Abstract

Key Words

Dairy-grazed pastures, Excretal inputs, IPCC-approach, Nitrous oxide, NZ-DNDC model, sheep-grazed pastures, temperate grasslands

Introduction

Nitrous oxide contributes about 16% (on a CO₂-equivalent basis) of New Zealand’s greenhouse gas emissions. The nitrous oxide (N₂O) emissions resulted from excretion by sheep and cattle onto grazed pastures which provides high localised concentrations of available N and carbon (C) in soils, and is the main source of anthropogenic N₂O emissions from New Zealand, contributing to about 85% of total N₂O emissions (Cameron et al. 2000).

Under UNFCCC (United Nations Framework Convention on Climate Change), New Zealand is required to produce an annual inventory of N₂O emission from all anthropogenic sources to assess the magnitude and change in total emissions since 1990. New Zealand ratified the Kyoto Protocol in December 2002 and this agreement has its own set of requirements, including the continuation of annual greenhouse gas reporting. New Zealand currently relies on the IPCC default methodology (National Inventory Report New Zealand 2004), and on animal population statistics for each region to estimate its N₂O emission inventory. Direct and indirect emissions from animal excreta (dung and urine) are estimated using N excreted by each animal type. IPCC default methodology is only a first approximation, because of i) uncertainty in emission factors, ii) uncertainty in indirect emissions, iii) limited data on the type and amount of N excreted by grazing animals, and iv) the spatial and temporal variability of N₂O emission. The methodology therefore appears to be too simplistic and generalised, ignoring all site-specific controls. It is also not sufficiently flexible to allow the assessment of mitigation options. Current emission rates from excretal input and from different soils in New Zealand have an uncertainty of ±65% (Sherlock et al. 2001), which must be reduced if changes since 1990 are to be reported internationally.

Accordingly, a more robust, process-based approach is required that is internationally acceptable and quantifies N₂O emissions at the field level more accurately than the IPCC methodology. Such an approach is needed to develop regional- and national-scale inventories with known levels of uncertainties. DNDC (DeNitrification DeComposition; Li et al. 1992) is a process-based model with reasonable data requirements that has been used to produce regional estimates for the US (Li et al. 1996), China (Li et al. 2001), Australia (Wang et al. 1997), Germany (Butterbach-Bahl et al. 2001), Canada (Smith et al. 2002), and the UK (Brown et al. 2001, 2002). Testing, modifications and use of DNDC have involved international researchers that have significantly advanced the development and applications of this model. DNDC has been validated against numerous datasets observed worldwide including emissions of CO₂, N₂O, (Figure 1) and CH₄. Although the model is suitable for simulation at appropriate temporal and spatial scales there were particular limitations in applying this model directly to New Zealand soils, which are distinctive and diverse within short distances, and have higher organic carbon contents than the world average (Saggar 2001). The New Zealand grazed pastoral system (grazing 24 hours a day) and climatic conditions are also different from other countries. The DNDC model has, therefore, been modified to represent New Zealand grazed pastoral systems (Saggar et al. 2002, 2004).

Figure 1. Comparison of modeled and observed N₂O fluxes from agriculture sites in the US, Canada, the UK, Germany, China, New Zealand, Costa Rica and Zimbabwe. Produced from Li and Saggar (2004).

Here we: i) present seasonal variations in N₂O emissions from two dairy pastures with contrasting soils and from a sheep pasture; ii) assess the ability of a modified DNDC model “NZ-DNDC” to simulate these emissions; and iii) compare the N₂O emissions from dairy- and sheep-grazed pastures using an empirical methodology recommended by the IPCC, currently used in preparing New Zealand’s annual N₂O emission inventory from grazed pastures.

Materials and methods

Experimental sites

The experimental sites were at Massey University, Turitea campus (40° 21' S, 175° 39' E). Average rainfall (1970–2000) at these sites is about 965 mm, which is fairly evenly distributed throughout the year, with driest months being January–March. The mean annual sunshine is ~1900 h. The mean annual air temperature is 12.8°C, and the coldest and warmest months are July (6.8°C) and January (18.1°C). Soil temperature and rainfall data during the study period are illustrated in Figure 2.

