Gaseous nitrogen loss from temperate perennial grass and clover ...

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Australian Journal of Agricultural Research, 2003, 54, 561–570

Gaseous nitrogen loss from temperate perennial grass and clover dairy pastures in south-eastern Australia R. J. EckardA,B, D. ChenA, R. E. WhiteA, and D. F. ChapmanA A

Institute of Land and Food Resources, The University of Melbourne, Parkville, Vic. 3010, Australia. B Corresponding author; email: [email protected]

Abstract. The use of nitrogen (N) fertiliser on dairy pastures in south-eastern Australia has increased exponentially over the past 15 years. Concerns have been raised about the economic and environmental impact of N loss through volatilisation and denitrification. Emissions of NH3, N2, and N2O were measured for 3 years in the 4 different seasons from a grazed grass/clover pasture, with or without 200 kg N fertiliser/ha, applied as ammonium nitrate and urea. Nitrogen-fertilised treatments lost significantly more N than the control treatments in all cases. More NH3 was lost from urea-fertilised treatments than from either the control or ammonium nitrate treatments, whereas ammonium nitrate treatments lost significantly more N through denitrification than the control or urea treatments in all seasons, except for summer. More NH3 was lost in summer than in the other seasons, whereas denitrification and N2O losses were highest in winter and lowest in summer. The total annual NH3 loss from the control, ammonium nitrate, and urea treatments averaged 17, 32, and 57 kg N/ha.year, respectively. Annual denitrification losses were estimated at around 6, 15, and 13 kg N/ha.year for the control, ammonium nitrate, and urea treatments, respectively. Total gaseous N losses were estimated to be 23, 47, and 70 kg N/ha.year from the control, ammonium nitrate, and urea treatments, respectively. Although the use of ammonium nitrate fertiliser would significantly reduce NH3 volatilisation losses in summer, this fertiliser costs 45% more per unit N than urea, so there is no economic justification for recommending its use over urea for the other seasons. However, the use of urea during the cooler, wetter months may result in significantly less denitrification loss. The results are discussed in terms of potential management strategies to improve fertiliser efficiency and reduce adverse effects on the environment. AR0210 eRGat. lsJe.Eocuksarnditrogen los fromdairypatsure

Additional keywords: ammonium nitrate, urea, volatilisation, denitrification, ammonia, nitrous oxide.

Introduction Over 70% of Australia’s dairy production is based on temperate grass/clover pastures grown on more than 9700 farms in south-eastern Australia (ADC 2000). The use of fertiliser nitrogen (N) on these pastures to increase herbage production and milk yield has increased exponentially over the past 15 years, with over 60% of dairy farmers now topdressing pasture with 25–50 kg N/ha at least once per year (Eckard et al. 1997, 2000; Eckard and Franks 1998). Because it is relatively cheap, urea is the main fertiliser used, but broadcasting urea onto grass swards that have high urease activity (Meyer et al. 1961) can result in large losses of N by ammonia (NH3) volatilisation (Jarvis et al. 1995). Consequently, concerns have been raised about the economic merit of urea use compared with other N sources such as ammonium nitrate. © CSIRO 2003

The environmental impact of increased N use is also of concern because of the large losses of N that can occur by emission of NH3 and the greenhouse gas nitrous oxide (N2O) to the atmosphere, and the leaching of nitrate to the groundwater (Jarvis et al. 1995). Nitrous oxide is produced in soils by nitrification and denitrification of soil N (Bremner 1997) and its emission is enhanced when soils are used for agriculture because of additional N added by legume fixation, fertiliser, crop residues, and animal faeces and urine (Granli and Bøckman 1994; Bouwman 1996). Ammonia produced as a result of the application of fertiliser and animal wastes to soil has a short lifetime in the atmosphere, is deposited on soils or waters nearby (Ferm 1998), and may also act as an indirect source of N2O through subsequent nitrification (Mosier et al. 1998). 10.1071/AR02100

0004-9409/03/060561

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Australian Journal of Agricultural Research

According to the Australian National Greenhouse Gas Inventory (AGO 2000), agriculture accounted for 79.7% of Australia’s total N2O emissions in 1999, an increase of 16% since 1990. With a global warming potential of 310 times that of carbon dioxide (IPCC 1996), N2O emissions could be an important consideration for the future sustainability of intensive dairy production systems. Estimates of N2O emissions associated with grazing and N fertiliser are not reliable (Granli and Bøckman 1994; Bouwman 1996; Mosier et al. 1998), and thus more information is required on emissions from different agricultural systems. Although studies on dairy pastures in New Zealand (Ledgard et al. 1999) showed that gaseous losses by denitrification and volatilisation increased up to 5-fold with the application of N fertiliser, few data are available quantifying gaseous N losses from intensive dairy pastures in south-eastern Australia (Eckard 1998). The objective of the experiment reported in this paper was to quantify gaseous N loss from a grazed temperate grass/clover pasture with and without 200 kg N fertiliser/ha, applied as ammonium nitrate and urea, in the 4 different seasons. Based on the results, suggestions are made for further improving N fertiliser efficiency and reducing adverse effects on the environment through changes in management.

R. J. Eckard et al.

Table 1. Selected properties of the soil (0–100 mm layer) prior to treatment application pH (1 :5 soil:water) Organic C (%) P (Olsen) (mg/kg) K (Skene) (mg/kg) S (mg/kg) Exchangeable cations (cmolc/kg) Ca Mg Na K Al Cation exchange capacity (cmolc/kg) Bulk density (g/cm3) Field capacity (% v/v) Total porosity (%) Air-filled porosity at field capacity (%)

5.10 4.30 40 183 28 5.90 1.46 0.22 0.47 1.37 9.40 1.10 41 58 17

Methods

Pastures were grazed by Friesian dairy cows for 2 days duration in each rotation 21 days after fertiliser application (DAF) in spring, and 28 DAF in autumn, winter, and summer. Cows were preconditioned on matching treatments for 2 days prior to each grazing of the trial plots. Stocking rates were calculated to achieve a post-grazing residual herbage mass of 1400 kg DM/ha within 2 days. For the remainder of the year, pastures were rotationally grazed according to the criterion described above. This resulted in 12 rotational grazing events per year at a stocking rate of 1.9 (control) and 2.8 (N fertilised) Friesian cows/ha, apart from the 3 summer grazing rotations where all treatments were stocked at 1.9 cows/ha, due to lower pasture growth rates.

