Soil Science Society of America Journal 66:647-652 (2002)
© 2002 Soil Science Society of America
DIVISION S-8—NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS
Cattle Slurry Applied Before Fertilizer Nitrate Lowers Nitrous Oxide and Dinitrogen Emissions
R. James Stevens* and
Ronald J. Laughlin
Department of Agriculture and Rural Development, Agricultural and Environmental Science Division, Newforge Lane, Belfast BT9 5PX, UK
* Corresponding author (jim.stevens{at}dardni.gov.uk)
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ABSTRACT
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Nitrous oxide emissions increase because of denitrification in the first few days after cattle (Bos taurus) slurry (CS) is applied to grassland soils fertilized with NO3. Denitrifying conditions are created when the readily decomposable C in the CS is oxidized by the soil microbial biomass when NO3 is present and O2 is deficient. Half of the readily decomposable C in CS can be volatile fatty acids (VFAs) that take up to 4 d to degrade. The timing of CS application relative to fertilizer-NO3 application could therefore affect the losses of N2O and N2. We used the 15N gas-flux method to measure N2O and N2 fluxes from grassland when CS containing 60 kg NH4-N ha-1 was applied 4, 3, 2, 1, and 0 d before the application of 60 kg N ha-1 of K15NO3. For a field experiment repeated in April, May, August, and October 1998, CS applied 3 or 4 d before KNO3 had no significant effect in any month on the flux of N2O in the 124 h after KNO3 application. On average over all months, the extra emission of N2O-N over the control was equivalent to 0.8, 1.1, and 2.9% of KNO3-N for prior applications of CS at 2, 1, and 0 d, respectively. When CS was applied 4 d prior to KNO3 there was no significant effect on the flux of N2 in any month. The maximum loss of N2O + N2 was 8.3% of the KNO3 applied (5 kg N ha-1) when CS and KNO3 were applied at the same time in April.
Abbreviations: CS, cattle slurry • VFA, volatile fatty acid • WFPS, water-filled pore space
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INTRODUCTION
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CODES OF GOOD AGRICULTURAL PRACTICE for managing livestock manures focus on how to avoid air pollution by odors and NH3 (Ministry of Agriculture, Fisheries and Food, 1998a), and water pollution by surface run-off (Ministry of Agriculture, Fisheries and Food, 1998b). For temperate grassland, refraining from applying manures during the winter months can lower the risk of surface run-off. Consequently, more manures have to be applied during the growing season. Farmers are advised to supply not more than half of the crop-N requirement from manures, the remainder being supplied from mineral fertilizers (Ministry of Agriculture, Fisheries and Food, 2000). Whenever fertilizer containing NO3 and CS are applied together or within a few days of each other, the potential exists for enhanced denitrification. Recent field studies have shown that N2O emissions from fertilizer NO3 were greatly increased in the first few days after application of CS (McTaggart et al., 1997; Clayton et al., 1997; Stevens and Laughlin, 2001). The increase in N2O emission was thought to be because of slurry constituents such as VFAs, being a readily available C source for heterotrophic denitrifiers (Paul and Beauchamp, 1989a).
When CS and KNO3 were applied at the same time, the increase in flux of N2O because of CS was maximal in the first measurement period (5–7 h), and persisted for up to 56 h (Stevens and Laughlin, 2001). Readily degradable substrates such as VFAs are rapidly decomposed under aerobic conditions (Cooper and Cornforth, 1978). Paul and Beauchamp (1989b) showed that VFA concentrations decreased significantly 10 mm below a manure layer in the first day after application and disappeared completely after 4 d. Delaying the application of NO3-containing fertilizers until a few days after CS has been applied may allow readily degradable substrates to be metabolized and prevent the reduction of freshly applied fertilizer NO3 but not native soil NO3. We conducted field experiments to measure the effect of applying CS before fertilizer NO3 on the fluxes of N2O and N2.
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MATERIALS AND METHODS
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Nitrogen-15 Gas-Flux Method
We used the 15N gas-flux method in the field to determine the effect of applying CS at various times prior to fertilizer NO3 on denitrification. The 15N gas-flux method measures N2O and N2 emissions while nitrification and denitrification proceed, as well as quantifying the relative contributions of each process to the flux of N2O (Stevens et al., 1997). Only the NO3 component was labelled with 15N as a previous study showed that no N2 was derived directly from the NH4 pool (Stevens and Laughlin, 2001).
