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N2O and CH4 emission and CH4 consumption in a sugarcane soil after variation in nitrogen and water application
Soil Biology and Biochemistry 31 (1999) 1931±1941
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N2O and CH4 emission and CH4 consumption in a sugarcane soil after variation in nitrogen and water application K.L. Weier* CSIRO Tropical Agriculture, Cunningham Laboratory, 306 Carmody Road, St. Lucia, Brisbane, QLD 4067, Australia Accepted 22 June 1999
Abstract Nitrous oxide (N2O) and methane (CH4) exchanges between the soil and the atmosphere are in¯uenced by management practices currently used in agriculture. This study was conducted to determine whether N2O and CH4 emission and CH4 consumption in a trash blanketed sugarcane soil could be in¯uenced by changed N fertiliser practice and variation in soil water content. Microplots were placed in rows of sugarcane and urea and ammonium sulphate (both 99 atom % 15N excess) applied to the soil surface, urea as both a split application of 80 kg N haÿ1 and a full rate of 160 kg N haÿ1 and ammonium sulphate at 160 kg N haÿ1. Sugarcane trash (15 t haÿ1) was applied to the surface of all microplots. Water was then added to all microplots to achieve water contents of 80% water-®lled pore space (WFPS) in half the plots and saturation in the other half. Initially, the application of urea at 160 kg N haÿ1 to microplots at 80% WFPS resulted in greater emission of (14N+15N)±N2O than was recorded from the split urea and the ammonium sulphate treatments at both moisture regimes. A mean value of 47 g N2O±N haÿ1 dÿ1 was measured for this treatment compared with 18.5 and 15.2 g and 4.9 and 7.7 g N2O±N haÿ1 dÿ1 for the split urea and ammonium sulphate waterlogged and 80% WFPS treatments respectively. Greater emissions of (14N+15N)±N2O were measured following 132 mm rainfall in February, with mean values of 214 and 143 g N2O±N haÿ1 dÿ1 being recorded for the waterlogged treatments of the split urea and ammonium sulphate treatments respectively. A decrease in the 15N±N2O emission from the split urea treatment was the only dierence recorded when comparing the initial (14N+15N)±N2O and 15N±N2O emissions from all treatments. 14N±N2O appeared to be the major gaseous N product from this treatment initially. Total emission of 15N±N2O ranged from 105 to 453 g N2O±N haÿ1, which represented between 25.3 and 23.4% of the total amount of N2O evolved. CH4 emission occurred from all microplots fertilised with urea whereas CH4 consumption was measured in plots fertilised with ammonium sulphate only. CH4 emission ranged from 297 to 1005 g CH4±C haÿ1 while CH4 consumption ranged from 442 to 467 g CH4±C haÿ1. The application of ammonium sulphate and using a split application of urea initially decreased N2O emissions but the delay in nitri®cation obtained from changing the type and rate of application of these fertilisers resulted in greater N2O emissions later in the season when the soil moisture regime had changed. CH4 emission and consumption were not aected by these management changes. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Nitrous oxide; Methane; Emission; Consumption; Microplot; Fertiliser; Residue; Sugarcane
1. Introduction Soils in agricultural regions can be an important source of atmospheric nitrous oxide (N2O) and maybe sinks for atmospheric methane when they become aerated (Bouwman, 1990). Application of nitrogen fertili* Fax: +61-617-3214-2288. E-mail address: [email protected] (K.L. Weier)
sers to soils increase N2O emissions (Eichner, 1990) while ammonium and nitrate based fertilisers have been found to suppress and stimulate CH4 consumption, respectively (Nesbit and Breitenbeck, 1992; Bronson and Mosier, 1994). The concentration of the 2 gases in the atmosphere is increasing at an annual rate of 0.8 and 0.25% respectively (Intergovernmental Panel on Climate Change, 1994). Both gases are estimated to be collectively responsible for about 20% of anticipated global warming (Rodhe, 1990) with
0038-0717/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 0 7 1 7 ( 9 9 ) 0 0 1 1 1 - X
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K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
Table 1 Characteristics of the soil underlying the sugarcane crop at the Ingham study site Site
Ingham a
Depth (cm)
0±5 5±20
BDa (Mg mÿ3)
1.18 1.48
Total N (%)
0.11 0.11
Total C (%)
1.47 1.55
Mineral-N (mg kgÿ1)
Textural Analysis (%)
NO3±N
NH4±N
sand
silt
clay
0.04 0.10
0.18 0.22
30 32
29 27
40 41
BD=bulk density.
