Estimating nitrous oxide emissions from flood-irrigated alkaline grey clays.
Concern is mounting regarding the quantities of nitrous oxide ([N.sub.2]O) entering the earth's atmosphere, as agriculture is a major contributor to the atmospheric [N.sub.2]O pool. Carbon dioxide, methane, and [N.sub.2]O combined represent 80% of emissions contributing to the global greenhouse effect (Russell 1991). Emissions of [N.sub.2]O are critical, as the greenhouse warming potential of [N.sub.2]O is estimated as 295 times greater than of C[O.sub.2] (IPPC 2001). Although agriculture may be a small contributor to the total [N.sub.2]O emission on a global scale (Weier 1998), it dominates anthropogenic [N.sub.2]O 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 [sup.15]N-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 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 [N.sub.2]O/[N.sub.2] 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 [N.sub.2]O (Gilliam et al. 1978; Koskinen and Keeney 1982). [N.sub.2]O/[N.sub.2] 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 [N.sub.2]O relative to unlimed soil. Moraghan and Buresh (1977) reported that [N.sub.2]O remained the dominant gaseous N form evolved at pH 6, but 84% of the [N.sub.2]O was reduced to [N.sub.2] at pH 8.
[N.sub.2]O 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 [N.sub.2]O emissions in the field, most studies have been conducted in the laboratory. Therefore, identification of surrogate measures of [N.sub.2]O emissions determined under laboratory conditions is one means of estimating [N.sub.2]O losses in the field. This paper develops a correlation between soil pH and [N.sub.2]O emissions that is then used to estimate [N.sub.2]O loss from field experiments where N loss has been determined in [sup.15]N balance studies.
Research published in relation to [N.sub.2] and [N.sub.2]O emissions from soil is collated in Table 1. Where the time course of [N.sub.2]O and [N.sub.2] emissions was published, the [N.sub.2]O/[N.sub.2] mole ratios were calculated for the highest level of [N.sub.2]O emission. It is recognised that [N.sub.2]O/[N.sub.2] 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 [N.sub.2]O/[N.sub.2] 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 [N.sub.2]O emanating from alkaline soils, the following procedures were followed. The [N.sub.2]O/[N.sub.2] mole ratio was estimated using the relationships with soil pH described above. The proportion of the total N lost as [N.sub.2]O-N [i,e. [N.sub.2]O/([N.sub.2]O + [N.sub.2])] was then calculated. This fraction was then multiplied by the apparent N fertiliser loss, as published in [sup.15]N 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 [N.sub.2]O/[N.sub.2] mole ratio
The [N.sub.2]O/[N.sub.2] mole ratio (Table 1) was related to soil pH (Fig. 1) using a negative exponential function. This relationship indicated that equivalent amounts of [N.sub.2]O and [N.sub.2] would be produced from soil of pH 6.04. Acidic soils were prone to produce more [N.sub.2]O relative to [N.sub.2]; alkaline soils produced relatively less [N.sub.2]O than [N.sub.2] during denitrification.
Koskinen and Keeney (1982) reported that soil pH may have a strong effect upon the composition of the N gases evolved, with a higher proportion of [N.sub.2]O to [N.sub.2] in soil of pH <6.0, whereas [N.sub.2] was the dominant N gas emitted from soil of pH > 6.9. Christensen et al. (1990) reported that [N.sub.2]O was the sole denitrification product from an acid soil (pH 3.8). Substantial amounts of [N.sub.2]O are emitted from acid soils but [N.sub.2] is the major gas emitted following denitrification in alkaline soils (Cooper and Smith 1963). The model of Focht (1974) predicts enhanced denitrification rates with higher soil pH, but with a decreased proportion of [N.sub.2]O. Soil pH of 7.7 produces the optimum conditions for reduction of [N.sub.2]O (Wicht 1996), whereby a small decrease in pH produces a substantial increase in nitrous oxide evolution. Hauck and Melsted (1956) indicated that soil pH influences the [N.sub.2]O/[N.sub.2] mole fraction, where abundant [N.sub.2]O production was observed below pH 6; liming this soil to pH 7.2 more than doubled the rate of denitrification, although less [N.sub.2]O was produced. Koskinen and Keeney (1982) concluded that the sequence of denitrification products follows the exhaustion of the more readily reducible N compounds and the final or dominant product of denitrification is affected by soil pH. Weier and Gilliam (1986) found that almost no [N.sub.2]O was emitted during incubation of flooded soil of pH >5.8, with [N.sub.2]O being the sole product of denitrification at pH <5. These results concur with the data in Fig. 1.