Site description

Gaseous loss measurements

The research was conducted at the Ellinbank Dairy Research Institute, in west Gippsland, Vic. (38°15´S, 145°93´E; mean annual rainfall 1114 mm), between May 1998 and March 2001. The soil at the site was a bleached-acidic, dermosolic, redoxic Hydrosol (Isbell 1996), Great Soil Group: Brown Podzolic soil (Stace et al. 1968). Pastures comprised predominantly perennial ryegrass (Lolium perenne L.) (53–60%) and white clover (Trifolium repens L.) (15–24%), with weeds and other grasses contributing an equal proportion of the balance of total biomass. Selected soil properties measured at the start of the experiment are presented in Table 1. To eliminate nutrient limitations, a basal dressing of fertiliser was applied in autumn each year at the rate of 60 kg phosphorus (P)/ha, 100 kg potassium (K)/ha, and 75 kg sulfur (S)/ha. Phosphorus and S were applied as single superphosphate (8.8% P; 11% S) and K was applied as potassium chloride (49% K). Soil organic C and water-soluble C content were analysed using the methods of Walkely and Black (1934) and Burford and Bremner (1975), respectively, and air-filled porosity at field capacity was calculated as described by Ruz-Jerez et al. (1994).

Ammonia volatilisation was measured by a micrometeorological mass balance method (Schjøerring et al. 1992). Borosilicate 100-mm sampling tubes (Mikrolab Aarhus, Holbjerg, Denmark) coated with oxalic acid (Leuning et al. 1985) were mounted on 3-m masts at 0.75, 1.5, 2.25, and 3.0 m above the pasture surface. Masts were located in the mid-point of each plot side, with 2 sampling tubes facing the treatment and 2 tubes facing the surrounding area at each height, resulting in 16 tubes per mast. Plots within each replicate were laid out in a contiguous block and therefore shared masts on their common boundaries. Replicates were separated by 30 m from each other. The method of Schjøerring et al. (1992) allows the calculation of the net emission (or deposition) of NH3 from each treatment by measuring the net horizontal flux across the plot boundary at several heights, and integrating to find the total vertical flux. Thus, the effect of any adjacent treatment or background on the plot NH3 emission is automatically accounted for. Coated sampling tubes were installed immediately after each N application and left in place for 28 days in summer, autumn, and winter, and 21 days in spring (referred to as the ‘ungrazed period’). A new set of coated tubes was installed at the start of each grazing and removed 14 days later (referred to as the ‘grazed period’). The tubes were eluted with 3 mL deionised water and the extracts analysed colorimetrically at 630 nm on a La Chat (Zellweger Analytics Inc., Milwaukee) Quickchem 8000 flow injection system (method number 10-107-06-1-A) (Anon. 1997). Denitrification loss, or the production of N2 + N2O, was measured using an acetylene inhibition method (Ryden et al. 1987; Ruz-Jerez et al. 1994). In this method the reduction of N2O is inhibited, and the N2O that accumulates is assumed equivalent to the N2 + N2O produced in the absence of acetylene.

Experimental design The experiment was laid out in a randomised block design with 3 treatments and 3 replicates. The effect of grazing subsequent to fertilising on gaseous N loss from the pasture plots was also studied. Plot size was 25 m by 25 m. Treatments were: (i) control (no added N), (ii) 200 kg N/ha.year as urea (46% N), and (iii) 200 kg N/ha.year as ammonium nitrate (34.5% N). The fertilisers were applied at the rate of 50 kg N/ha per application in May (autumn), July (winter), October (spring), and February (summer) for 3 consecutive years.

Gaseous nitrogen loss from dairy pasture


Starting on the day of each N application through to 14 days after grazing commenced, 24 soil cores (250 mm diam. by 150 mm deep) were randomly collected weekly from each plot, and tightly packed intact into a gas-tight 1-L incubation chamber individually for each plot. Using a gas flow meter, each chamber was flushed for 2 min with 10% acetylene, with an O2 concentration adjusted to the in situ soil O2 level and balanced by N2. Each week, one chamber per plot (9 chambers) was buried in the soil for 24 h. The N2O produced in the head space after 24 h was sampled and analysed by gas chromatography as described by Ryden et al. (1987). Nitrous oxide production was measured using the same sampling procedure, incubating without acetylene, with soils taken from a single replicate at each sampling. Annual calculations Annual volatilisation and denitrification losses were calculated as the sum of the measured losses for the 4 ungrazed and grazed periods, made over 42 days in summer, autumn, and winter and 35 days in spring, plus interpolated losses for the remainder of the year. The interpolated losses were based on the average daily loss rates measured for each season during the 2-day grazing and 12 days subsequent growth period (grazed period), scaled up for the remainder of each season when no measurements were made (i.e. 50, 50, 56, and 48 days in autumn, winter, spring, and summer, respectively). Because pastures were grazed on the same rotation and stocking rate as the ‘grazed period’ within each season, it was assumed that N was lost from each treatment at the same rate for the 3 months (season) surrounding the ‘grazed period’. Meteorological measurements An automatic weather station at the site recorded monthly total rainfall, monthly average soil temperature (at 100 mm depth), air temperature, wind speed (m/s), and soil air-filled porosity (at 150–300 mm depth) (Fig. 1). Potential evapotranspiration (ETp) was calculated using a modified Penman-Monteith equation (Smith 1992).