Australia's contribution to this global increase about 958 kt N2O±N yrÿ1 and 6.5 Mt CH4 yrÿ1 respectively (Galbally et al., 1992). The net contribution of Australian agriculture to these totals has been estimated at about 140 and 2049 kt yrÿ1 respectively (Howden et al., 1991; Galbally et al., 1992). The contribution from the Australian sugar industry to these estimates has been largely unknown. Values for N2O emissions have been calculated from denitri®cation losses using an approximate N budget for the crop (Vallis and Keating, 1994) However, this required some knowledge of the proportion of gaseous N loss that was occurring as N2O and N2 and this has only recently been documented (Weier et al., 1996; 1998). For CH4, values both in Australia and globally, have only been calculated from the burning of sugarcane with CH4 emission occurring during the smouldering phase (Crutzen and Andreae, 1990). However, there are no known recorded measurements of CH4 production from waterlogged sugarcane ®elds or from areas where trash blanketing is the normal management practice. In the only study so far that has attempted to document N2O emission and CH4 emission and consumption from sugarcane areas, Weier (1998) estimated that, for the 1994 season, a total of 10.7 kt N2O±N yrÿ1 was emitted from bare soils, trash blanketed soils and after burning. For CH4, 6.7 kt CH4±C yrÿ1 was emitted after burning and 34 kt CH4±C yrÿ1 was consumed by the trash blanket. Net uptake was therefore occurring in sugarcane ®elds. My objective was to determine the eect of dierent N fertilisers and dierent rates of N application on N2O and CH4 ¯uxes from trash blanketed sugarcane soils. These eects were investigated at 2 soil moisture contents.
2. Materials and methods 2.1. Study site The experimental site was established on a sugarcane farm, 21 km west of Ingham (18.7S, 146.2E), north Queensland, following harvesting of the crop at the
end of September. The climate of the area is tropical with a mean annual rainfall of 2275 mm. Site selection was based on soil type and the use of a trash blanket as a management practice. The soil type was a bleached grey clay (Stace et al., 1968), of principal pro®le form Ug 3.2 (Northcote, 1965), some characteristics of which are given in Table 1. 2.2. Field procedure Microplots were formed by pushing PVC cylinders (23.5 cm i.d. by 25 cm long) into the soil to a depth of 20 cm between the stools in the rows of sugarcane. The experimental design was a factorial with 2 N fertilisers at 3 rates, 2 soil water contents, 4 intensive sampling periods and 3 replications, making a total of 72 microplots. On 5 October, 1995, urea (99 atom % 15 N excess) was broadcast on the soil surface of 48 microplots, with 24 receiving 160 kg N haÿ1 and 24 receiving 80 kg N haÿ1. A further 80 kg N haÿ1 was applied to the latter microplots on 17 December, 1995, following the second intensive sampling period. Sugarcane trash (15 t haÿ1) was applied to the soil surface of the 48 microplots. On 13 November, 1995, ammonium sulphate (99 atom % 15N excess) was broadcast on the soil surface of the remaining 24 microplots at a rate of 160 kg N haÿ1. Sugarcane trash (15 t haÿ1) was immediately placed on the soil surface of these microplots. Potassium bromide (200 kg Brÿ haÿ1) was added to the soil surface of all microplots, at the same time as the N fertiliser, as a guide to the extent of solute leaching of nitrate below the end of the microplot. Water was added to all 72 microplots following the 14.00 h gas sampling on d 1 and similarly on d 2, 3 and 4. Thirty-six microplots received enough water to achieve saturation and 36 received sucient water to acquire a water-®lled pore space (WFPS) of 80% to the depth of the microplot base. In each case, 24 of the 36 were urea treated and 12 ammonium sulphate treated. Water-®lled pore space is synonymous with relative saturation and was calculated as WFPS=[(gravimetric water content soil bulk density)/total soil porosity], where soil porosity=[1ÿsoil bulk density/2.65] and 2.65 equals the assumed particle density of soil (Mg mÿ3). Rainfall
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
1933
15
N±(N2O+N2) by isotope mass spectrometry (Mosier et al., 1986). Total (14N+15N)±N2O and 15N±N2O emission and CH4 consumption and emission from the microplots over the 104 d study was calculated by applying the Trapezoidal Rule (Origin 4.0, Microcal Software Inc., MA) which uses an approximate integration to calculate the area under the curve. After each intensive 5 d sampling, 18 soil cores (6 from each fertiliser N treatment) were destructively sampled and returned to the laboratory for analysis. Total N and C in the soil and trash were measured by dry combustion using a Europa ANA mass spectrometer. Brÿ was determined colorimetrically after extraction of the soil with 10 mM MgSO4 (Marti and Arozarena, 1981). Mineral N was determined after extraction of the soil with 2 M KCl (Catchpoole and Weier, 1980) and NO3±N and NH4±N measured colorimetrically (Henzell et al., 1968). 15N±(NO3+NH4) was measured by diusion using the procedure of Brooks et al. (1989). 3. Results 3.1. Nitrous oxide (unlabelled and labelled)
Fig. 1. (14N+15N)±N2O emission over 104 d from microplots placed in sugarcane rows within a green-cane, trash blanketed ®eld. Microplots were fertilised with: (a) ammonium sulphate at 160 kg N haÿ1 (b) urea at 80 kg N haÿ1 on 5 Oct. 95 and again on 17 Dec. 95 and (c) urea at 160 kg N haÿ1. Rainfall over the sampling period is shown at the top of the graph. Vertical bars show standard errors of the mean [WL=waterlogged; 80=80% WFPS].