[FIGURE 1 OMITTED]
Other edaphic factors may also affect the [N.sub.2]O/[N.sub.2] mole fraction. Weier et al. (1993) observed a wide range of [N.sub.2]O/[N.sub.2] mole fractions that normally decreased over time. The lowest ratios were observed in soils high in soluble C and with high water-filled pore space. The highest ratios were associated with higher nitrate concentration. The [N.sub.2]O/[N.sub.2] mole fraction normally declines as soil moisture increases and aeration decreases (Eriksen and Holtan-Hartwig 1993).
The [N.sub.2]O/[N.sub.2] mole fraction also increases with nitrate, and particularly nitrite, concentrations (Firestone et al. 1979; Smith and Tiedje 1979). Galsworthy and Burford (1978) showed that nitrous oxide was the dominant gas evolved from soils amended with 400 mg N[O.sub.3]-N/kg and the [N.sub.2]O/[N.sub.2] mole fraction changed little with time.
The rate of denitrification may also be affected by soil pH. Bollag et al. (1970) indicated that denitrifying microorganisms grow slowly and rates of denitrification are depressed below pH 6; the optimum pH range for the isolates they examined was neutral to slightly alkaline. Cooper and Smith (1963) suggested that in acid soils, nitrate reduction may be the rate-limiting step, whereas nitrite reduction may limit denitrification in alkaline soils. Koskinen and Keeney (1982) considered that C availability, rather than soil pH, limited denitrification in most situations, whereas pH may have a stronger effect upon the composition of the gases evolved.
Importantly, the 2 reports that relate to field observations (Table 1, Fig. 1) indicate extremely low [N.sub.2]O emissions, but both relate to flooded fields where [N.sub.2] would normally be the sole product of denitrification. Simpson et al. (1984) found only negligible amounts of [N.sub.2]O in flooded alkaline grey clay, and Denmead et al. (1979) found similarly low [N.sub.2]O emissions from flooded acidic red clay.
Soil profile characteristics can play an important part in the reduction of [N.sub.2]O. In the field [N.sub.2]O is largely evolved from banded fertiliser placed at depth in the soil and this [N.sub.2]O may then be reduced as it diffuses through the soil (Gilliam et al. 1978). Where shallow samples of soil are used in laboratory studies, there may be little opportunity for further reduction of [N.sub.2]O. Possibly, the laboratory studies may overestimate emission of [N.sub.2]O from soil and may not be indicative of [N.sub.2]O emissions in the field; the high [N.sub.2]O/[N.sub.2] ratios described in Table 1 and Fig. 1 may be a consequence of the experimental conditions. Gilliam et al. (1978) indicated that low [N.sub.2]O/[N.sub.2] mole fractions result from [N.sub.2]O formation at depth and this [N.sub.2]O is reduced as it moves to the soil surface and this complicates the prediction of the [N.sub.2]O/[N.sub.2] mole fraction in agricultural soils. Nitrate and soluble organic compounds may be leached through the soil to more anaerobic regions where denitrification prevails, although the production of N gases generally decreases with depth (Khan and Moore 1968).
Estimation of [N.sub.2]O emission
Based on the relationship depicted in Fig. 1, emission of about 5 kg [N.sub.2]O-N was predicted from several alkaline grey clay soils during a cotton season, according to the calculations reported in Table 2. A mean loss of 2.0 kg [N.sub.2]O-N/ha (range 1.6-2.6) was calculated using the N fertiliser recovery data from the 3 soil types reported by Rochester and Constable (2000). [N.sub.2]O was estimated to comprise about 2.4% of the N gases evolved, based on the fitted curve in Fig. 1. The higher values of [N.sub.2]O emission calculated were associated with finer textured soils, where poorer aeration and drainage produced higher rates of denitrification. Currently, it is assumed that because of the high soil pH, fine soil texture, and waterlogging through imperfect drainage, virtually all [N.sub.2]O is reduced to [N.sub.2] and only traces of [N.sub.2]O are emitted.
