DETERMINANTS OF NITROUS OXIDE EMISSION FROM AGRICULTURAL DRAINAGE WATERS D. S. REAY1,∗ , A. C. EDWARDS2 and K. A. SMITH1 1
School of GeoSciences, University of Edinburgh, Darwin Building, Mayfield Road, Edinburgh, EH9 3JU, UK; 2 Macaulay Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK
(*author for correspondence, e-mail:
[email protected]; phone: +44-131-6507723; fax: +44-131-6620478)
Abstract. Emissions of the powerful greenhouse gas nitrous oxide (N2 O) from agricultural drainage waters are poorly quantified and its determinants are not fully understood. Nitrous oxide formation in agricultural soils is known to increase in response to N fertiliser application, but the response of N2 O in field drainage waters is unknown. This investigation combined an intensive study of the direct flux of N2 O from the surface of a fertilised barley field with measurement of dissolved N2 O and nitrate (NO3 ) concentrations in the same field’s drainage waters. Dissolved N2 O in drainage waters showed a clear response to field N fertilisation, following an identical pattern to direct N2 O flux from the field surface. The range in N2 O concentrations between individual field drains sampled on the same day was large, indicating considerable spatial variability exists at the farm scale. A consistent pattern of very rapid outgassing of the dissolved N2 O in open drainage ditches was accentuated at a weir, where increased turbulence led to a clear drop in dissolved N2 O concentration. This study underlines the need for carefully planned sampling campaigns wherever whole farm or catchment N2 O emission budgets are attempted. It adds weight to the argument for the downward revision of the IPCC emission factor (EF5 -g) for NO3 in drainage waters. Keywords: denitrification, emission factor, greenhouse gas, leaching, nitrification
1. Introduction Agricultural soils are a major global source of N2 O, currently estimated to be of the order of 4 Tg N per year (Mosier et al., 1998). Formation of N2 O in soil is primarily caused by microbial nitrification and denitrification, and depends mainly on N concentration, soil water content and temperature (Sahrawat and Keeney, 1986; Skiba et al., 1998; Smith et al., 1998). An aspect of modern farming systems that stimulates N2 O production and emission is the application of N in fertilisers and manures. When high rates of N2 O formation in soils coincide with soil water contents above field capacity, substantial amounts of N2 O may enter field drainage systems (e.g. Dowdell et al., 1979; Harrison and Matson, 2003). Additionally, dissolved NO3 may provide in the drainage water an additional source of N2 O through denitrification in down-stream aquatic sediments (Seitzinger and Kroeze, 1998). Water, Air, and Soil Pollution: Focus 4: 107–115, 2004. C 2004 Kluwer Academic Publishers. Printed in the Netherlands.
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Several recent studies have reported supersaturation of N2 O in agricultural drainage waters, either in the waters of discrete field drains, open drainage ditches, or in underlying aquifers (e.g. Hasegawa et al., 2000; Hack and Kaupenjohann, 2002; Hiscock et al., 2002). Other studies have examined the direct flux of N2 O from agricultural soils, particularly in response to N fertiliser application (e.g. Bouwman, 1996; H´enault et al., 1998; MacKenzie et al., 1998). However, the relationship between N2 O concentration in drainage waters and N2 O formation in soil, immediately following fertiliser N application, has not been fully examined for the intensive agricultural systems common to parts of Europe. Reay et al. (2004) reported the apparently poor relationship between the amount of N fertiliser applied to an intensively farmed area of Midlothian, and the subsequent changes in dissolved NO3 and N2 O in the field drainage water. However, this earlier study did not include measurement of direct N2 O flux from the field surface. We have now measured simultaneously direct N2 O emissions from a field receiving N fertiliser, and concentrations of dissolved N2 O in the field drain serving the same field. Additionally, we have examined the spatial variation in dissolved N2 O across the whole farm drainage network and its subsequent outgassing in the open ditches.