at the experimental site was recorded over the duration of the study. Soil temperature was recorded at each gas sampling using thermometers inserted 5 cm below the soil surface. Gas sampling commenced at 06.00 h on 14 November, 1995 (d 1). Covers were placed on the microplots for 1 h and gas samples taken from the headspace after 0 and 60 min. This procedure was repeated every 8 h for 5 d. Further 5 d periods of intensive gas sampling occurred from 12±16 December, 1995; 3±7 January, 1996 and 20±24 February, 1996, during which water was applied as above. During the weeks between intensive gas samplings, gas samples were collected at 06.00 h on Tuesday and Friday of each intervening week. However, no water treatments were reimposed on the plots for the bi-weekly samplings between intensive measurements. The gas samples were analysed for N2O and CH4 by g.c. (Mosier et al., 1991; Weier et al., 1991), 15N±N2O by Europa Trace Gas Analyzer (Stevens et al., 1993) and
Initial (14N+15N)±N2O emission, over the ®rst 5 d of intensive gas sampling, was greatest from microplots at 80% WFPS and fertilised with urea at 160 kg N haÿ1 (Fig. 1). Values ranged from 9 to 142 g N2O±N haÿ1 dÿ1, with a mean value of 47 210 g N2O±N haÿ1 dÿ1. For the urea treatment, with a split application of fertiliser, average values of 18.5 2 10.5 and 15.2 2 8.0 g N2O±N haÿ1 dÿ1 were obtained for the waterlogged and 80% WFPS treatments, respectively. However, at the ®nal sampling in this period, ¯uxes of 164 and 127 g N2O±N haÿ1 dÿ1 were observed from the respective water treatments. For the microplots fertilised with ammonium sulphate, values from the waterlogged and 80% WFPS treatments ranged from 0.8 to 11.2 (mean 4.9 20.8) and 2.8 to 15.3 (mean 7.7 21.2) g N2O±N haÿ1 dÿ1, respectively. Except for a ¯ux of 248 g N2O±N haÿ1 dÿ1 at the end of November from the 80% WFPS treatment receiving the split N application, a period of low N2O evolution then occurred from all microplots. In early January, 1996, when water application for the third intensive gas sampling was supplemented by rainfall, a maximum ¯ux of 144 g N2O±N haÿ1 dÿ1 was observed from the 80% WFPS urea treatment receiving 160 kg N haÿ1. Fluxes of 150 and 110 g N2O±N haÿ1 dÿ1 were also measured from the waterlogged and 80% WFPS split urea treatments respectively. However, the greatest emissions occurred from waterlogged microplots following 132 mm rainfall at the beginning of February 1996. Evolution of N2O did
1934
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
Table 2 The amounts of (14N+15N)±N2O and 15N±N2O evolved following application of fertiliser N to trash covered microplots in sugarcane rows. Treatments imposed were: (1) ammonium sulphate and urea applied at 160 kg N haÿ1, (2) urea applied at an initial rate of 80 kg N haÿ1 with a further 80 kg N haÿ1 added after the second intensive sampling, (3) water applied over 5 d at 4 times during each study period to achieve conditions of waterlogging and 80% WFPS and (4) natural moisture conditions between periods of intensive sampling. Values were obtained using an approximate integration to obtain the area under the curve Fertiliser
Resident time (d)
N rate (kg haÿ1)
Water treatment
15
(14N+15N)±N2O (g haÿ1)
Ammonium sulphate
104 104 143 143 143 143
160
WLa 80% WFPS WL 80% WFPS WL 80% WFPS
154 241 105 362 453 118
1082 951 415 1143 1934 1562
Urea Urea a
160 2 80
N±N2O (g haÿ1)
WL=waterlogged.