Low rates of [N.sub.2]O emission have been reported by Avalakki et al. (1995a), who observed that [N.sub.2]O comprised only 0.5% of the N emissions from a Vertosol, even when straw was added. Also, Mosier et al. (1986) observed in irrigated maize that <3% of applied N fertiliser was denitrified, although about 70% of this N was evolved as [N.sub.2]O, and concluded that the importance of denitrification may have been overemphasised in some agricultural systems.
More significant [N.sub.2]O emissions have been reported. Murakami and Kumazawa (1987) reported that [N.sub.2]O comprised 92% of the evolved N gases, but when this soil was kept at 90% of water-holding capacity, [N.sub.2]O emission was reduced to 24% of the N gases evolved. Ryden and Lund (1980) measured [N.sub.2]O fluxes of up to 1 kg N/ha.day from vegetable-cropped fields, with the highest emission occurring at the first irrigation post-fertilisation and emissions declining during the cropping cycle. [N.sub.2]O emission accounted for the loss of 4.2 kg N/ha (8% of total N emission from this system). Of the 250 kg ammonia-N/ha applied by Bremner et al. (1981), 16 kg/ha was lost as [N.sub.2]O, mostly in the first 6 weeks of the 20-week experiment; the unfertilised control lost only 2 kg N/ha over the same period. N losses from the control and fertilised soil were similar after 14 weeks. Colbourn et al. (1984) estimated that 0.3 kg N/ha.day was denitrified over a 23-day period; 93% of this loss was as [N.sub.2] but 30% of the 100 kg N/ha applied was denitrified over 2 months.
Because [N.sub.2]O is one gas in a sequence of gases formed during denitrification, it is possible that [N.sub.2]O not reduced to [N.sub.2] will be emitted from the soil as [N.sub.2]O. Mosier et al. (1995) proposed that a figure of 1.25([+ or -]1)% of the N applied be used to estimate [N.sub.2]O emission from fertiliser. This is close to the values estimated from alkaline cotton-growing soils in Table 2 where losses averaged 1.1% (range 0.9-1.4%) of N fertiliser applied.
Alkaline cotton-growing soils in Pakistan (Mahmood et al. 2000; Table 3) may lose similar amounts of N and [N.sub.2]O as in Australia due to the similar N fertiliser losses and soil pH. However, wheat grown in an acidic soil in Australia may lose substantially larger quantities of [N.sub.2]O (Freney et al. 1992; Table 3).
In comparison, Weier (1998) estimated that losses of [N.sub.2]O from Australian sugarcane fields were of the order of 4400 t [N.sub.2]O-N/year (1994). The area under sugarcane was quoted at 365000 ha, indicating that more than 12 kg [N.sub.2]O-N/ha was emitted from the soil via denitrification in that year. The proportion of N gases emitted as [N.sub.2]O was between 0.45 and 0.78, which reflects the more acidic soils on which sugarcane is grown in Australia. Weier (1998) estimates that a further 28 kg [N.sub.2]O-N/ha was emitted from the area where cane trash was burnt following harvest.
Possible strategies to reduce [N.sub.2]O emissions
Alternatively, the use of nitrification inhibitors has not only reduced the total quantity of N denitrified, but the [N.sub.2]O component of N gases emitted may be altered. Rochester et al. (1994) reported that total [sup.15]N recovery in the plant/soil system was significantly increased (by 43% in one instance) with the use of etridiazole; N and estimated [N.sub.2]O loss would be reduced by 24% (Table 3). Mikkelsen et al. (1986) reported that the application of etridiazole substantially reduced denitrification loss (by 64%); leaching was also reduced through delayed nitrification in the free-draining soil. Further, Somda et al. (1990, 1991) reported that etridiazole significantly suppressed microbial reduction of nitrate and nitrite under anaerobic laboratory conditions and production of [N.sub.2]O was almost eliminated, and [N.sub.2] production was reduced by 19%. Mills and McElhannon (1984) demonstrated in the laboratory that etridiazole substantially reduced the evolution of [N.sub.2] and [N.sub.2]O, and a lower proportion of [N.sub.2]O relative to [N.sub.2] was evolved from soil treated with etridiazole. Importantly, the addition of a nitrification inhibitor (wax-coated calcium carbide) reduced N loss (and estimated [N.sub.2]O emission) by almost 50% in a wheat cropping system in Australia (Freney et al. 1992; Table 3).
