CSIRO PUBLISHING Soil Research, 2014, 52, 833–840 http://dx.doi.org/10.1071/SR13311
Quantification of NOx and NH3 emissions from two sugarcane fields Bennett C. T. Macdonald A,C, O. Tom Denmead A, and Ian White B A
CSIRO Land and Water, Canberra, Australia. The Fenner School of Environment and Society, Australian National University, Canberra, Australia. C Corresponding author. Email:
[email protected] B
Abstract. This paper reports emissions of NOx and NH3 from a rain-fed, fertilised, residue-blanketed sugarcane field at Mackay, Queensland. Emissions were measured using a micrometeorological flux-gradient technique for the whole of the 2006–07 season and for the first 2 months of the 2007–08 season. Nitrogen (N) fertiliser was applied as urea at a rate of 150 kg N ha–1 into slits 100–150 mm deep. Previous work at the site found that N2O emissions accounted for ~5 kg N ha–1, or 3% of the applied N in the 2006–07 season. In the present study, NOx and NH3 were emitted in both the 2006–07 and 2007–08 seasons and accounted for ~1.5 kg N ha–1, or ~1% of applied N. The main driver of NOx emissions appeared to be the availability of a soil mineral N source. However, the maximum N2O and NOx fluxes were offset by nearly 20 days, which indicated different emission pathways. After the soil mineral N was exhausted, the emissions of NOx were reduced. Emissions of NH3 continued at around the same rate for the whole of the growing season. Water-filled pore space, which was a main driver of N2O emissions, did not seem to influence the measured emissions of NOx or NH3. Additional keywords: agricultural emissions, ammonia, nitrous oxide, nitrogen oxides. Received 25 October 2013, accepted 11 August 2014, published online 12 November 2014
Introduction Emissions of nitrogen (N) gases from agriculture to the atmosphere represent a loss of N and have important environmental and economic effects (Galloway 2003). The gases of concern are the direct greenhouse gas nitrous oxide (N2O) and the indirect greenhouse gases ammonia (NH3), nitric oxide (NO) and nitrogen dioxide (NO2), the last two together known as NOx. Over a 100-year period, NH3 and NOx have a global warming potential comparable to methane (Aneja et al. 2009), that is, ~21 times that of carbon dioxide (CO2). In addition, 1% of the NH3 and the NOx emitted to the atmosphere, once deposited upon landscape, will be converted to N2O (Eggleston et al. 2006). Nitrous oxide is a powerful greenhouse gas with a global warming potential 310 times that of CO2. Further, deposition of gaseous NH3 and NOx and aerosol particles over the landscape can modify landscape N dynamics (Vitousek et al. 1997). Research over the last two decades has shown that substantial emissions of N gases, principally N2O, N2, NOx and NH3, can occur from sugarcane soils to the atmosphere (Freney et al. 1992; Matson et al. 1996; Weier et al. 1996, 1998; Weier 1998; Denmead et al. 2010). Denmead et al. (2010) made the first report of N2O emissions for sugarcane crops on two different soils at Murwillumbah, New South Wales (acid sulfate soil), and Mackay, Queensland (alluvial soils), Australia. The difference between the fluxes at each site was large, 45.9 kg N2O-N ha–1 year–1on the acid sulfate soil site and 4.7 kg N2O-N ha–1 year–1 on the alluvial soils. Intensive (30-min time-step) Journal compilation CSIRO 2014
seasonal measurements of NOx and NH3 are not available from sugarcane production. Early research into gaseous emissions of N in the Australian sugarcane industry showed that the annual emissions of NH3-N from surface applications of urea to plant-residue-covered or ‘trash-blanketed’ soils could be as much as 40% of the N applied (Freney et al. 1992). Current practices that bury the fertiliser in the soil have been shown to reduce NH3 losses substantially (Prasertsak et al. 2002) from 2.2 to 0.31 kg NH3-N ha–1 day–1 over a 27-day period. Matson et al. (1996) reported NO-N losses of 0.0072 kg NO-N ha–1day–1 from tropical sugarcane soils fertilised with urea. Macdonald et al. (2011) reported a shortterm study of NOx losses from sugarcane production on acid sulfate soils, in which the fertiliser was placed into slots. The soil water content was an important driver of the NOx emission. When the soil was wet (13 days), 2.46 kg NOx-N ha–1day–1 was emitted, and when the soil was dry (10 days), only 0.025 kg NOx-N ha–1day–1 was lost. It is clear from these studies that significant losses of N via NH3 and NOx could be occurring in tropical sugarcane agricultural systems. A limitation of all of these studies is that they have been short term, sometimes conducted on essentially bare soils, and the methodological approach may not have captured the entire emission and the peak rate. These early studies were short term because the aim was to measure emissions of NH3 from bare soils in the first few days after fertilisation, and the measurement of NOx is difficult because of the reactivity of the gas (Taylor et al. 1999; Watt et al. 2004). Emissions of NOx and NH3 could www.publish.csiro.au/journals/sr
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B. C. T. Macdonald et al.
