gas concentration gradients in snow following the method described by van Bochove et al. (1996). Concentrations of N2O were measured

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three times during the snow accumulation (29 Jan., 11 Feb., 3 Mar. 1998) and twice during the snow ablation (18 and 30 Mar. 1998). The snow ablation period was characterized by the initial melt and the main melt. The initial melt period corresponded with a loss of 5 to 10% of the snow water equivalent (SWE) during the early, often intermittent, melt. The main melt period corresponded with large losses of SWE by melt water discharge. Multilevel gas-sampling probes (two per plot) were installed into the soil prior to the onset of snow cover as described in details by van Bochove et al. (2000). Samples were taken in 7.5-mL vials at 10 and 35 cm below ground level and at 0, 5, 10, 30, 50, 70, and 90 cm above the soil depending on snow cover height. Standard mixtures of N2O in N2 were taken into the field and treated in exactly in the same manner as the field samples during transport, storage, and analysis. The N2O vertical concentration gradients in snow (⳵C/⳵z ) were defined from the average concentrations of six measurements per treatment at each depth. The gradients were mostly linear and calculated from the average concentration values from the soil surface to the highest point in the snow cover. Gas fluxes (N2O, ng m⫺2 s⫺1) from the soil to the atmosphere were calculated using Fick’s first law of diffusion:

FN2O ⫽ Ds

冢冣

⳵C Mmol ⳵z Vmol

[2]

where Ds (m2 s⫺1) is the effective diffusion coefficient of gas in snow. The coefficient of diffusion of gas in snow, Ds, was calculated as a function of the bulk porosity of snow (cm3 airfilled pore space cm⫺3 snow matrix) and a constant coefficient of gas diffusion in air of 1.39 ⫻ 10⫺5 m2 s⫺1 (Sommerfeld et al., 1993). The SWE (cm) of the snowpack was determined on three snow cores at each sampling date (Adirondack corer, Gamma Instrument, Hempstead, NY). The bulk density (d, g cm⫺3) for any core was then calculated from the SWE and the core height of each sample (H⬘, cm) as SWE/H⬘. The bulk porosity of the snowpack (p, cm3 cm⫺3) was calculated from the expression: p ⫽ 1⫺ (d/di), where di is the ice density (0.917 g cm⫺3). Values of bulk snow porosity and snow depth were, respectively, 0.68 cm3 cm⫺3 and 121 cm on 29 January, 0.67 cm3 cm⫺3 and 113 cm on 11 February, 0.66 cm3 cm⫺3 and 110 cm on 3 March, 0.61 cm3 cm⫺3 and 114 cm on 18 March, and 0.56 cm3 cm⫺3 and 78 cm on 30 March. The gas samples were analyzed for N2O concentrations within 2 d of sampling for the syringes and within 2 wk for the vials by means of gas chromatographs fitted to electron capture detectors (Model 3400, Varian, Walnut Creek, CA; Model 5890 Series II, Hewlett-Packard, North Hollywood, CA) (Lessard et al., 1996; van Bochove et al., 1996).

Ancillary Measurements Soil samples (0–15 cm) were taken at intervals outside chamber frames in each plot, returned to the laboratory and extracted within 4 h with 0.1 M KCl (10 g soil in 50 mL). The extracts were frozen until analysis for NO3⫺-N using an ion chromatograph (Model 4000i, Dionex, Sunnyvale, Ca) and NH4⫹–N by colorimetry (Nkonge and Ballance, 1982). Rainfall was measured daily with standard rain gauges at the St-Lambert Research Farm weather site located 200 m from the experimental plots.

Denitrifying Enzyme Activity and Denitrification Rate The DEAs and DRs were measured four times during 1997. At each date (9 June, 2 July, 18 August, and 8 September),

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Fig. 1. (a) Temporal variations of N2O flux from soils cropped with silage maize amended with mineral fertilizer (control) and pig slurry at 60 (PS60) and 120 Mg ha⫺1 (PS120). Fluxes were measured with closed chambers except for five dates in 1998 prior to 15 April. Mineral fertilizer was applied on 29 May and pig slurry on 30 June 1997. Error bars ⫽ standard deviations. (b) Daily precipitation during the experiment at St-Lambert, Que´bec, Canada. In 1998, measurements began on 15 April.