140

Chemical analysis Pastures were sampled prior to each grazing by randomly hand-plucking across each plot to the target grazing residual dry matter (DM). Samples were oven-dried at 60°C for 48 h, weighed, and analysed for N by near infrared spectroscopy (Eckard et al. 1988; Schenk and Westerhaus 1991). Subsamples of the soils from each sampling for assessing denitrification and N2O emission were analysed for NH4+ (Berthelot colour reaction) and NO3– (cadmium reduction) using a flow injection autoanalyzer (Anon. 1997). Data analysis All data were subjected to analysis of variance (GENSTAT 5 Committee 1995) using a factorial treatment structure, N (control, urea, and ammonium nitrate) × season, and blocking structure, paddock (3 replicates) × year (1998–99, 1999–2000, 2000–01), split for season. Treatment differences were compared using least significant differences (l.s.d.s).

Results Climatic conditions The mean monthly air temperature decreased from 10°C at the start of the experiment in May to a minimum of 8°C in July, then increased to a maximum of 20°C in February before decreasing to 12°C in April (Fig. 1). During the same period, the mean soil temperature at 100 mm depth followed the same pattern, but fluctuated from 10°C in July to 22°C in February (Fig. 1). Rain fell throughout the study period, but the wettest periods were in May–October and the driest in January–March (Fig. 1). In wet periods, ETp (~33 mm/month) was much lower than rainfall (~92 mm/month), whereas in summer, although ETp

Rainfall

ETP

Soil temperature

Air temperatures

Wind speed

Air-filled porosity

25

120

100 15 80

60

10

40

Soil and air temperature (oC) Wind speed (m/s)

Rainfall and ETP (mm/month) Air-filled porosity (v/v%)

20

5 20

0

0

May

June

July

Aug.

Sept.

Oct.

563

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

Fig. 1. Monthly total rainfall (mm), potential evapotranspiration (ETp, mm), average soil temperature (°C at 100 mm), air temperature (°C), wind speed (m/s), and air-filled porosity (v/v % at 150–300 mm depth), measured at the experimental site averaged over 3 years.

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(~98 mm/month) was greater than rainfall (~70 mm/month), the difference between ETp and rainfall was much less (Fig. 1). These results are reflected in the data for air-filled porosity, with the soil being considerably wetter in the wet season (~23%) than in the dry season (~37%) (Fig. 1). Average wind speed was slightly higher in spring and summer (~6 m/s) than in autumn and winter (~4 m/s) (Fig. 1). Soil and pasture composition There were marked increases in the NH4+ concentration of the soil within a week of each fertiliser application and each grazing. There were also marked increases in soil NO3– within a week of ammonium nitrate applications, but less marked increases following the application of urea (Fig. 2a, b). Water-soluble carbon varied significantly (P < 0.05) between the sampling in May (1.14 mg/g) and November (0.47 mg/g) 1998, but differences between treatments for a sampling date were non-significant. The average pasture N concentration was significantly (P < 0.05) higher in autumn and winter than in summer and spring (Table 2). The N concentration in the control treatment was significantly (P < 0.05) lower than that in both N treatments in all seasons, whereas there were no significant differences in N concentration between N fertilised treatments, regardless of season.

Ammonium (µg N/g)

35

(a)

30 25 20 15 10 5 0

Nitrate (µg N/g)

25

(b)

20 15 10 5 0

Fig. 2. (a) Ammonium and (b) nitrate concentrations in the 0–150 mm layer of soils from the fertilised pasture treatments. Data averaged over 3 years for each sampling date. The arrows at N and G denote the times of fertiliser application and commencement of grazing, respectively.

R. J. Eckard et al.

Table 2.

Nitrogen concentration (% DM basis) in the pastures prior to grazing Data averaged for each season over 3 years

Season

Control

Ammonium nitrate

Urea

l.s.d. (P = 0.05)

Autumn Winter Spring Summer

4.30 3.80 3.20 2.70

4.60 4.20 3.30 3.30

4.70 4.30 3.40 3.10

0.20 0.20 0.20 0.20

l.s.d. (P = 0.05) Average

0.68 3.50

0.68 3.90

0.68 3.90

0.10

Ammonia volatilisation In the periods immediately after fertiliser application when the plots were ungrazed (0–28 DAF in summer, autumn, and winter or 0–21 DAF in spring), there was no significant loss of NH3 from the control or the ammonium nitrate treatments (range: –0.8–1.1 kg N/ha) (Fig. 3a). Over the 3 years of the study the control treatments showed a very small net deposition of NH3 (0.2–0.8 kg N/ha.year) from the surrounding areas. Significant (P < 0.05) amounts of NH3 were lost from the urea treatment (range 1.8–4.1 kg N/ha), and significantly (P < 0.05) more NH3 was lost in summer than in the other seasons (Fig. 3a). Total ammonia volatilisation loss following 4 applications of urea (ungrazed periods) was significantly (P < 0.05) higher (at 10.3 kg N/ha.year) than from both the control and ammonium nitrate treatments (Table 3). During the grazing periods (29–42 DAF in summer, autumn, and winter and 22–35 DAF in spring), NH3 was lost from all treatments, ranging over –0.2–1.9, 0.5–2.5, and 0.8–3.2 kg N/ha for the control, ammonium nitrate, and urea treatments, respectively (Fig. 3b). In general, the grazed fertilised treatments lost more NH3 than the grazed control, and there was little difference in NH3 loss between the ammonium nitrate and urea treatments (Fig. 3b). Ammonia loss in autumn tended to be lower than that in the other seasons (Fig. 3b). On average, NH3 lost from the control, ammonium nitrate, and urea treatments each year during the 8 weeks of grazing totalled 4.0, 6.2, and 10.0 kg N/ha, respectively. As a result of fertiliser application and subsequent grazing, significantly (P < 0.05) more NH3 was lost from the urea treatment than from the control and ammonium nitrate treatments in every season, with the smallest loss occurring after the autumn application; the respective losses ranged over –1.0–1.7, 0.7–3.5, and 3.0–7.0 kg N/ha (Fig. 3b). The calculated annual NH3 losses from the control, ammonium nitrate and urea treatments were all significantly different (P < 0.05) and averaged 17, 31, and 57 kg N/ha, respectively (Table 3).