2. Methods 2.1. STUDY
SITE
The study site (Figure 1) comprised a whole-farm drainage catchment located at the headwater of a tributary of the Ythan river catchment, Aberdeenshire, UK. The farm is a mixed arable/grassland system typical of those in the region (Domburg et al., 2000), and most fields receive significant annual N fertiliser applications (>100 kg N). Previous surveys of the study site have identified the network of field drains and drainage ditches serving the farm. A detailed survey of dissolved N2 O concentration in the farm drainage waters was carried out, and direct and indirect emissions of N2 O from a field planted with winter barley were measured. This field, marked in Figure 1, was bordered by the main ditch draining the farm, and received an application of 250 kg ha−1 (NH4 )2 SO4 (53 kg N ha−1 ) on 25 March 2003. 2.2. FIELD
FLUX MEASUREMENTS
Six static chambers (40 cm diameter) were situated randomly and gas samples were taken at regular intervals, before and, for a total period of 4 weeks after N fertilisation, starting on 24 March 2003. Methods of chamber installation and gas sampling were identical to those described by Smith et al. (1995), with chamber closure periods of approximately 45 min.
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Figure 1. Site map of the farm study site. Sample points in the transect of dissolved N2 O concentrations are marked with their individual site numbers (see Table I for details). The marked winter barley field (∼6 ha) was used for measurement of direct and indirect N2 O flux following field fertilisation.
2.3. WATER
SAMPLING
Drainage water was collected at all the identified field drain outfalls on the farm, and at regular intervals (∼10 m) along the open ditches, downstream from the outfalls, to examine the loss of dissolved N2 O to the atmosphere. The water emerging from the field drain serving the winter barley field was sampled at the same time as samples were collected from the gas flux chambers located on the field surface. Drainage water samples (250 mL) were collected as described in Reay et al. (2003). Water samples were taken to the Macaulay Institute in Aberdeen within 2 h, where triplicate 5-mL sub-samples were placed in 22-mL glass vials with butyl rubber seals. All water samples were preserved with mercuric chloride and stored at 4◦ C until analysis within 1 month of collection. 2.4. A NALYSIS Nitrous oxide was analysed by gas chromatography using an Agilent 6890 GC fitted with a 1.8-m Hayesep Q column and electron capture detector. Water samples were analysed for dissolved NO3 − and ammonium (NH4 + ) by colorimetry, using a Skalar auto analyser. Concentrations of dissolved N2 O in water samples were calculated by analysis of N2 O in the vial headspace (Reay et al., 2003). The relationship
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TABLE I Sample site characteristics
Site number
Type
Dissolved N2 O concentration (µg N L−1 )
Flow rate (L s−1 )
N2 O throughput (µg N s−1 )
24 23 22
Field drain Field drain Field drain
28.2 19.2 76.7
0.05 1.33 0.05
1.4 25.6 3.8
15 13 12 11
Field drain Field drain Field drain (Barley field) Field drain
30.7 17.0 8.1 36.6
0.10 0.25 1.00 1.33
3.1 4.3 8.1 48.7
8
Field drain
46.9
1.00
46.9
7 6
Open farmyard ditch Open ditch
8.5 3.8
1.50 7.50
12.7 28.5
3
Weir
4.0
7.50
30
Water samples taken on 29th August 2002. Standard errors for dissolved N2 O concentrations were 200 times that in equilibrium with air. However, the low flow volumes of the field drainage at sites 24 and 22 meant that the actual throughput of N2 O was relatively small. A majority of the field drains entered the main drainage ditch close to the farmyard (Figure 1). Again, dissolved N2 O concentrations in these field drains were often many times greater than that at air equilibrium. 3.2. N 2 O
LOSS IN OPEN DITCHES
On entry to the open ditches, concentrations of dissolved N2 O decreased markedly (Table I). The significant decline in concentration ( p < 0.01) in the open ditch sampling sites (6 and 7) was primarily due to dilution of the field drain water by that in the larger open ditch. At some sites a contributing factor may also have been
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Figure 2. Profile of dissolved N2 O concentration along a transect of the open drainage ditches comprising: (a) Open ditch downstream of the entry of field drain at site 13 and (b) Open drainage ditch serving farmyard and downstream of the entry of field drain site 8. Entry points of individual field drains are marked. Error bars represent standard error (n = 3). Note differences in scales for X and Y axes.