Another means of overcoming the loss of N fertiliser and thus emission of [N.sub.2]O is to include legume cropping in agricultural systems as a means of reducing N fertiliser inputs. Rochester et al. (2001) indicated in [sup.15]N balance studies that loss of legume N was substantially less than of fertiliser N applied at the time of incorporation of legume stubble. A reduced input of N fertiliser would be expected to result in a commensurate reduction in [N.sub.2]O loss, as estimated in Table 4. Loss of N and estimated [N.sub.2]O emission from green-manured legumes (field peas and lablab) were smaller than with legume stubble following grain harvest (fababean and soybean), but considerably more N was recovered from legume residues than from fertiliser N (Table 4). It is possible that N is lost from the crop or stubble before it is incorporated into the soil; hence soil pH may have little bearing on the composition of nitrogen gases derived from this source at this time. However, losses of N or [N.sub.2]O may not be substantial even where legume stubble of high N content is incorporated, although Aulakh et al. (1991) observed higher rates of [N.sub.2]O emission from organic residues of low C/N (legumes) compared with high C/N (maize or wheat) residues in the short term. Previously, Aulakh et al. (1983) had concluded that incorporation of clover residues substantially reduced gaseous N loss compared with fallowing.
Further in-field experimentation is required to measure the emission of [N.sub.2]O from alkaline soils, particularly those that support cotton crops that require substantial inputs of N fertiliser or where legume crops have been included in the cropping system.
Although measurements of the [N.sub.2]O/[N.sub.2] mole fraction from alkaline soils are low, the evolution of [N.sub.2]O from these soils may be significant. Because of the large quantities of fertiliser N applied to these cropping soils, estimates indicate that a small proportion of the denitrified fertiliser N is emitted as [N.sub.2]O, but this may amount to about 2 kg [N.sub.2]O-N/ha or 1% of the N fertiliser normally applied to alkaline grey clays for cotton cropping.
Table 1. Description of soils and [N.sub.2]O/[N.sub.2] mole ratios reported in various studies It was assumed that 1:5 soil: water was used for all pH determinations where not specified [N.sub.2]O/ Reference Conditions Soil type Soil pH [N.sub.2] Eriksen and Holtan- Laboratory Clay loam 5.5 2.9 Hartwig 1993 5.9 0.2 Koskinen and Laboratory Silt loam 4.6 6.0 Keeney 1982 5.4 5.2 6.0 1.2 6.9 0.4 Gilliam et al. 1978 Laboratory Loam 6.2 4.0 Fine silt 6.3 0.01 Silty clay 7.4 0.01 Loam 7.9 0.00 Firestone et Laboratory Loamy sand 6.4 1.20 al. 1979 Loam 7.0 0.44 Avalakki et al. Laboratory Black earth 8.0 0.05 1995a, 1995b Laboratory Red-brown earth 6.3 0.266 Black earth 8.0 0.064 Keeney et al. 1979 Laboratory Silt loam 6.8 0.50 Hauck and Melsted Laboratory Silt loam 5.9 6.77 1956 7.2 0.45 Bronson and Laboratory Clay loam 7.9 <0.01 Mosier 1991 Yeomans and Laboratory Clay loam 7.5 0.23 Bremner 1988 Silty clay 7.7 0.22 Silty clay loam 8.1 0.18 Weier and Laboratory Histosol 4.2 13.0 Gilliam 1986 Histosol 4.6 3.3 Histosol 4.7 >20 Histosol 4.7 >20 Histosol 5.0 9.5 Histosol 5.4 1.94 Histosol 5.6 0.2 Histosol 5.6 2.0 Histosol 5.7 0.3 Histosol 5.8 0.1 Denmead et al. 1979 Flooded Red clay 5.8 0.01 field Simpson et al. 1984 Flooded Grey clay 8.2 <0.01 field Table 2. Estimated loss of fertiliser-derived [N.sub.2]O-N during a cotton season from an application of 180 kg N/ha Soil and nitrogen fertiliser recovery (NFR) data are from Rochester and Constable (2000) Soil