6

(a)

5


Table 3.

Average annual N loss (kg N/ha) from pastures receiving N fertilisers in each of 4 seasons Values calculated using actual losses for the ungrazed treatments and measured plus extrapolated losses for grazing periods

Control Ammonium nitrae Urea

4

Control

3 2 1

Ammonia volatilisation (kg N/ha)

0

(b) 4 3 2

Ammonium nitrate

Urea

l.s.d (P = 0.05)

UngrazedA GrazedB Total volatilised

–2.0 19 17

NH3 volatilised 2.6 28 31

10.3 46 57

3.0 10 11

Ungrazed Grazed Total denitrified

1.6 05 06

N denitrified 5.4 10 15

03.5 10 13

1.6 30 50

Volatilisation + denitrification –0.4 8.0 13.8 23 36 54 23 44 68

3.6 12 14

5

1

Ungrazed Grazed Total N loss

0

A

10

565

0–28 DAF in summer, autumn, and winter, and 0–21 DAF in spring (105 days total). B Calculations based on average daily rates measured for each season during a 14-day grazing period (29–42 DAF in summer, autumn, and winter, and 22–35 DAF in spring; 56 days total), scaled up for the remainder of each season when no measurements were made (i.e. 50, 50, 56, and 48 days in autumn, winter, spring, and summer, respectively).

(c)

8 6 4 2 0

Autumn

Winter

Spring

Summer

Fig. 3. Ammonia volatilised from a grass/clover pasture after 0 (control) or 4 applications of 50 kg N/ha.year, applied as urea or ammonium nitrate. (a) Ungrazed, 0–28 days after fertilisation (DAF) in autumn, winter, and summer, and 0–21 DAF in spring; (b) grazed, 29–42 DAF in autumn, winter, and summer, and 22–35 DAF in spring; (c) total, 0–42 DAF in autumn, winter, and summer, and 0–35 DAF in spring. Data averaged over 3 years. Vertical lines denote l.s.d. at P = 0.05 for comparisons of treatment within season (above the bar) and treatment × season (in the bar).

Nitrogen lost by denitrification The amount of N lost as a result of denitrification from the ungrazed control, ammonium nitrate, and urea treatments in the periods 0–28 DAF in summer, autumn, and winter, or 0–21 DAF in spring, ranged over 0.3–0.5, 0.4–2.1, and 0.4–1.2 kg N/ha, respectively (Fig. 4b). In autumn, winter, and spring, significantly (P < 0.05) more N was lost by denitrification from the two fertiliser treatments than from the control, and more N was lost from the ammonium nitrate treatment than from the urea treatment. In summer there were no significant treatment effects on denitrification (Fig. 4a). In general, denitrification losses were higher in winter and lower in summer, but the only significant

(P < 0.05) seasonal effect occurred in the ammonium nitrate treatment, where considerably more N was lost in winter than in summer. Total N losses by denitrification from the control, ammonium nitrate, and urea treatments during the 4 ungrazed periods each year were all significantly different (P < 0.05) and amounted to 1.6, 5.4, and 3.5 kg N/ha.year, respectively (Table 3). During the grazing periods (29–42 DAF in autumn, winter, summer, and 22–35 DAF in spring), loss of N by denitrification from the control, ammonium nitrate, and urea treatments ranged over 0.2–0.4, 0.2–0.7, and 0.3–0.7 kg N/ha, respectively. Denitrification losses were consistently higher from treatments receiving N fertiliser than from the control treatments (Fig. 4b), and these differences were significant (P < 0.05) in all seasons except summer. However, there was no significant difference in N loss between the ammonium nitrate and urea treatments. As was the case with the ungrazed treatment periods, loss of N by denitrification measured during grazing was low in summer. Calculated N losses by denitrification from the control, ammonium nitrate, and urea treatments each year during all grazing periods averaged 5, 10, and 10 kg N/ha.year, respectively (Table 3). Total denitrification losses from the control, ammonium nitrate, and urea treatments, from the time of N application until 2 weeks after subsequent grazing, ranged over 0.5–0.8, 0.7–2.6, and 0.8–1.7 kg N/ha, respectively (Fig. 4c). The


2.5

(a)

Control

Ammonium nitrate

R. J. Eckard et al.

12

Urea

Total gaseous loss (kg N/ha)

566

2.0 1.5 1.0 0.5

Denitrification loss (kg N/ha)

0.0

1.0

Ammonium nitrate

Urea

10 8 6 4 2 0 −2

(b)

Control

Autumn

Winter

Spring

Summer

Fig. 5. Total gaseous nitrogen (NH3 + N2 + N20) loss from a grass/clover pasture after 0 (control) or 4 applications of 50 kg N/ha.year, applied as urea or ammonium nitrate, 0–42 days after fertilisation (DAF) in autumn, winter, and summer, and 0–35 DAF in spring. Data averaged over 3 years. Vertical lines denote l.s.d. at P = 0.05 for comparisons of treatment within season (above the bar) and treatment × season (in the bar).

0.8 0.6 0.4 0.2

the control treatments losing significantly (P < 0.05) less N than both N-fertilised treatments.

0.0 3.0

(c)

2.5

Total gaseous loss of nitrogen

2.0

Total gaseous N loss from the 3 treatments in the periods 0–42 DAF (autumn, winter, summer) and 0–35 DAF (spring), ranged over 0–2.3, 2.4–5.6, and 4.6–7.7 kg N/ha for the control, ammonium nitrate, and urea treatments, respectively (Fig. 5). The N-fertilised treatments lost significantly (P < 0.05) more gaseous N than the control treatments in all seasons except summer. Total gaseous N loss was consistently higher from the urea treatment than from the ammonium nitrate treatment in all seasons, although these differences were only significant (P < 0.05) in autumn and summer. There were no significant seasonal effects on total gaseous losses from the control treatments, but the ammonium nitrate treatments lost significantly (P < 0.05) less N in summer than in spring and winter, and the urea treatments lost significantly (P < 0.05) more N in summer than in autumn. In general, N loss from all treatments was least in autumn. Calculated annual losses showed that more N was lost when the plots were grazed than during ungrazed periods (for urea, 54 v. 14 kg N/ha.year), NH3 volatilisation was the dominant gaseous loss process (for urea 57 v. 13 kg N/ha.year), and significantly (P < 0.05) more N was lost following the application of urea than when ammonium nitrate was applied (68 v. 44 kg N/ha.year; Table 3).