rapid outgassing of dissolved N2 O due to the fall between field drain outfall and ditch bottom (up to 30 cm), and the resulting high degree of water turbulence. Figure 2 shows the downstream profile of dissolved N2 O in the open drainage ditch waters of the farm. These drainage waters can carry high concentrations of both dissolved N2 O and NO3 , with implications for the overall greenhouse gas budget of the farm. A consistent loss of dissolved N2 O was evident (Figure 2a) downstream from the field drain at site 13, with this trend being even more distinct in the open drainage ditch serving the farmyard (Figure 2b). Outgassing of dissolved N2 O from the water, downstream from site 8, was very rapid. The initially high dissolved N2 O concentration in the ditch water immediately below the entry of the field drain being reduced by more than two thirds within 100 m. A comparison of the total throughput of dissolved N2 O from all the site field drains (total of 142 µg N2 O–N sec−1 , Table I), to that passing along the open ditch at site 6 (28.5 µg N2 O–N sec−1 ), revealed an overall loss of 80% of the dissolved N2 O loading between the field drain outfalls and site 6. Such a trend of N2 O loss in open drainage ditches has been reported previously for a comparable site in Midlothian (Reay et al. 2003). The data presented here further underline the rapid N2 O loss in these systems; this should be considered when attempting to quantify overall N2 O emissions at farm or catchment scale. More outgassing was also observed as the drainage waters passed over a weir (38 cm drop) at the very bottom of the farm drainage catchment (Site 3, Figure 1). A
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Figure 3. (a) Time course of direct N2 O flux from winter barley field and dissolved N2 O concentration in field drain (no. 12) serving the same field, following field fertilisation (250 kg (NH4 )2 SO4 ha−1 , equivalent to 60 kg N ha−1 ). Error bars for field fluxes represent standard error (n = 6), standard errors for dissolved N2 O (n = 3) smaller than symbols. (b) Time course of dissolved NO3 − and NH4 + in field drain (no. 12) serving the winter barley field. Chamber sampling began at 10.30 hrs on 25 February 2003, with fertiliser application at 14.30 hrs on the same day.
loss of around 20% of the dissolved N2 O concentration took place here, indicating the influence that water turbulence has on the rate of N2 O loss from surface waters. 3.3. N
FERTILISER APPLICATION
The spring application of N fertiliser to the barley field resulted in a relatively small, direct N2 O flux from the soil surface (Figure 3a). However, the dry conditions preceding this application (no rain for the week prior to application) are likely to have resulted in less favourable conditions for denitrification, and so also for appreciable N2 O formation. Given that the N was applied as ammonium sulphate
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(equivalent to 53 kg N ha−1 ) it is likely that nitrification was a significant source of N2 O production in this soil during the study period. Though a small response in field surface N2 O emissions was measured within the first 24 h post-N application, the largest and most sustained response was seen between about 5 and 15 days after application. However, due to high spatial variability in chamber N2 O flux measurements no significant changes ( p > 0.4) in mean N2 O fluxes were detected. The greatest spatial variation was seen at around 15 days after application, values ranging from 80 g N ha−1 day−1 across the six flux chambers. After this time emission rates fell to pre-N application levels and remained relatively low until the end of the study period. The 5-day lag between N application and the peak in field N2 O emission is not uncommon for such agricultural soils (e.g. Dobbie and Smith, 2003), and reflects the time required for the applied N to initiate elevated nitrification and/or denitrification rates in the soil. The peak also coincided with some limited rainfall (1.5, 4 and 7 mm on the fourth, fifth and sixth days following fertilisation, respectively), which could have increased both the availability of the applied N and promoted N2 O formation via denitrification. Figures 3a and 3b show the dissolved N2 O and inorganic N concentrations measured in the field drain serving this barley field. Concentrations of dissolved N2 O showed a clear and significant increase ( p < 0.01) following N application and increased production of N2 O in the barley field soil. Concentrations more than quadrupled within the 5 days following field fertilisation, reaching a peak concentration of 5 µg N L−1 and coinciding almost exactly with the peak in direct N2 O emissions from the field surface. Similarly, a rise in concentration of dissolved NH4 + , and to a lesser extent NO3 − , was also observed approximately 5 days after fertiliser application, though these changes were not statistically significant. The apparent rise in dissolved inorganic N concentration occurred against a background of steadily decreasing NO3 − and NH4 + concentrations during the course of the study. The positive relationship between N2 O emitted from the field surface and N2 O in drainage water was highly significant ( p < 0.001) over the period of field fertilisation. Following N application, N2 O emissions from the field were equivalent to 1300 g N from the field as a whole (220 g ha−1 ) over the study period, while the indirect loss of N2 O via the field drainage waters accounted for just 9 g of N2 O–N (