Fluxes of N2O and N2 into the headspace of an enclosure were measured according to Mosier and Schimel (1993). Each circular enclosure (240-mm i.d.) consisted of a base section and a lid fabricated in polyvinyl Cl of a 6-mm thickness. The base section was 120 mm in depth and was inserted 100 mm into the soil 4 wk before flux measurement. The lid was 30 mm in depth and was fitted with a gas sampling port. When the lid was in place the headspace volume was 
3 L. The exact volume of each base section was determined at the end of the measurement period by filling with dry sand. At each sampling time the lid of the enclosure was fitted to the base section using a rubber band (40-mm wide) to form a gas-tight seal.
Site
The experiment was conducted on a grassland soil at the Agricultural Research Institute of Northern Ireland, Hillsborough, County Down (54°26' 45'' N lat., 6°04' 50'' W long.). The soil is an acid brown earth (fine loamy, acid, Typic Dystrochrept) with a pH of 6.0 and sand, silt, and clay contents of 480, 310, and 210 g kg-1 of mineral matter, respectively. It contained 120 g kg-1 oven-dry soil of organic matter in the 0- to 75-mm depth. The sward was dominated by perennial ryegrass (Lolium perenne L.) and had been under a three-cut silage regime receiving 300 kg N ha-1 yr-1 for the previous 6 yr. Soil and climatic conditions are given in Table 1.
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Table 1. Soil and climatic conditions during the experimental period at the grassland site in Northern Ireland where the interaction between cattle (Bos taurus) slurry and fertilizer NO3 was studied on four occasions in 1998.
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Treatments
During 1998, we repeated an experiment to measure the fluxes of N2 and N2O on four occasions (April, May, August, and October). A new area of grassland was used on each occasion. Treatments applied on each occasion were: CS 4 d before K15NO3 (CS-4d); CS 3 d before K15NO3 (CS-3d); CS 2 d before K15NO3 (CS-2d); CS 1 d before K15NO3 (CS-1d); CS at the same time as K15NO3 (CS0d); and unlabeled NH4HCO3 at the same time as K15NO3 (Control). The treatments were applied in a randomized block design with five replicates. Sufficient CS (25 L) for the four occasions was collected in February 1998 from the storage tank underneath the slatted floor of a house containing beef cattle at the Agricultural Research Institute of Northern Ireland, Hillsborough, County Down. The CS (90 g kg-1 dry matter) was passed through a 6-mm sieve to remove gross particulate material and stored at 2°C. For each occasion, a portion of this CS was diluted with water until the dry matter content was about 50 g kg-1 and then passed through a 1-mm sieve. Some properties of these slurries are shown in Table 2. Organic C and total N were determined by wet oxidation according to Ministry of Agriculture, Fisheries and Food (1981). Volatile fatty acids were determined by gas chromatography using a 10 m by 0.53 mm column of Carbowax 20M (Hewlett Packard HP-20M, Hewlett Packard, Palo Alto, CA) at 90°C and flame ionization detection.
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Table 2. Properties of the cattle slurries used for studying their interaction with fertilizer NO3 on four occasions in 1998.
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The rate of CS application was 40 m3 ha-1 (180 mL per enclosure) to supply 60 kg NH4-N ha-1. Ammonium bicarbonate served as a control, supplying no degradable C but the same amount of water and NH4-N as CS. The pH of the NH4HCO3 was adjusted to the same pH as that of the slurry. Potassium nitrate was applied in aqueous solution (100 mL per enclosure) to supply 60 kg NO3-N ha-1 enriched at 60 atom% in 15N. The 15N-labeled KNO3 was purified of any 15NO-2 according to Malone and Stevens (1998). The day prior to the application of CS treatments, the grass within each enclosure was clipped to a height of 20 mm above the soil surface and removed. The aliquots of fertilizer solution were applied uniformly over the soil surface within each enclosure using a standard 100-mL pipette. A 100-mL pipette with a 2-mm outlet was used for applying the aliquots of CS. Immediately after the first CS application, a transparent plastic box (450 by 320 by 240 mm) was propped 60 mm above each of the thirty enclosures to serve as a rain shelter. Water loss by evapotranspiration from an enclosure was assumed to be the similar whether or not CS was applied. The boxes were removed after the CS and K15NO3 treatments were applied on Day 0.