temperature throughout summer is relatively constant (~308C) and decreases during winter to 238C (Fig. 1). Mean annual rainfall is 1665 mm, ~69% of which is received in December–March (Fig. 1). The soil at both sites is a Chromosol (Isbell 1996) and the surface soil (300 mm) has 1.7 mg carbon kg–1, pH 4.7 and a mean mineral N concentration 10 mg N kg–1. After residue blanketing, ~9.4 t ha–1 of cane residue remained on the soil surface. The experimental period was divided into two main seasons, the dry (April–November) and the wet (December–March) seasons (Fig. 1). Measurements at both sites began after harvest towards the end of the dry season preceding fertilisation, and continued into the wet season. Daily soil water-filled pore space (0–300 mm) measured with a CS616 water content reflectometer (Campbell Scientific, Logan, UT, USA) responded to the daily rainfall inputs throughout the year, and in the first experiment, its average was ~68% during the first 130 days of the experiment (Fig. 2). During dry periods, the water-filled pore space declined (minimum 36%), and this was evident for the first 20 days after fertilisation and again around day 60 (Fig. 2). The daily maximum temperature during the first 130 days of measurement was constant (30 1.58C).
comprise a significant fraction of the unaccounted losses in soil N balance sheets throughout the cropping season. This paper reports near-continuous, 30-min measurements of NOx and NH3 emissions and determines the strength of the emissions from a Chromosol (Isbell 1996) used for sugarcane production at Mackay, Queensland. Materials and methods
Micrometeorological measurements A commonly used flux-gradient technique was employed to measure emissions of both NOx and NH3. Denmead et al. (2010) undertook a N2O emission experiment at the site and they provide a detailed description of the technique used in this experiment. The micrometeorological flux gradient technique is similar to the measurement approach used by Watt et al. (2004) and Taylor et al. (1999) to measure NO and NO2 fluxes above grasslands and bare soils. The emission rate of the gas, Fg, is calculated from the relationship: Fg ¼
ku ðrg;1 rg;2 Þ ln½ðz2 dÞ=ðz1 dÞ ½yðz2 dÞ yðz1 dÞ
600
30
500
25
Sampling period site 2
Sampling period site 1
300
20 15
5
0
0 00
7
07
/2 12
/2 0 11
7 00
/2 0 10
/2 09
/2 0
07
07
7 00
/2 0 07
/2 06
/2 0 05
00 /2 04
03
/2
00
7
7
07
07
/2 0 02
00
/2 0
01
/2
00 /2
12
11
07
100
07
10
6
200
Mean maximum temperature (°C)
35
400
ð1Þ
where k is the von Karman constant, u* is the friction velocity, rg is the average gas concentration each half-hour, z is height above
700
6
Monthly rainfall (mm)
Experimental site The 2006–07 field experiment was on a sugarcane farm close to the city of Mackay (218090 S, 1498070 E), Queensland, in a block of fifth-ratoon sugarcane (site 1), and the residues were returned to the soil by green-cane residue-blanketing (GCRB). The experiment was funded only for the 2006–07 season, but we were able to make a short-term measurement campaign during the 2007–08 season to verify the emission measurements in 2006–07. This second experiment was on the same farm on an adjacent block of second-ratoon sugarcane (site 2) and used to verify the cumulative NOx and NH3 losses over 2 months after fertiliser application. GCRB had been performed since 1996 and both sites received the same fertiliser treatment (150 kg N ha–1 as urea, buried in slits 100–150 mm below the soil surface). GCRB is the normal practice at Mackay and in most of the industry, where furrow irrigation is not employed. The sugarcane crop is not burnt before harvest; the non-stalk material is returned to the paddock as a ‘blanket’ and the stalk is removed to the mill for processing. At site 1, emission measurements commenced on 8 November 2006 when the plants were 0.45 m high, fertiliser was applied on 19 November 2006, and the experiment continued until 7 September 2007 when the plants were 4.1 m high. After measurements ceased, the field was harvested and scheduled for ploughing out and fallowing. Subsequently, measurements were made in an adjacent field (site 2). At site 2, measurements commenced on 1 October 2007 (~3 weeks after harvest) when the plants were 0.4 m high, fertiliser was applied on 19 October 2007, and measurements continued until 5 January 2008 when the plants were 1.9 m high. Mean annual temperature in this area is 22.38C, with the lowest mean monthly temperature in July (16.88C) and the highest in January (26.48C). The maximum monthly
08
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Fig. 1. Monthly rainfall (mm, columns) and mean monthly maximum temperature (8C, line) during field measurements at site 1 and site 2.