1.5 1.0 0.5 0.0

Autumn

Winter

Spring

Summer

Fig. 4. Nitrogen lost from a grass/clover pasture by denitrification after 0 (control) or 4 applications of 50 kg N/ha.year, applied as urea or ammonium nitrate. (a) Ungrazed, 0–28 days after fertilisation (DAF) in autumn, winter, and summer, and 0–21 DAF in spring; (b) grazed, 29–42 DAF in autumn, winter, and summer, and 22–35 DAF in spring; (c) total, 0–42 DAF in autumn, winter, and summer, and 0–35 DAF in spring. Data averaged over 3 years. Vertical lines denote l.s.d. at P = 0.05 for comparisons of treatment within season (above the bar) and treatment × season (in the bar).

losses from the fertilised treatments were significantly (P < 0.05) greater than that from the control in all seasons except summer. Although the total loss by denitrification from the ammonium nitrate treatment was consistently higher than that from the urea treatment, the difference was significant (P < 0.05) only in winter (Fig. 4c). Estimated daily denitrification rates (total measured loss/days) were high in winter and spring at 17–18, 60–62, and 41–44 g N/ha.day for the control, ammonium nitrate, and urea treatments, respectively, and low in summer at 11, 16, and 18 g N/ha.day, respectively. Annual denitrification losses estimated for the control, ammonium nitrate, and urea treatments were 6, 15, and 13 kg N/ha, respectively (Table 3),

Emission of nitrous oxide Total N2O losses, measured from the unreplicated soil samples incubated without acetylene (Table 4), were between 0.9 (summer) and 2.7 (winter) times higher from N-fertilised treatments than from the controls. Apart from



Table 4. Nitrous oxide emission (kg N/ha) from a grass/clover pasture after fertiliser application and subsequent grazing Data averaged from weekly sampling of a single alternating replicate in each season over 3 years. Measurements were made over 42 days in summer, autumn, and winter and 35 days in spring Season

Control

Ammonium nitrate

Urea

Autumn Winter Spring Summer

0.4 0.5 0.7 0.4

0.8 1.7 0.6 0.4

0.8 1.0 1.0 0.3

the lower losses in summer, there were no obvious seasonal trends in N2O losses, although substantially more N2O was lost from the ammonium nitrate treatments than from the urea treatments in winter, with the reverse trend in spring. Discussion and conclusions Ammonia volatilisation The volatilisation of NH3 from N-fertilised pasture is well documented in Europe (Ryden 1986; Jarvis et al. 1989; Bussink 1992), New Zealand (Ball and Ryden 1984; Ledgard et al. 1999), and South Africa (McKenzie and Tainton 1993). Studies in Australia were reported from south-eastern Queensland (Vallis et al. 1982; Harper et al. 1983), South Australia (Packrou et al. 1997), and more recently from tropical Australia (Prasertsak et al. 2001). Published estimates of NH3 loss following application of urea to pasture vary between 10 and 30% of the N applied (Vallis et al. 1982; Harper et al. 1983; Whitehead 1995; Prasertsak et al. 2001), whereas volatilisation of NH3 from ammonium nitrate fertiliser is usually negligible on most non-calcareous soils (Whitehead and Raistrick 1990). Consistent with these studies, the loss of NH3 was relatively small when ammonium nitrate was applied, and appreciable when urea was used (Fig. 3a), particularly in the summer when temperatures and wind speed were high and rainfall low (Fig. 1). The small negative NH3 losses from the controls imply a net deposition of NH3, as the method used measures NH3 loss relative to the background (Schjøerring et al. 1992). Background NH3 concentrations in the air on dairy farms are likely to be high with stock on surrounding paddocks, pond storage of effluent, and the use of N fertilisers. Under such conditions a net deposition of ammonia may be expected on ungrazed, unfertilised pastures. In terms of NH3 lost from the fertilised treatments during ungrazed periods (0–28 DAF in autumn, winter, summer, and 0–21 DAF in spring), losses from the urea treatment (5–8% of N applied) were within the range cited by Packrou et al. (1997) for temperate pastures in Australia. However, the losses measured were well below the 20–28% reported for pastures at subtropical (Vallis et al. 1982; Harper et al. 1983) and tropical latitudes in Australia (Prasertsak et al.