Flux Measurement
Thirty, two-part enclosures were used for the experiment on each occasion. Fluxes of N2 and N2O were measured for 2-h periods at 4, 8, 28, 52, 76, and 124 h after K15NO3 was applied. After 2 h, samples of the headspace of each enclosure were taken using a gas-tight syringe fitted with a push-button valve. A 12-mL sample was transferred to an evacuated (<100 Pa), septum-capped 12-mL vial for the analysis of N2 and N2O. The isotopic composition of the N2 and the concentration and isotopic composition of the N2O were determined by continuous-flow isotope-ratio mass spectrometry (Stevens et al., 1993), using a Europa Scientific 20-20 Stable Isotope Analyzer with Trace Gas Preparation System (PDZ Europa, Crewe, England) and Gilson autosampler (Anachem, Luton, England). The flux of N2O was calculated from the change in N2O concentration with time. Cumulative fluxes were calculated by integration, assuming linear change in flux rates between observation times. The flux of N2 was calculated assuming that the labeled N2 evolved into the headspace was derived from the same single, uniformly-labeled NO3 pool as the N2O. The fraction of labeled N2 in the headspace (dN2) was calculated from the difference in 30R (30N2/28N2) between normal and enriched headspace (
30R) and the mole fraction of 15N in the NO3 pool (15XN) according to the equation of Mulvaney (1984).
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The isotopic composition of the N2O was used to apportion the N2O flux between nitrification and denitrification, and to calculate the enrichment of the denitrifying NO3 pool. When N2O is derived from nitrification of the NH4 pool at natural abundance and from denitrification of the uniformly-labeled NO3 pool, the distribution of 15N atoms within the N2O molecules in the mixture is nonrandom. The extent of the nonrandomness is reflected in the molecular ratios for N2O of 45R (45N2O/44N2O) and 46R (46N2O/44N2O). Using 45R and 46R, the enrichment of the denitrifying pool (aD) and the fraction of the N2O derived from this pool were calculated from the equations of Arah (1997). The value of 15XN was taken to be the same as that for aD.
Statistical Analysis
Analysis of variance was carried out using Genstat (1993) to determine the significance of treatments on the fluxes of N2O and N2. When an effect was significant (P < 0.05), the least significant difference (LSD) for comparing any two means was calculated at P = 0.05.
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RESULTS AND DISCUSSION
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Effect of Interval between Cattle Slurry and Potassium Nitrate Applications on Losses of Nitrous Oxide and Dinitrogen
Cattle slurry had a significant positive effect on N2O emission in each of the 4 mo of application (Fig. 1)
. The magnitude of the effect depended on the interval between CS and KNO3 addition. Cattle slurry applied at the same time as KNO3 (Treatment CS0d) significantly increased the flux of N2O compared with the control in all months. A similar effect was found for this treatment in the previous year at the same site (Stevens and Laughlin, 2001). The fluxes of N2O were also significantly different from the control in April and May for the CS-1d treatment, and in April for the CS-2d treatment. The application of CS 3 or 4 d prior to KNO3 had no significant effect on N2O emission in any month. Maximum fluxes of N2O always occurred within the first 48 h after CS application.
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Fig. 1. The effect of cattle slurry applied 4, 3, 2, 1, and 0 d before KNO3 on the flux of N2O from grassland on four occasions in 1998. (LSD values are for comparing any two means at P = 0.05).
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Cattle slurry had a significant positive effect on N2 emission in April, May, and August (Fig. 2)
. As for N2O, the magnitude of the effect depended on the interval between CS and KNO3 addition. Fluxes of N2 were significantly higher than the control in April, May, and August for the CS0d treatment, and in April and May for the CS-1d treatment. The application of CS 2 or 3 d prior to KNO3 had a significant effect on N2 emission only in April. When CS was applied 4 d before KNO3 there was no significant effect in any month. Again as for N2O, the maximum flux of N2 always occurred within the first 48 h after CS application.
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Fig. 2. The effect of cattle slurry applied 4, 3, 2, 1, and 0 d before KNO3 on the flux of N2 from grassland on four occasions in 1998 (LSD values are for comparing any two means at P = 0.05).
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When CS was applied at the same time as KNO3 the maximum flux of N2O within 29 h after CS application reached a value of 484 µmol N2O-N m-2 h-1 (Fig. 1). In a previous study in 1997 conducted on the same site, the flux of N2O reached a maximum of 175 µmol N2O-N m-2 h-1 within a day of CS application (Stevens and Laughlin, 2001). Soils were wetter and CS rates were higher in 1998 than in 1997, hence conditions were more conducive to denitrification. In 1998, the overall average water-filled pore space (WFPS) was 77% (Table 1) compared with 68% in 1997. The application rate of CS in 1998 was almost double (40 m-3 ha-1) that in 1997 (22 m-3 ha-1). Other field studies have shown that N2O emission increases with increasing slurry application rate (Egginton and Smith, 1986; Velthof and Oenema, 1993).