Emissions of NOx and NH3 from sugarcane fields
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835
40
(a)
Maximum temperature Minimum temperature Precipitation
35
30 150 25 100 20
Temperature (°C)
Daily rainfall (mm)
200
50 15
0
10
0.40
(b)
Soil moisture content (v/v)
0.35
0.30
0.25
0.20
5 cm 10 cm 30 cm
0.15
0.10 0
20
40
60
80
100
120
140
Days after fertilisation Fig. 2. (a) Daily rainfall (mm), and minimum and maximum temperature (8C) for the first 127 measurement days at site 1 (2006–07 growing season). (b) Mean daily soil water-filled pore space at 5, 10 and 30 cm for the same period.
the ground and d the height of the zero-plane displacement, and y is a correction for atmospheric stability represented by the Obukhov length (L). The subscripts 1 and 2 denote lower and upper measuring heights, respectively. In our studies, z1 was set at 0.5–1 m above the top of the cane and z2 at 1.5 m above that. A 3D sonic anemometer (CSAT3; Campbell Scientific) mounted at 1.5–2 m above the top of the crop was used to measure u*. Appropriate forms for u*, y and L are given by Prueger and Kustas (2005). Gases were sampled alternately at each measurement height for 7.5 min, and the line-switching was initiated by solenoid valves, which were controlled by a CR5000 logger (Campbell Scientific). After the sampling lines were switched by the triggering of the solenoid valves, the samples for the next
1.5 min were discarded. The average concentration of each gas per half-hour, rg, was the average of 12 min of measurement, which equates to two measurement periods at each height. Gas concentrations were measured with an EC9842 oxides of nitrogen/ammonia analyser (Ecotech Pty Ltd, Knoxfield, Vic.). The field in which the measurements were made was surrounded by other fields that were harvested and fertilised at the same time. Ancillary meteorological measurements made on a continuous basis include air temperature and humidity with an HMP 45 (Vaisala, Helsinki) and soil heat-flux plates (Campbell Scientific); soil water content with three CS616 water content reflectometers, converted to water-filled pore space; and soil temperature (TCAV; Campbell Scientific) at three depths (50, 100, 300 mm), net radiation (Q7.1 Campbell Scientific), and rainfall
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(CS700-L Rain Gauge Campbell Scientific). These data were logged using a Campbell Scientific CR5000 data logger. Measurements of NOx and NH3 The concentrations of NOx and NH3 were measured every minute using an EC9842 oxides of nitrogen/ammonia analyser; the EC9842 uses a gas phase chemiluminescence detector to perform continuous analysis of NH3, NOx and Nx (= NOx-N + NH3-N). Within the analyser, the sample was separated into two streams, one going to a thermal oxidiser (HTO-1000N) and the other to an NH3 scrubber. Ammonia was measured by the conversion of NH3 in the sample to NO, and that of NO2 to NO in the thermal oxidiser, followed by detection in a chemiluminescent reaction. The analysis of NO by means of chemiluminescence is based on the luminescence of an activated molecular NO2 species (Ecotech 2006). Samples exiting the thermal oxidiser represent Nx and samples exiting the NH3 scrubber represent NOx-N. The signal difference when measuring the output of the two sample streams represents the NH3-N in the sample. A significant issue for the measurement of both of these gases can be the change in concentration between the sampling of the air and its subsequent measurement. To limit such errors, all of the gas line tubing was made from clear polytetrafluoroethylene (PTFE; 6.35 mm ID) and the tubing length was minimised by placing the analysers within 10 m of the mast used for the micrometeorological measurements and gas sampling. The analyser was operated in an insulated, air-conditioned trailer and located downwind of the sampling mast. Inside the trailer, the sample tubing was connected to a diaphragm pump, which drew air to the instruments at a flow rate of 6 L min–1. All of the internal piping within the analyser trailer was made of either stainless steel or PTFE tubing. Calibration and maintenance of the analysers was done according to the instructions detailed in the service manual (Ecotech 2006). Nitrous oxide and soil nitrate data The N2O emission data were sourced from Denmead et al. (2010) and were used to measure the relative contributions of NOx, NH3 and N2O from the studied sugarcane fields. The soil nitrate (NO3–) was sourced from Wang et al. (2008), who undertook a companion study in the adjacent sugarcane block. Results and discussion Methodological considerations Our aim was to measure rates of gas exchange for each 30-min period over the duration of observation, but because of the inapplicability of the micrometeorological theory in light winds and highly stable and highly unstable turbulent atmospheric conditions, data were rejected whenever u* was