The three sites are part of Massey University’s dairy- and sheep-farm systems, which represent highly productive (~16 Mg DM ha^-1 y^-1) legume-based pastures on different soil types (Weathered Fluvial Recent, Karapoti fine sandy loam, Dairy 1, and Argillic-fragic Perch-gley Pallic, Tokomaru silt loam, Dairy 3). These sites were selected because their soils provide a contrast in texture and hydraulic properties. The pastures are regularly fertilised with N and P, and are 3 km apart. Dairy 1, Dairy 3 and sheep farms received annual urea applications amounting to 130, 190 and 80 kg N ha^-1. Dairy-pastures were rotationally grazed for periods of 12 to 48 hours at a time. Intervals between grazing varied from 2 to 6 weeks, depending on herbage accumulation. Sheep were set-stocked with ewes and lambs (15-22 stock units ha^-1), which resulted in daily excretal inputs. A representative area of each farm (5 × 5 m) was fenced to exclude grazing, and excretal and fertiliser inputs, and was used as a control. The control areas were mown to 20 mm height at each grazing event, with clippings removed.

Figure 2. Distribution of daily soil temperature and monthly rainfall during the study period and 30-year mean monthly rainfall.

Nitrous oxide measurements

Nitrous oxide measurements were made periodically between April 2001 and February 2002 from the grazed and ungrazed areas at each site. To account for the spatial variability, 18 replicate chambers were randomly located c. 20 m apart along a Z-shaped transect to measure the fluxes of N₂O from the grazed area (~1 ha); two chambers were located in the ungrazed area (~0.005 ha). To protect the chambers from damage, they were removed from the sites while stock were present. Full descriptions of the chambers, collection and analyses of gas samples, calculation of N₂O flux, and soil sampling and analyses are presented elsewhere (Saggar et al. 2002, 2003, 2004).

Environmental variables and soil water content

At the Tokomaru site, an automated meteorological station was set up to monitor daily rainfall, wind velocity, soil temperature and volumetric soil water content (SWC), which was monitored using calibrated Delta-T theta probes (MLZx) at depths of 20, 50, 100 and 300 mm. For the Karapoti site, data were taken from an existing automated meteorological station 2 km away; gravimetric SWC was measured at 0–50 and 50–100 mm depths, only on gas sampling days. Field-moist soil samples were weighed, oven-dried (105º C) to constant mass, and weighed again. The final mass M_s, and the difference between the field-moist and dry masses M_w were used to calculate the gravimetric SWC = (M_w/M_s)×100. The volumetric SWC was then calculated by multiplying the gravimetric SWC with the soil bulk density (Saggar et al. 2004). Soil temperature was measured at both sites with a thermocouple at 20 mm depth at 30-min intervals. Water-filled pore space (WFPS) was calculated as the ratio of the volumetric SWC to the total pore space (Saggar et al. 2002, 2004).

Modelling

DNDC was modified to better represent New Zealand’s grazed pasture systems. Modifications were made to version 6.7 of the model, the most recent version at the time the research was begun [a newer version has subsequently been released that may address some of the deficiencies found in version 6.7]. Details of the modifications are given in Saggar et al. (2004). The modified model, hereafter named “NZ-DNDC”, was then used to simulate N₂O emissions from the pastures grazed by dairy cattle.

Results And Discussion

Seasonality of N₂O fluxes

Figures 3a and 3b show measured N₂O emissions for each site. Large spatial and temporal variations were observed in the N₂O fluxes measured from the grazed area in both the pastures studied (Figs 3a, b). Large fluxes were generally observed after each grazing and rainfall event, and were followed by a decline. The spatial variations in N₂O fluxes observed for the grazed sites throughout the year were large (Figs 3a, b), with coefficient of variation values ranging between 56 and 262%. The N₂O fluxes and their spatial variability at both ungrazed sites were low, with coefficient of variation values ranging between 35 and 59%. Spatial variability in N₂O emissions is naturally large in most soils (Folorunso and Rolston 1984; Choudhary et al. 2002) because of soil heterogeneity and the episodic nature of N₂O emissions, and variability increases as a result of animal grazing and unevenly distributed excretal returns (Carran et al. 1995; Saggar et al. 2002, 2004).

(A)

(B)

Figure 3. Measured means and ranges of nitrous oxide emissions from grazed and ungrazed pastures from (A) Karapoti and (B) Tokomaru soils.