The cumulative losses of N2O and N2 (Table 3) varied greatly between months because denitrification rate was affected by soil conditions (Table 1). Cumulative losses were controlled primarily by soil moisture. The highest loss occurred in April when the WFPS was 83% and the lowest loss occurring in May when the WFPS was 69%. Losses in August and October were intermediate when the WFPS values were 77 and 78% respectively, the loss in August being higher than in October probably because of the higher soil temperature. Half the variation in denitrification rates in the field can be explained by soil moisture, the rate of denitrification increasing exponentially between 60 and 90% WFPS (Linn and Doran, 1984). Soil temperature appeared to be the main factor affecting the shape of the profiles of N-gas flux (Fig. 1 and 2), the time to maximum flux being shortest in May and August when soil temperatures were highest.
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Table 3. The losses of N2O and N2 in the 124 h after the application of KNO3 to grassland, which had received cattle slurry 0, 1, 2, 3, and 4 d previously, on four occasions during 1998.
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Sources of Nitrous Oxide
The flux of N2O was partitioned between nitrification and denitrification (data not shown). As in the previous study at this site (Stevens and Laughlin, 2001), denitrification was always the dominant source of the N2O. On average over all months, denitrification produced 91% of the N2O in control treatments and 94% in the CS treatments.
Time Interval for Metabolism of Readily Degradable Substrates
Cattle slurry applied 4 d before KNO3 had no significant effect on the cumulative flux of N2O or N2 in any month (Table 3). An interval between CS application and fertilizer NO3 addition provides the soil biomass with the opportunity to metabolise the readily degradable substrates such as VFAs in the CS (Table 2). In our previous study (Stevens and Laughlin, 2001), a slurry-induced CO2 flux was measured in the first 2 d after CS was applied at half the rate used in the present study. Volatile fatty acids are present in anaerobically-stored animal slurries (Cooper and Cornforth, 1978) constitute 60% of the soluble C in CS, and are readily available C sources for heterotrophic denitrifiers (Paul and Beauchamp, 1989a). The metabolism of VFAs in CS added to soil consumed NO3 and was complete after 4 d at 15°C (Paul and Beauchamp, 1989b).
Apart from delaying the application of NO3-containing fertilizers until after CS application, the other main strategy for lowering the interaction would be to remove the readily degradable substrates from the CS by anaerobic digestion (Pain et al., 1990a) or aerobic digestion (Pain et al., 1990b). In laboratory experiments, where denitrification from anaerobically digested slurry was compared with undigested slurry, losses of N because of denitrification were lowered by 49 (Stevens et al., 1995) and 90% (Petersen et al., 1996). In a field experiment, N2O emissions from anaerobically digested slurry were less than the emissions from undigested slurry and similar to the emissions from fertilizer N (Petersen, 1999).
Relevance of the Interaction between Cattle Slurry and Potassium Nitrate to Nitrogen Efficiency
Agronomic studies to measure N efficiency on grassland have failed to find significant interactions between CS and fertilizer N. Pain et al. (1986) concluded that the combined effect of CS and NH4NO3 was additive. Stevens et al. (1987) found no significant interaction between CS and Ca(15NO3)2 when yield, N removal, and 15N recovery were measured in a field experiment with ryegrass. In the present study, the maximum loss of KNO3-N as N2O plus N2 was 8.3% (5 kg N ha-1) with the April application for CS0d (Table 3). The total available N for grass growth was therefore decreased from 90 to 85 kg N ha-1, assuming that half of the NH4-N in surface-applied CS volatilized as NH3 (Stevens and Laughlin, 1997). The effect of such a decrease on herbage yield would be difficult to measure with statistical significance in the field because of spatial variability.
Relevance of the Interaction between Cattle Slurry and Potassium Nitrate to Emission Factors
The annual emission factor for N2O, as proposed by the Intergovernmental Panel on Climate Change, is 1.25% of the mineral N applied as fertilizer or manure (Mosier et al., 1998). The present study emphasizes that CS can interact with fertilizer NO3 and result in excedence of this emission factor by variable amounts, depending on the interval between CS and fertilizer applications and on the time of year (Table 3). On average over all months of application, 40 m3 ha-1 of CS applied at the same time as KNO3 caused an additional 2.9% of the KNO3-N to be emitted as N2O. This extra emission could largely be avoided by applying the KNO3 at least 3 d after the CS.
Received for publication January 22, 2001.
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REFERENCES
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ABSTRACT
INTRODUCTION
