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Australian Journal of Soil Research, 2003, 41, 197–206
Estimating nitrous oxide emissions from flood-irrigated alkaline grey clays Ian J. Rochester Australian Cotton Cooperative Research Centre, CSIRO Plant Industry, Cotton Research Unit, LB 59, Narrabri, NSW 2390, Australia 2390; email:
[email protected]
Abstract Concern has mounted over recent decades regarding the emission of nitrous oxide (N2O) to the atmosphere through human activities. Modern agriculture has contributed to this with elevated use of nitrogenous fertilisers and irrigation. Irrigated cotton grown on alkaline heavy clay soils often uses nitrogen fertiliser inefficiently, due largely to N loss (commonly 50–100 kg N/ha) through denitrification. However, the amount of denitrified N emitted as N2O has rarely been measured. This paper derives estimates of the quantities of N2O emitted from N fertiliser applied to alkaline grey clays. A negative exponential function between the N2O/N2 mole fraction and soil pH was derived from a search of laboratory and field studies published by numerous authors using a wide range of soil types. A greater proportion of N2O relative to N2 is emitted from acid soils; approximately equivalent amounts of each gas are emitted from soil of pH 6.0. For the alkaline grey clays (pH 8.3–8.5), the N2O/N2 mole fraction was about 0.024. The quantities of N2O emitted from alkaline grey clays during the growth of a cotton crop were estimated by applying this relationship to 15N balance studies where N fertiliser losses had been measured. Using this approach, about 2 kg N/ha (~1.1% of the N applied) was calculated to be lost as N2O during the cotton-growing season. This is similar to the value of 1.25% commonly used to estimate N2O emissions from N fertiliser, but this estimation should only be applied to alkaline soils; a larger percentage of the fertiliser N denitrified from acid soils should be emitted as N2O-N. These estimates of N2O emissions require validation with field experimentation. The low (negligible) values for N2O emission from flooded fields compared with laboratory observations are discussed. It is possible that high N2O emissions observed under laboratory conditions result from the shallow depth of soil, decreasing the opportunity for N2O to be further reduced as it diffuses through the soil profile. Management strategies that have the potential to reduce N2O emissions are discussed. SR0268 NI.itJro. uRocshxedtisermisionsform aklainlesoils
Additional keywords: soil pH, denitrification, nitrogen fertiliser, cotton.
Introduction Concern is mounting regarding the quantities of nitrous oxide (N2O) entering the earth’s atmosphere, as agriculture is a major contributor to the atmospheric N2O pool. Carbon dioxide, methane, and N2O combined represent 80% of emissions contributing to the global greenhouse effect (Russell 1991). Emissions of N2O are critical, as the greenhouse warming potential of N2O is estimated as 295 times greater than of CO2 (IPPC 2001). Although agriculture may be a small contributor to the total N2O emission on a global scale (Weier 1998), it dominates anthropogenic N2O emissions and it is possible that such emissions can be reduced with improved irrigation and N fertiliser management. Recent field studies have indicated that substantial quantities of 15N-labelled fertiliser can be lost from flood-irrigated grey clays used for cotton cropping (Freney et al. 1993; Rochester et al. 1994, 1996). Denitrification is believed to be the dominant process contributing to these losses, which commonly exceed 50% of applied N (Freney et al. 1993). High rates of N fertiliser (up to 200 kg N/ha) may be required to optimise lint yield. Hence, N fertiliser losses may exceed 100 kg N/ha during a growing season. Soils of high © CSIRO 2003
10.1071/SR02068