three soil cores (5-cm i.d.) per plot for DR and triplicate soil samples per plot for DEA were taken in the interrow at the 0- to 10-cm depth and stored at 4⬚C within 4 h for 24 to 48 h until incubations began. The DEA was determined according to Martin et al. (1988). Soil subsamples (10 g) were incubated in two replicates for 4 h at 25⬚C in 10 mL of 50 mM phosphate buffer containing 100 ␮g mL⫺1 chloramphenicol. Optimal enzyme activity was obtained by adding 10 mM NO3 and 10 mM glucose as electron acceptor and donor, respectively. Incubations were performed under an N2 atmosphere using acetylene inhibition of N2O reductase (90% N2; 10% C2H2). For all assays, no lag phase in N2O production was observed and the rate of production was linear for the entire 4-h incubation period. Denitrifying enzyme activity was calculated using the increase in N2O concentration (gas chromatograph, Model 5890 Series II, Hewlett-Packard) during the 4-h incubation. The DR was measured using the acetylene blockage technique (Yoshinari and Knowles, 1976). At the time of incubation, soil cores were placed in 1-L glass jars and acclimated at 25⬚C for 2 h. Jars were then sealed and 100 mL of the headspace were replaced by the same volume of C2H2 through a rubber septum fitted to the lid. Denitrification rates and DEA were calculated using the increase in N2O concentration (gas chromatograph, Model 5890 Series II, Hewlett-Packard) between 1 and 17 h following the addition of C2H2. Statistical analysis was performed on log-transformed flux values. Treatment effects were examined for each sampling

day using the interaction between block and treatment as the error term in the General Linear Models procedure of SAS (SAS Institute, 1989).

RESULTS AND DISCUSSION FN2O in the Fertilized Control Plots Soil surface N2O fluxes on the control plots receiving mineral fertilizer were low, ranging from 0 to 25 ng N2O m⫺2 s⫺1 (Fig. 1a). Wagner-Riddle et al. (1997) also reported small average FN2O on barley (Hordeum vulgare L.; 19 ng m⫺2 s⫺1), canola (Brassica napus L.; 31 ng m⫺2 s⫺1), and maize (28 ng m⫺2 s⫺1) during the first ≈2 mo following mineral fertilization in southern Ontario, Canada. Application of NH4NO3 fertilizer to agricultural soils can result in large N2O emissions, as reported by Mosier et al. (1982) (0–111 ng m⫺2 s⫺1), Ryden (1983) (0–556 ng m⫺2 s⫺1), and Hansen et al. (1993) (0–1111 ng m⫺2 s⫺1). The absence of such high fluxes in our study suggests that FN2O values were limited by one or many of the factors controlling N2O production and emission. Nitrification occurred in June when fertilizer NH4⫹ was transformed to NO3⫺ (Fig. 2a and 2b). However, there was no concomitant increase in soil N2O concentration


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Fig. 2. Temporal variations of (a) NH4ⴙ–N and (b) NO3⫺-N for soils cropped with silage maize amended with mineral fertilizer (control) and pig slurry at 60 (PS60) and 120 Mg ha⫺1 (PS120). Mineral fertilizer was applied on 29 May and pig slurry on 30 June 1997. Error bars ⫽ standard deviations.

(Fig. 3) or FN2O (Fig. 1a), suggesting that nitrification was not a measurable source of N2O. Levels of DEA and DR were also low during the growing season in the control plots (Fig. 4). Accordingly, FN2O values were only slightly increased on consecutive days in early July following heavy rainfalls (Fig. 1a and b). Myrold and Tiedje (1985) and Højberg et al. (1994) concluded that C supply rather than NO3⫺ availability often controls denitrification in agricultural mineral soils. Absence of increases in soil N2O concentration and FN2O at a time of simultaneous occurrence of high levels of soil water content and inorganic N suggests that denitrification was limited by a low concentration of electron donors. This interpretation is supported by the low soil surface CO2 fluxes recorded (Rochette et al., 2000) and by the small quantities of residue C returned to the soil in continuous silage maize systems (≈570 kg C ha⫺1 yr⫺1).

FN2O in the Slurry-Amended Plots Annual applications of PS to the soil for 18 yr had positive residual effects on the DEA compared with the long-term fertilized control plots, as indicated by measurements made on 9 June 1997 prior to the annual

addition of PS (Fig. 4b). However, this increased enzymatic potential in manured plots did not result in increased levels of soil N2O concentration (Fig. 3), and FN2O in the field (Fig. 1a) and in the laboratory (Fig. 4a), presumably due to N or C limitation or to the absence of rainfall-induced anaerobic conditions in June. The response of FN2O to the addition of PS (30 June) was strong and rapid, as indicated by the high fluxes (PS60 ⫽ 70 ng m⫺2 s⫺1; PS120 ⫽ 430 ng m⫺2 s⫺1) measured 18 h after application (Fig. 1a). During the 20 d following PS application, FN2O values on manured plots were greater than on control plots in seven of nine measurements (P ⬍ 0.05). These results agree with reports of increased FN2O following application of various slurries or effluents in the field (McTaggart et al., 1997; Sharpe and Harper, 1997; Yamulki et al., 1998) and in the laboratory (Comfort et al., 1990; Paul et al., 1993; Sommer et al., 1996; Dendooven et al., 1998). After PS addition, intense N transformations occurred in the soil as the slurry NH4⫹ was rapidly nitrified (Fig. 2). Presumably, nitrification was directly involved in N2O production. Dendooven et al. (1998) reported that nitrification contributed to 33% of the N2O produced when a soil amended with pig slurry was incubated in