In both soils, the N₂O emissions for the grazed areas in winter were higher than those in spring. The emissions in autumn were lower than in spring by about one-third. However, highest emissions (almost 5 to 10 times of those in winter) were measured in both soils for about a week after a heavy rainfall in January-02 following a grazing event. The emissions dropped rapidly to the levels observed in autumn as the soils dried out. These highest summer N₂O fluxes could be attributed to a combination of high WFPS and high soil temperature (~20°C) and mineral-N. Significant N₂O fluxes can occur at high temperatures, our temperature response being similar to that of Dobbie and Smith (2001). The high N₂O fluxes observed during winter could have been due to a combination of WFPS exceeding 0.60, a high soil mineral-N content and low plant uptake of N. The low rainfall in autumn resulted in low soil water content (WFPS <0.60), and the lowest N₂O emissions. The spring season was characterised by significant rainfall events, rapidly fluctuating soil water content (WFPS between 0.40 and 0.95), mild temperatures, and increased plant growth that resulted in medium levels of N₂O emissions. This seasonal pattern of N₂O fluxes is consistent with the data of Ruz-Jerez et al. (1994) and Carran et al. (1995). The lowest emissions were obtained during the periods when WFPS was <0.60.

Overall, N₂O emissions were slightly lower in the well-drained fine sandy loam soil than in the poorly drained silt loam soil. The emissions from the ungrazed pastures were <20% of those from the grazed pastures. These results suggest that in grazed pastures it is the animal excreta deposited in the form of dung and urine that provide high concentrations of available N and C, and are the principal source for N₂O production.

Effect of SWC, mineral N and extractable C

The SWC was expressed as WFPS, which normalises for differences in bulk density and particle density between soils. Our results show that, of the measured variables, WFPS was the most strongly influential on N₂O fluxes at both grazed sites. Generally, N₂O emissions at the grazed sites were high in both soils when the WFPS was above ‘field capacity’ (Saggar et al. 2002, 2003), indicating that formation of anaerobic sites following rainfall, a fundamental requisite for denitrification, was mainly responsible for these high N₂O fluxes. The field-capacity WFPS values appear to be the critical levels above which conditions were sufficiently anaerobic to enhance N₂O emissions significantly in the grazed sites. Although both nitrification and denitrification may have contributed to the emissions, the very high fluxes associated with WFPS values, >0.60, were more likely to have come mainly from denitrification. Davidson (1991) showed that nitrification was the dominant source of N₂O when WFPS was <0.60 and denitrification was the predominant source at WFPS >0.60. Dobbie and Smith (2001) suggested the exponential increase in N₂O flux with WFPS above 0.60 pointed to denitrification being responsible for N₂O production in Scottish soils. Further indication of the importance of WFPS in controlling denitrification, and hence N₂O emission, is the abrupt increase in N₂O fluxes when the WFPS increased above 0.66 or 0.62 after a significant rainfall event (data not reported).

Changes in WFPS above field capacity had no discernible effect on emissions at the ungrazed sites, where NO₃^- and extractable C levels were very low (Saggar et al. 2004), indicating the emissions also depended on the supply of mineral N and soluble C substrate from animal excreta. These results show N₂O emissions at the ungrazed sites are obviously limited by the lack of substrate.

Effect of rainfall

Our measurements show that N₂O emissions change with changes in soil moisture resulting from rainfall. Rainfall increases soil moisture and forms anaerobic sites, which result in denitrification and increased emissions. Therefore, annual emissions would be different in El-Nino and La-Nina years. The NZ-DNDC simulations (Figure 4) suggest that the model does account for these climatic variations in rainfall.

Figure 4. Effect of the amount of rainfall on nitrous oxide emissions.

Effect of grazing regimes

Pastures are grazed either by set-stocking or rotationally, and the number of grazing events varies according to pasture growth rates and farm management. Pasture growth varies with season, being highest in spring and lowest in winter (Saggar & Hedley 2001). Although total excretal inputs are the same in both grazing regimes, the intensity of input varies, which may result in different N₂O emission rates. Set-stocked pastures receive very small daily excretal inputs, compared with the rotationally grazed pastures where the inputs depend on the number of grazing events. Our model simulations suggest that with increased grazing events the amount of N₂O emitted decreases (Figure 5). The rate of decrease in emission also varies with region. The IPCC approach is independent of grazing events because it uses a constant emission factor, which accounts for only the quantity of excretal input, but not the frequency.