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2001). Recent grazing studies by Ledgard et al. (1999) reported volatilisation losses from unfertilised and urea-fertilised (200 kg urea N/ha.year) grass/clover pastures in New Zealand of 15–17 and 30–45 kg N/ha.year, respectively. The range of 17–57 kg N/ha.year found in this study is consistent with those results. However, as the annual NH3 losses in Table 3 were based on only 161 days of actual measurement, and interpolated for the other 204 days, these data should be treated with some caution and, accordingly, compared with other published data. Ammonia losses measured during and subsequent to grazing (29–42 DAF in summer, autumn, and winter and 22–35 DAF in spring) largely reflected the differences in stocking rate (2.8 v. 1.9 cows/ha) and pasture N concentrations (Table 2) between treatments, with both N treatments losing similar quantities of NH3 except in summer. More than 75% of the N in urine of dairy cows grazing N-fertilised pasture is in the highly labile urea form (Bussink 1992). Consequently, with the enzyme urease liberally present in pasture and soil (Meyer et al. 1961), 4–38% of the urinary N deposited could volatilise as NH3 within 2–3 days of grazing (Whitehead 1995). Losses of NH3 associated with grazing were also lowest in autumn, when rainfall was high and evaporation was low (Fig. 1). The significantly (P < 0.05) greater loss of NH3 from the grazing of urea treatments in summer (Fig. 3a) could have resulted from some carryover effects from the N application under the drier conditions. For most of the rest of the year, ETp was low and rainfall in the weeks following N application appeared sufficient to wash the N fertiliser into the soil (Black et al. 1987; Bussink 1994). In the summer period, ETp was at its highest and rainfall and soil moisture were low (Fig. 1), which may have resulted in some urea remaining unhydrolysed on the soil surface. Total NH3 losses from urea treatments, during the autumn, winter, and spring period, were lower than in summer due to a combination of lower temperatures and higher rainfall (Fig. 1) (Ryden 1986; Bussink 1992). Denitrification Denitrification in soil is controlled by a number of factors, including O2 partial pressure (Aulakh et al. 1992; Ruz-Jerez et al. 1994), water-soluble carbon (Burford and Bremner 1975), nitrate concentration, pH, and temperature (Ryden 1983). The application of ammonium nitrate resulted in higher denitrification losses in autumn, winter, and spring than the application of urea (Fig. 4a). This could be a reflection of the higher soil nitrate concentration in the ammonium nitrate treatment (Fig. 2b). Denitrification losses were highest in the winter period, when rainfall and soil moisture were highest (Fig. 1). Air-filled porosity was below the critical level of 17% (Ruz-Jerez et al. 1994) for extended periods each year

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between August and September, indicating that soil conditions were conducive to denitrification at this time of the year. Although soil temperatures were lowest during this period (Fig. 1), they were above the critical limit of 4–6°C below which Ryden (1986) reported a marked reduction in denitrification rate. However, as soil temperatures in winter were consistently below 10°C, the rate of denitrification was likely to have been temperature-limited (Ruz-Jerez et al. 1994). In contrast, denitrification losses during summer were low, largely due to extremely dry soil conditions linked to lower rainfall and high ETp (Fig. 1). Data from New Zealand, where summer rainfall was also low (Ruz-Jerez et al. 1994), show similar low denitrification rates in summer of 11–18 g N/ha.day. Denitrification losses attributable to grazing (Fig. 4b) were lowest in summer due largely to the high air-filled porosity then (Fig. 1). Total denitrification losses from the controls were 1.5–3.6 times lower than losses from N-fertilised plots (Fig. 4c), indicating a substantial increase in denitrification with the use of N fertiliser, and in particular ammonium nitrate. Ruz-Jerez et al. (1994) reported a 6-fold increase in denitrification rate between zero N fertiliser and 400 kg N/ha applied to pastures in New Zealand. As denitrification rates vary considerably both spatially and temporally in response to temperature, rainfall, and urine deposition, the interpolation of weekly measurements of denitrification, made over 161 days per year, to annual losses should be treated with some caution. Also it must be noted that the acetylene inhibition method (Ryden et al. 1987) measures N2 plus N2O production from soil cores within an incubation chamber, and does not necessarily estimate actual emission from the soil surface. Another limitation of the soil core incubation method is that a larger surface area of soil is exposed to air than would be the case for intact soil. This could result in lower estimates of denitrification from saturated soils, particularly as the multiple soil cores are circular, exposing a greater surface area of soil to oxygen within the chamber. To minimise this effect in the current study, we transferred soil cores to the chambers through a PVC tube of similar diameter to the soil corer. The chambers were then flushed with a mixture of N2 and O2 in the ratio of the measured soil gas composition, in addition to the 10% acetylene. Despite possible limitations in the techniques, our calculated total denitrification losses of 6–15 kg N/ha.year compare well with results recorded elsewhere. Ruz-Jerez et al. (1994) reported losses of around 3.4 kg N/ha for a grass/clover system and 19.3 kg N/ha (4.8% of N applied) where 400 kg N/ha was applied. Ledgard et al. (1999) reported total denitrification losses from dairy pasture in New Zealand of 3–5 and 10–25 kg N/ha.year, from the application of zero N (grass/clover) and 200 kg N/ha, respectively. Using a similar technique to that used in this study, Ryden (1983) measured annual denitrification losses

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of 11 kg N/ha following the application of 250 kg N/ha in 4 equal split applications. Total gaseous loss Total gaseous losses (volatilisation plus denitrification) from the urea treatments were consistently higher than those from the ammonium nitrate and control treatments during all stages of measurement and seasons (Fig. 5). These differences were most evident in summer, when NH3 volatilisation losses from urea made an overriding contribution to total gaseous loss. As might be expected, there was no net total gaseous loss from the ungrazed control treatments. Although some denitrification was occurring, presumably from residual soil nitrate (Fig. 2) from prior grazing and from legume N2 fixation, the net deposition of NH3 on these treatments resulted in no net gaseous N loss (Table 3, Fig. 5). During the grazing phases, volatilisation and denitrification N losses from the control treatments were higher than those from the ungrazed treatment phases presumably due to urinary deposition. This was particularly evident during the spring period when urinary N deposited on the pasture was likely to be higher than during the rest of the year because of additional N from soil mineralisation and clover growth. Our derived values for annual gaseous loss (volatilisation and denitrification) from the control, ammonium nitrate, and urea treatments of 23, 44, and 68 kg N/ha.year, respectively, are of similar magnitude to those reported by Ledgard et al. (1999) for a New Zealand dairy pasture. They reported losses of 18–22 and 40–70 kg N/ha.year from unfertilised and fertilised (200 kg urea-N/ha) grass/clover pastures, respectively. Economic and environmental impact Although NH3 volatilisation has been of some concern as an economic loss to dairy farmers, its environmental impact in south-eastern Australia has been largely ignored. However, it is likely to be as much an issue in Australia in regions where dairy farming is concentrated, as it is in the United Kingdom and the Netherlands, for example, where NH3 emissions from fertilised pastures of 8–41 kg N/ha have been reported (Jarvis et al. 1989; Bussink 1992). Ammonia has a short lifetime in the atmosphere, being deposited through wet and dry deposition back to the soil or water (Ferm 1998), re-entering the soil–plant N cycle, further acidifying the soil through nitrification, and acting as an indirect source of N2O gas, or NO3 for leaching (Mosier et al. 1998). Nitrous oxide emissions from agricultural systems are of increasing concern due to its global warming potential and effect on the ozone layer. In the current study, total N2O losses, measured in the absence of acetylene (Table 4), showed a notable increase when 200 kg N was applied. Clearly a limitation of the current study is that the weekly sampling regime may have missed some large temporal