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the laboratory. However, Bergstrom et al. (1994) observed that increases in N2O production in response to the addition of NH4⫹ and available C to soil were the result of increased denitrification of nitrified NH4⫹–N rather than by the nitrification process itself. Pig slurry, which is rich in available C and NH4⫹, may have induced conditions similar to those described by Bergstrom et al. (1994) and led to increased FN2O from denitrification.

Indeed, measurements of DR made 2 d after PS addition (2 July) indicated that DR was increased by one to two orders of magnitude over June levels (Fig. 4a). Nitrate-N (Fig. 2b) and respiration rates (Rochette et al., 2000) were much above background levels during the postapplication FN2O burst. Focht et al. (1979) and Lessard et al. (1996) also observed a correspondence between periods of increased soil NO3⫺ and increased

Fig. 3. Temporal variations of N2O concentrations at 5, 10, 20, and 40 cm during snow-free season and at 10 and 35 cm during winter in soils cropped with silage maize amended with mineral fertilizer (control) and pig slurry at 60 (PS60) and 120 Mg ha⫺1 (PS120). Mineral fertilizer was applied on 29 May and pig slurry on 30 June 1997.


N2O production or emission following application of organic materials to soils. Respiration rate is not only a good indicator of the availability of soil C for the denitrifiers but is also a determinant of soil O2 levels. Respiration rates explained up to 46% of the variability in denitrification in soils amended with fermentation waste materials (Rice et al., 1988). The close match between the periods during which soil respiration rate, (Rochette et al., 2000), inorganic N, and FN2O were increased suggests that denitrification was a major contributor to FN2O. Other factors related to the addition of large quantities of PS, such as the increased soil moisture and surface sealing, may also have contributed to increase denitrification by limiting O2 diffusion. Doubling the quantity of PS added to the soil increased FN2O (P ⬍ 0.05) on seven of the first nine sampling dates following application compared with the single dose (Fig. 1). However, the increase was not linear, as the second additional 60 Mg ha⫺1 increment resulted in greater emissions than the first one. This response of FN2O contrasted with those of NH4⫹ (Fig. 2a), NO3⫺ (Fig. 2b), and respiration rates (Rochette et al., 2000), which were linear with PS dose during the postapplication period. Nonlinear response of FN2O to soil inorganic N levels may be explained by the fact that mineral N contents were determined on bulk soil samples, while DRs are controlled by the rate of NO3⫺ diffusion to anaerobic microsites where denitrification occurs (Højberg et al., 1994). Chantigny et al. (1998) observed that FN2O were 50 to 200% greater for a fertilization rate of 180 kg N ha⫺1 than for a rate of 120 kg N ha⫺1. They concluded that when N rate exceeds the plant needs, unused fertilizer N can be available for denitrification and N2O production. Furthermore, larger NO3⫺ contents in PS120 than in PS60 (Fig. 2b) may have reduced the conversion of N2O in N2 (Firestone et al., 1979), thereby increasing FN2O. Accordingly, Weier et al. (1993) measured smaller N2/N2O ratios as products of denitrification with increasing doses of NO3⫺-N. After 19 July (20 d after application), FN2O values were increased during three short episodes following rainfall events (Fig. 1a and 1b). Rainfall-induced episodes of N2O emissions result from denitrification under the anaerobic conditions created by the wetting front moving down the soil profile (Mosier and Hutchinson, 1981; Cates and Keeney, 1987; Davidson, 1992). Low FN2O were measured after 15 August even after large precipitations (Fig. 1a and 1b). The decreasing response of FN2O to rainfall as the season progresses has been observed in other studies (Lessard et al., 1996). It has been shown that this phenomena was the result of lower availability of C (Sexstone et al., 1985; Groffman et al., 1988) or N (Ryden, 1983; Chantigny et al., 1998) later in the season. Concentrations of N2O in soil atmosphere (Fig. 3) increased linearly with depth during most of the snowfree season on all treatments. The slopes of the relationships (⳵C/⳵z) varied across dates and treatments from 0.003 to 0.581 ␮L L⫺1 cm⫺1 with r 2 ⬎ 0.9 for 70% of the sampling dates. However, the occurrence of large gradients were not always associated with high FN2O as measured with chambers. Accordingly, the gradients