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clay content can rapidly become anaerobic when flood irrigated, providing ideal conditions for denitrification. However, little is known of the proportions of the various N gases emitted following denitrification. Published research reveals that high N2O/N2 mole ratios are commonly associated with soils of low pH. Several authors indicate that where several soils of varying pH are compared, those with higher pH commonly emit less N2O (Gilliam et al. 1978; Koskinen and Keeney 1982). N2O/N2 mole ratios reported from investigations of alkaline soils are commonly low or negligible (Simpson et al. 1984; Avalakki et al. 1995b). Hauck and Melsted (1956) showed that liming an acid soil reduced emission of N2O relative to unlimed soil. Moraghan and Buresh (1977) reported that N2O remained the dominant gaseous N form evolved at pH 6, but 84% of the N2O was reduced to N2 at pH 8. N2O emissions may be reduced by managing, reducing, or avoiding denitrification. The use of nitrification inhibitors can maintain fertiliser N as ammonium, thereby avoiding loss of nitrate-N (Eriksen and Holtan-Hartwig 1993). Alternatively, legume cropping can substantially reduce the need for the high rates of fertiliser N commonly used in commercial cotton production (Rochester et al. 2001). Although it was shown that N loss was reduced under this system, significant denitrification may still occur when leguminous residues of high N content decompose. By improving drainage through better field design and irrigation management, waterlogging and denitrification can be reduced. Because of the difficulties in quantifying N2O emissions in the field, most studies have been conducted in the laboratory. Therefore, identification of surrogate measures of N2O emissions determined under laboratory conditions is one means of estimating N2O losses in the field. This paper develops a correlation between soil pH and N2O emissions that is then used to estimate N2O loss from field experiments where N loss has been determined in 15N balance studies. Methods Research published in relation to N2 and N2O emissions from soil is collated in Table 1. Where the time course of N2O and N2 emissions was published, the N2O/N2 mole ratios were calculated for the highest level of N2O emission. It is recognised that N2O/N2 mole ratios vary with time and soil moisture content (Hauck and Melsted 1956; Aulakh et al. 1984). Where the method of determining soil pH was not specified, it was assumed that 1: 5 soil : water was used for the determination, as used for the cotton-growing soils reported here. A relationship between the N2O/N2 mole ratio and the pH of each soil was determined using the Sigma Plot program (Anon. 2000). A negative exponential function most closely fitted the data. In order to estimate the quantities of N2O emanating from alkaline soils, the following procedures were followed. The N2O/N2 mole ratio was estimated using the relationships with soil pH described above. The proportion of the total N lost as N2O-N [i.e. N2O/(N2O + N2)] was then calculated. This fraction was then multiplied by the apparent N fertiliser loss, as published in 15N balance studies (Freney et al. 1993; Rochester et al. 1994, 1996, 2001). The apparent N fertiliser loss reported in Tables 2, 3, and 4 includes ammonia volatilisation loss, but this is a very minor source of N loss from fertilisers placed below the soil surface in this cropping system (~1% of N applied; Denmead et al. 1977).
Results and discussion pH effect on the N2O/N2 mole ratio The N2O/N2 mole ratio (Table 1) was related to soil pH (Fig. 1) using a negative exponential function. This relationship indicated that equivalent amounts of N2O and N2 would be produced from soil of pH 6.04. Acidic soils were prone to produce more N2O relative to N2; alkaline soils produced relatively less N2O than N2 during denitrification.
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Table 1. Description of soils and N2O/N2 mole ratios reported in various studies It was assumed that 1: 5 soil : water was used for all pH determinations where not specified Reference
Conditions
Soil type
Soil pH
N2O/N2
Eriksen and Holtan-Hartwig 1993
Laboratory
Clay loam
Koskinen and Keeney 1982
Laboratory
Silt loam
Gilliam et al. 1978
Laboratory
Firestone et al. 1979
Laboratory
Avalakki et al. 1995a, 1995b
Laboratory Laboratory
Keeney et al. 1979 Hauck and Melsted 1956
Laboratory Laboratory
Loam Fine silt Silty clay Loam Loamy sand Loam Black earth Red-brown earth Black earth Silt loam Silt loam
Bronson and Mosier 1991 Yeomans and Bremner 1988
Laboratory Laboratory
Weier and Gilliam 1986
Laboratory
Denmead et al. 1979 Simpson et al. 1984
Flooded field Flooded field
5.5 5.9 4.6 5.4 6.0 6.9 6.2 6.3 7.4 7.9 6.4 7.0 8.0 6.3 8.0 6.8 5.9 7.2 7.9 7.5 7.7 8.1 4.2 4.6 4.7 4.7 5.0 5.4 5.6 5.6 5.7 5.8 5.8 8.2
2.9 0.2 6.0 5.2 1.2 0.4 4.0 0.01 0.01 0.00 1.20 0.44 0.05 0.266 0.064 0.50 6.77 0.45 20 >20 9.5 1.94 0.2 2.0 0.3 0.1 0.01 5.8, with N2O being the sole product of denitrification at pH