Figure 5. Simulated nitrous oxide emissions as influenced by grazing regimes.

Comparisons of modelled with measured emissions

NZ-DNDC simulated well the average daily N₂O fluxes from the control and grazed plots (Figures 6a, b). However, it slightly overestimated spring (October) emissions. It also underestimated the very high emissions observed at both the grazed sites in summer (January) due to high temperature and moisture after a rainfall event; however, it simulated well the following low emissions that dropped quickly as the soils dried out.

(A)

(B)

Figure 6. Comparison of modelled and measured nitrous oxide emissions from (a) Karapoti and (b) Tokomaru soils.

The modified NZ-DNDC model was very well able to predict the annual measured emissions from both the grazed and ungrazed farms. NZ-DNDC estimated an annual net emission of 2.51 and 1.88 kg N₂O-N ha^-1 (ungrazed areas), and 10.41 and 12.40 kg N₂O-N ha^-1 (grazed areas) from the well-drained and poorly drained soils, respectively. Grazing by dairy cattle markedly increased total N₂O emissions in both soils. The excretal plus fertiliser N inputs in the Karapoti and Tokomaru grazed pastures were 396 and 345 kg N ha^-1, respectively. Annual emissions of N₂O-N were 1.99% of the excretal and fertiliser N in the well-drained soil, and 2.53% in the poorly drained soil. NZ-DNDC annual-emission estimates for both farms were within 10% of the measured values intrapolated over the year, and within their uncertainty range.

Figure 7 compares the measured annual emissions from two dairy farms and a sheep farm with emissions estimated using the IPCC approach and NZ-DNDC model. We observed very significant differences in emission rates between sheep-grazed and dairy pasture. Estimates based on New Zealand refined IPCC methodology (National Inventory Report New Zealand 2004), for the two dairy-grazed pastures, were about 25 to 60% lower than our modelled and measured values. Modelled and IPCC estimates were, however, the same for the sheep-grazed pasture. Our measured values for the sheep-grazed pasture were slightly lower than those predicted by the model, possibly because no measurements were made during a high-rainfall December period. It was also not possible to shift chambers daily to capture daily excretal inputs. We have now modified our chambers to allow sheep to graze over the chambers, and so capture daily excretal inputs.

Figure 7. Annual-measured, NZ-DNDC-predicted and IPCC-calculated nitrous oxide emissions from two dairy-grazed sites and one sheep-grazed site.

Both the dairy farms had similar grazing management, pasture production and rainfall, but differed in soil texture. The measured emissions were about 20% higher in the poorly drained Tokomaru silt loam soil than in well-drained Karapoti fine sandy loam soil. The NZ-DNDC model was able to pick up these differences in emissions resulting from differences in soil texture, the IPCC methodology cannot account for such influences.

Conclusions

The overall comparisons of predicted and measured annual emissions (Figure 7) indicate NZ-DNDC should be applicable to the simulation of N₂O emissions from New Zealand grazed pastures. More testing is now needed on a range of different soils, and with sheep as well as other cattle-grazed pastoral systems. Our ultimate goal is to be able to estimate emissions accurately on a regional and national scale based on available climatic data, soil types, numbers and types of grazing animals and their excretal N inputs.

NZ-DNDC offers a solid beginning for this goal and a base for future development.

We are currently using available climate, soil types, and number of and types of animals and their excretal N input information to estimate regional scale N₂O emissions. Our current research also focuses on using NZ-DNDC to monitor the efficacy of mitigating fertiliser- and urine-induced N₂O emissions from grazed pasture systems, using nitrification inhibitors, and on developing best management practices for efficient effluent application to reduce N₂O emissions.

Acknowledgements

We thank Massey University for access to dairy farms and farm management information, Natasha Rodda and Jacquiline Townsend for technical assistance, Robbie Andrew for model simulations, Drs Dave Scotter for assistance in soil moisture parameters and Des Ross for comments. The New Zealand Foundation for Research Science and Technology funded this research.

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