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Fig. 4. (a) Denitrification rate (soil cores) and (b) denitrifying enzyme activity (soil slurries) in soils cropped with silage maize amended with mineral fertilizer (control) and pig slurry at 60 (PS60) and 120 Mg ha⫺1 (PS120) measured in the laboratory on soil samples taken at four dates in 1997. Mineral fertilizer was applied on 29 May and pig slurry on 30 June 1997. Error bars ⫽ standard errors.

correlated with FN2O only in the PS120 treatment (r 2 ⫽ 0.69). Burton et al. (1997) also reported a poor correlation between independent measurements of ⳵C/⳵z and FN2O. A poor relationship between vertical concentration gradients and FN2O suggests that other factors such as the coefficient of gas diffusion (Ds) or the distribution of N2O production and consumption sites throughout the soil profile changed during the growing season.

Nitrous Oxide Fluxes during Winter Nitrous oxide fluxes were low but constant (2–9 ng m⫺2 s⫺1) in all treatments during the winter and snow melt periods of 1997-1998 (Fig. 1a). Although these fluxes are equivalent to the lowest winter fluxes reported between 1994 and 1997 in the same region (van Bochove et al., 2000), they represent cumulative losses of 0.1 to 0.4 kg N ha⫺1. Nitrous oxide production and emission can be large (≈200 ng m⫺2 s⫺1) in soils during winter when microbial biomass remains active at nearfreezing temperatures (van Bochove et al., 1996; Wagner-Riddle et al., 1998; Ro¨ver et al., 1998). Occurrence of large FN2O during winter were reported following incorporation of readily decomposable organic material (legume residues, animal manures) in the preceding fall (Wagner-Riddle et al., 1997). In this study, low levels of mineral N (Fig. 2a and 2b), microbial biomass, soil respiration (Rochette et al., 2000), and N2O

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Fig. 5. Cumulative N2O–N losses from soils cropped with silage maize amended with mineral fertilizer (control) and pig slurry at 60 (PS60) and 120 Mg ha⫺1 (PS120). Mineral fertilizer was applied on 29 May and pig slurry on 30 June 1997.

production (Fig. 1a) during fall of 1997 suggest that organic substrates required to sustain denitrification were low at the onset of winter.

Cumulative losses of Nitrous Oxide-Nitrogen Cumulative N2O–N losses were calculated by linearly interpolating N2O emissions between sampling dates, assuming that measurements made between 0900 and 1200 h were a good estimator of average daily FN2O. In the manure plots, only the N2O emissions from within the slurry application band were considered. Cumulative losses of N2O–N for the 12-mo period following organic or mineral fertilization were 0.93 kg N ha⫺1 in control, 1.55 kg N ha⫺1 in PS60, and 4.16 kg N ha⫺1 in PS120 plots (Fig. 5). The FN2O values were already high 18 h after PS addition when the first measurement was taken (Fig. 1a). Sharpe and Harper (1997) have shown that large FN2O can occur within 6 to 12 h following irrigation with swine effluent. Tortoso and Hutchinson (1990) have reported similar results using mineral fertilizer. Therefore, the absence of measurements during the first hours following PS application may have resulted in an underestimation of the cumulative N2O. Considering the very low FN2O on control plots, we also assumed that the emissions on unfertilized plots or so-called background emissions were zero. Based on that assumption, we calculated that total N2O–N emissions represented 0.62, 1.23, and 1.65% of total N applied in control, PS60, and PS120 plots, respectively. Emission factors expressed as the fraction of applied N that is lost as N2O have been proposed by Bolle et al. (1986) (0.5–2%), Mosier (1993) (1%), and Bouwman (1996) (1.25%). Our values are within the range of these estimates for the manured plots but are lower for the mineral fertilizer, supporting that FN2O was probably C-limited in control plots.

CONCLUSIONS Overall, the results of this study have shown that pig slurry application can result in greater N2O emissions than mineral fertilizer in soils with low available C content. Incorporation of PS into the soil increased levels

of inorganic N and available C. These conditions resulted in greater abundance of denitrifying enzymes and denitrification rates in soils. The close match between the periods during which inorganic N, soil respiration rate, and FN2O were increased suggested that denitrification was mostly responsible for increased FN2O following PS application. ACKNOWLEDGMENTS This work was supported by the PERD program of Agriculture and Agri-Food Canada. We thank F. Gagne´, P. Jolicoeur, N. Bissonnette, R. Baillargeon, P. Drouin, and J. Lizotte for assistance in soil surface gas flux measurements, soil analyses, statistical analyses, and plot maintenance. The numerous discussions with Dr. M.H. Chantigny are also gratefully acknowledged.

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