series Soil Clay NFRB N pH (A) content (%of N fertiliser (%) applied) lost (kg/ha) Gommel medium clay 8.3 53 68 57.6 Gommel medium clay 8.5 59 52 86.4 Gommel heavy clay 8.4 69 40 108.0 Mean 8.4 60 53 84.0 Soil series [N.sub.2]O/ [N.sub.2]O/ [N.sub.2]O-N [N.sub.2]O-N [N.sub.2]C ([N.sub.2]O+ emitted emitted [N.sub.2]) (kg/ha) (% of N applied) Gommel medium clay 0.0283 0.0275 1.58 0.9 Gommel medium clay 0.0207 0.0203 1.75 1.0 Gommel heavy clay 0.0242 0.0236 2.55 1.4 Mean 0.0242 0.0236 1.98 1.1 (A) Soil pH (1:5 soil:water). (B) Nitrogen fertiliser recovery. (C) Ratio derived from the fitted curve in Fig. 1. Table 3. Estimated emission of fertiliser-derived [N.sub.2]O-N during the growth of cotton and wheat crops The application of nitrification inhibitors reduced N fertiliser loss and estimated [N.sub.2]O emission Reference Soil NFR (A) N (% of N fertiliser applied) lost (kg/ha) Mahmood et al. (2000), Sandy clay loam, 58 (C) 71.8 cotton, Pakistan pH 7.9 Rochester et al. (1994), Gommel heavy 35 52 cotton, Australia clay, pH 8.4 50 (D) 40 Freney et al. (1992), Red-brown 50 50 wheat, Australia earth, pH 6.2 73 (E) 27 Reference [N.sub.2]O/ [N.sub.2]O/ [N.sub.2] (B) ([N.sub.2]O+ [N.sub.2]) Mahmood et al. (2000), 0.0532 0.0505 cotton, Pakistan Rochester et al. (1994), 0.0242 0.0236 cotton, Australia 0.0242 0.0236 Freney et al. (1992), 0.773 0.436 wheat, Australia 0.773 0.436 Reference [N.sub.2]O-N [N.sub.2]O-N emitted emitted (kg/ha) (% of N applied) Mahmood et al. (2000), 3.63 2.1 cotton, Pakistan Rochester et al. (1994), 1.23 1.5 cotton, Australia 0.94 1.2 Freney et al. (1992), 21.8 21.8 wheat, Australia 11.8 11.81 (A) Nitrogen fertiliser recovery. (B) Ratio derived from the fitted curve in Fig. 1. (C) 173 kg urea-N applied/ha. (D) Nitrification inhibitor (Terrazole) applied; 80 kg N/ha applied. (D) Nitrification inhibitor (wax-coated calcium carbide) applied; 100 kg N/ha applied. Table 4. Estimated emission of [N.sub.2]O from [sup.15]N-labelled fertiliser or legume stubble as described by Rochester et al. (2001) The soil was a Gommel medium clay, pH 8.3 and 53% clay. The summer and winter crop stubbles were incorporated 5 and 11 months, respectively, before the microplots were destructively sampled in October 1996; fertiliser was applied at the same time that the crop stubbles were incorporated Previous Source N NFR (A) N crop of applied (% of N fertiliser [sup.15] N (kg/ha) applied) lost (kg/ha) Wheat Fertiliser 100 18 82 Cotton Fertiliser 100 11 89 Fababean Stubble 80 60 32 Field pea Stubble 107 78 24 Soybean Stubble 220 72 62 Lablab Stubble 247 91 23 Previous [N.sub.2]O/ [N.sub.2]O/ [N.sub.2]O-N [N.sub.2]O-N crop [N.sub.2] ([N.sub.2]O+ emitted emitted (B) [N.sub.2]) (kg/ha) (% of N applied) Wheat 0.0283 0.0275 2.26 2.26 Cotton 0.0283 0.0275 2.45 2.45 Fababean 0.0283 0.0275 0.88 1.10 Field pea 0.0283 0.0275 0.66 0.62 Soybean 0.0283 0.0275 1.71 0.78 Lablab 0.0283 0.0275 0.63 0.26 (A) Nitrogen fertiliser recovery. (B) Ratio derived from the fitted curve in Fig. 1.
Drs P. Grace, K. Weier, D. Barrett, M. Peoples, G. Constable, and S. Milroy provided helpful comments on the manuscript.
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Manuscript received 11 June 2002, accepted 11 November 2002
Ian J. Rochester
Australian Cotton Cooperative Research Centre, CSIRO Plant Industry, Cotton Research Unit, LB 59, Narrabri, NSW 2390, Australia 2390; email: firstname.lastname@example.org
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|Author:||Rochester, Ian J.|
|Publication:||Australian Journal of Soil Research|
|Date:||Mar 1, 2003|
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