variations in denitrification rate and N2O production. Given the high spatial variability in N2O emissions from grazed pasture, these unreplicated results should be treated as indicative only and the data should not be interpolated to provide annual loss estimates. Further research is required to quantify annual N2O emissions from grazed dairy pastures over a range of stocking rates and N regimes, to assess the global warming potential of various N management regimes. Such research should aim to measure actual N2O lost from the intact soil surface, and measurements should be sufficiently continuous to capture the large temporal variation in emissions. Many N balance studies have shown reduced N efficiency with increasing N fertiliser inputs, with N efficiencies ranging from 60% (Aarts et al. 1992; Ledgard et al. 1999; Eckard et al. 2001). At the same time, studies have shown a potential 70% reduction of gaseous N emission from intensively managed dairy farming systems through implementation of a range of best N management practices (Whitehead 1995; Velthof et al. 1998; Eckard et al. 2001). These measures include strategic timing and placement of N fertiliser application, changes to grazing management, use of high energy–low protein supplements, and integration of clover N2 fixation with N fertiliser management. Using ammonium nitrate instead of urea in summer may result in a significant reduction in loss of N by NH3 volatilisation, and urea use in winter may result in lower denitrification loss. However, as the price of ammonium nitrate N is currently 45% more than of urea N there is little economic justification for recommending the use of ammonium nitrate over urea for the other seasons. Another option may be to apply urea to taller pasture at least 3 days before grazing, as this would reduce the effect of wind speed at ground level during hydrolysis (McKenzie and Tainton 1993), with a greater amount of the ammonia trapped in the plant canopy and assimilated by the foliage (Denmead et al. 1976). The efficacy of these and other management options requires further research under local conditions, to develop best management practices to minimise gaseous N losses from N-fertilised dairy pastures. Acknowledgments The authors acknowledge the support of the Victorian Department of Natural Resources and Environment, the University of Melbourne, the Australian Research Council (grant no. C19804739), the Dairy Research and Development Corporation, Pivot Ltd, and Incitec Ltd for financing the study. In particular the authors thank Paul Durling for technical support. References Aarts HFM, Biewinga EE, van Keulen H (1992) Dairy farming systems based on efficient nutrient management. Netherlands Journal of Agricultural Science 40, 285–299.


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ADC (2000) ‘Australian dairy industry in focus 2000.’ pp. 3–5. (Australian Dairy Corporation: Melbourne) AGO (2000) National Greenhouse Gas Inventory: fact sheet 1998 no. 4: agriculture. (The Australian Greenhouse Office: Canberra, ACT) Anon. (1997) ‘Methods list for Quickchem Automated Analyzers.’ p. 24. (La Chat, Zellweger Analytics, Inc.: Milwaukee, WI) Aulakh MS, Doran JW, Mosier AR (1992) Soil denitrification—significance, measurement and effects of management. Advances in Soil Science 18, 1–57. Ball PR, Ryden JC (1984) Nitrogen relationships in intensively managed temperate grasslands. Plant and Soil 76, 23–33. Black AS, Sherlock RR, Smith NP (1987) Effect of timing of simulated rainfall on ammonia volatilization from urea, applied to soil of varying moisture content. Journal of Soil Science 38, 679–687. Bouwman AF (1996) Direct emission of nitrous oxide from agricultural soils. Nutrient Cycling in Agroecosystems 46, 53–70. Bremner JM (1997) Sources of nitrous oxide in soils. Nutrient Cycling in Agroecosystems 49, 7–16. Burford JR, Bremner JM (1975) Relationships between the denitrification capacities of soils and total, water-soluble and readily decomposable soil organic matter. Soil Biology and Biochemistry 7, 389–394. Bussink DW (1992) Ammonia volatilization from grassland receiving nitrogen fertiliser and rotationally grazed by dairy cattle. Fertilizer Research 33, 257–265. Bussink DW (1994) Relationships between ammonia volatilization and nitrogen fertilizer application rate, intake and excretion of herbage nitrogen by cattle on grazed swards. Fertilizer Research 38, 111–121. Denmead OT, Freney JR, Simpson JR (1976) A closed ammonia cycle within a plant canopy. Soil Biology and Biochemistry 8, 161–164. Eckard RJ (1998) A critical review of research on the nitrogen nutrition of dairy pastures in Victoria. pp. 1–31. (ILFR, The University of Melbourne, and Agriculture Victoria, Ellinbank, DNRE) ISBN 0 7311 4270 5. Eckard RJ, Chapman DF, White RE, Smith A, Chen D, Durling PA (2001) Nitrogen balances in high rainfall, temperate dairy pastures of south eastern Australia. In ‘Proceedings of the XIX International Grasslands Congress, São Pedro, Brazil, February 2001’. pp. 169–170. (Brazilian Society of Animal Husbandry: Piracicaba, Brazil) Eckard RJ, Dalley D, Crawford M (2000) Greenhouse gas sources and sinks from dairy production systems in Australia. In ‘Management options for carbon sequestration in forest agricultural and rangeland ecosystems’. (Eds R Keenan, AL Bugg, H Ainslie) pp. 58–72. (Workshop Proceedings CRC for Greenhouse Accounting, ANU: Canberra, ACT) Eckard RJ, Franks DR (1998) Strategic nitrogen fertiliser use on perennial ryegrass and white clover pasture in north-western Tasmania. Australian Journal of Experimental Agriculture 38, 155–160. Eckard RJ, McKenzie FR, Lane PR (1997) The ‘Pendulum Paradigm’—trends in nitrogen fertiliser use on temperate grass/clover pastures. In ‘Proceedings of the XVIII International Grasslands Congress, Canada, June 1997’. pp. 30.3–30.4. (Extension Service, Saskatchewan Agriculture and Food: Saskatoon, Saskatchewan) Eckard RJ, Miles N, Tainton NM (1988) The use of near infra-red reflectance spectroscopy for the determination of plant nitrogen. Journal of the Grassland Society of Southern Africa 5, 175–177. Ferm M (1998) Atmospheric ammonia and ammonium transport in Europe and critical loads—a review. Nutrient Cycling in Agroecosystems 51, 5–17. Genstat 5 Committee (1995) ‘Genstat for Windows reference manual supplement.’ (Numerical Algorithms Group, Oxford Science Publications: Oxford, UK)

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Granli T, Bøckman OC (1994) Nitrous oxide from agriculture. Norwegian Journal of Agricultural Science Suppl. No. 12, 1–128. Harper LA, Catchpoole VR, Vallis I (1983) Ammonia loss from fertilizer applied to tropical pastures. In ‘Gaseous loss of nitrogen from plant–soil systems’. (Eds JR Freney, JR Simpson) pp. 195–214. (Martinus Nijhoff/Dr W Junk Publishers: The Hague, The Netherlands) IPCC (1996) ‘Revised 1996 IPCC guidelines for National Greenhouse Gas Inventories; Vol. 1, Greenhouse Gas Inventory reporting instructions.’ (OECD, IPCC, IEA: Geneva) Isbell RF (1996) ‘Australian soil and land survey handbook: the Australian Soil Classification.’ (CSIRO Publishing: Melbourne) Jarvis SC, Hatch DJ, Roberts DH (1989) The effects of grassland management on nitrogen losses from grazed swards through ammonia volatilization: the relationship to excretal N returns from cattle. Journal of Agricultural Science 112, 205–216. Jarvis SC, Scholefield D, Pain B (1995) Nitrogen cycling in grazing systems. In ‘Nitrogen fertilization in the environment’. (Ed. PE Bacon) pp. 381–419. (Marcel Dekker, Inc.: New York) Ledgard SF, Penno JW, Sprosen MS (1999) Nitrogen inputs and losses from clover/grass pastures grazed by dairy cows, as affected by nitrogen fertilizer application. Journal of Agricultural Science 132, 215–225. Leuning R, Freney JR, Denmead OT, Simpson JR (1985) A sampler for measuring atmospheric ammonia flux. Atmosphere and Environment 19, 1117–1124. McKenzie FR, Tainton NM (1993) Pattern of volatilized nitrogen loss from dryland kikuyu pastures after fertilization. African Journal of Range and Forage Science 10, 86–91. Meyer RD, Olson RA, Rhoades HF (1961) Ammonia losses from fertilized Nebraska soils. Agronomy Journal 53, 241–244. Mosier A, Kroeze C, Nevison C, Oenema O, Seitzinger S, van Cleemput O (1998) Closing the global N2O budget: nitrous oxide emissions through the agricultural nitrogen cycle. Nutrient Cycling in Agroecosystems 52, 225–248. Packrou N, Dillon P, Stanger G (1997) Impact of pastoral land use on groundwater quality. Final Report LWRRDC Project No. CCW9, Water Industry Research Award. Centre for Groundwater Studies Report no. 64, South Australia. Prasertsak P, Freney JR, Denmead OT, Saffigna PG, Prove BG (2001) Significance of gaseous nitrogen loss from a tropical dairy pasture fertilized with urea. Australian Journal of Experimental Agriculture 41, 625–632.

Ruz-Jerez BE, White RE, Ball PR (1994) Long-term measurement of denitrification in three contrasting pastures grazed by sheep. Soil Biology and Biochemistry 26, 29–39. Ryden JC (1983) Denitrification loss from a grassland soil in the field receiving different rates of nitrogen as ammonium nitrate. Journal of Soil Science 34, 355– 365. Ryden JC (1986) Gaseous losses of nitrogen from grassland. In ‘Nitrogen fluxes in intensive grassland systems’. (Eds HG van der Meer, JC Ryden, GC Ennik) (Martinus Nijhoff Publishers: Dordrecht, The Netherlands) Ryden JC, Skinner JH, Nixon DJ (1987) Soil core incubation system for the field measurement of denitrification using acetylene inhibition. Soil Biology and Biochemistry 19, 753–757. Schenk JS, Westerhaus MO (1991) Population definition, samples selection and calibration procedures for near infrared reflectance spectroscopy. Crop Science 31, 469–474. Schjøerring JK, Sommer SG, Ferm M (1992) A simple passive sampler for measuring ammonia emission in the field. Water, Air, and Soil Pollution 62, 13–24. Smith M (1992) Expert consultation on revision of FAO methodologies for crop water requirements. (Land and Water Development Division, FAO: Rome) Stace HCT, Hubble GD, Brewer R, Northcote KH, Sleeman JR, Mulcahy MJ, Hallsworth EG (1968) ‘A handbook of Australian soils.’ (Rellim Technical Publications: Glenside, S. Aust.) Vallis I, Harper LA, Catchpoole VR, Weier KL (1982) Volatilization of ammonia from urine patches in a subtropical pasture. Australian Journal of Agricultural Research 33, 97–107. Velthof GL, van Beusichem ML, Oenema O (1998) Mitigation of nitrous oxide emission from dairy farming systems. Environmental Pollution 102, 173–178. Walkely A, Black IA (1934) An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Science 37, 29–38. Whitehead DC (1995) ‘Grassland nitrogen.’ (CAB International: Wallingford, UK) Whitehead DC, Raistrick N (1990) Ammonia volatilisation from five nitrogen compounds used as fertilisers following surface application to soils. Journal of Soil Science 41, 387–394.

Manuscript received 13 June 2002, accepted 17 March 2003

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