not occur immediately after rainfall ceased, but 7 d later for all treatments. The largest values were recorded from microplots, which received a second application of urea in December, 1995, with a peak emission of 317 g N2O±N haÿ1 dÿ1 and a mean value of
214 2 89 g N2O±N haÿ1 dÿ1. For microplots fertilised with ammonium sulphate, the mean value was 143 2 162 g N2O±N haÿ1 dÿ1 even though a peak emission of 408 g N2O±N haÿ1 dÿ1 was recorded. Much lower emissions were recorded for microplots, which were fertilised with urea at 160 kg N haÿ1. Total emission of (14N+15N)±N2O from the microplots over the study period ranged from 415 g N2O±N haÿ1 for the waterlogged urea 160 kg N haÿ1 treatment to 1934 g N2O±N haÿ1 for the waterlogged, split urea treatment (Table 2). 3.2. N-labelled gases 15
Fig. 2. 15N±N2O emission over 104 d from microplots placed in sugarcane rows within a green-cane, trash blanketed ®eld. Microplots were fertilised with: (a) ammonium sulphate at 160 kg N haÿ1 (b) urea at 80 kg N haÿ1 on 5 Oct 95 and again on 17 Dec 95 and (c) urea at 160 kg N haÿ1. Rainfall over the sampling period is shown at the top of the graph. Vertical bars show standard errors of the mean [WL=waterlogged; 80=80% WFPS].
N±N2O emission from the microplots virtually followed the same pattern observed for total emission of (14N+15N)±N2O, with the only dierence occurring in initial N2O evolution from the split urea application (Fig. 2). Here, the initial N2O ¯uxes, at both water contents, had disappeared which indicated that all measured N2O ¯uxes had occurred as 14N±N2O. Total emission of 15N±N2O from all microplots over the 104 d study ranged from 105 to 453 g N2O±N haÿ1 which comprised 25.3 and 23.4% of the total N2O emissions (Table 2). The in¯uence of soil moisture on the gaseous loss process can be seen from the quantities of (14N+15N)±N2O, 15N±N2O and 15N± (N2O+N2) evolved from the microplots at 06.00 h on 13 February, 1996 (Table 3). 15N±N2O emission was not large, comprising between 3.6 and 36.2% of the total (14N+15N)±N2O evolved and between 0.05 and 14.2% of the 15N±(N2O+N2) evolved. This suggests that, under these conditions, 14N±N2O was the major component of N2O being evolved and that 15N±N2, produced following denitri®cation, was the major gaseous N product. 3.3. Methane emission and consumption Methane emission occurred from the urea 160 kg N haÿ1 treatment during the ®rst and second intensive
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
1935
Table 3 The quantities of (14N+15N)±N2O, 15N±N2O and 15N±(N2O+N2) evolved from the N fertilised, trash covered microplots at 06.00 h on Tuesday, 13 February, 1996 following 132 mm of rainfall in the preceding week. Values are the means of 3 replications with standard errors in parentheses Fertiliser
N rate (kg haÿ1)
Water treatment
(14N+15N)±N2O (g N haÿ1 dÿ1)
15
N±N2O (g N haÿ1 dÿ1)
15
Ammonium sulphate
160
Urea
160
Urea
2 80
WLa 80% WFPS WL 80% WFPS WL 80% WFPS
143 (132) 58 (40) 5 (2) 11 (3) 214 (73) 13 (4)
26 (25) 21 (14) 1 (0.6) 0.4 (0.1) 61 (25) 3 (2)
183 569 1024 764 1334 1185
a
N±(N2O+N2) (g N haÿ1 dÿ1) (74) (344) (381) (460) (209) (287)
WL=waterlogged.
gas samplings following water application to the microplots and in the intervening period between these samplings (Fig. 3). Total emissions during this period were 651 g CH4±C haÿ1 for the waterlogged treatment and 438 g CH4±C haÿ1 for the 80% WFPS treatment. Over the remainder of the study, only limited periods occurred when either consumption or emission of CH4 was occurring. For the ammonium sulphate and split
urea treatments, some consumption and emission occurred in the former whereas CH4 emission occurred mainly from the latter. Consumption of CH4 occurred during the study period in microplots fertilised with ammonium sulphate, with values ranging from 442 to 467 g CH4±C haÿ1 (Table 4). For the other 2 treatments, CH4 was evolved from the microplots with concentrations between 297 and 1005 g CH4±C haÿ1. There was no signi®cant eect of water treatments on CH4 consumption in the microplots fertilised with ammonium sulphate. However, large, but inconsistent dierences in CH4 emissions were found between water treatments for microplots fertilised with urea. 3.4.
Fig. 3. CH4 consumption and emission over 104 d from microplots placed in sugarcane rows within a green-cane, trash blanketed ®eld. Microplots were fertilised with: (a) ammonium sulphate at 160 kg N haÿ1 (b) urea at 80 kg N haÿ1 on 5 Oct. 95 and again on 17 Dec. 95 and (c) urea at 160 kg N haÿ1. Rainfall over the sampling period is shown at the top of the graph. Vertical bars show standard errors of the mean [WL=waterlogged; 80=80% WFPS; se=standard error].
15
N recovery
Total recovery of 15N from all microplots for all treatments decreased over time, with recoveries generally higher from microplots, which were brought to a moisture content of 80% WFPS (Tables 5±7). Values over the 4 samplings for this treatment ranged from 97 to 20% for ammonium sulphate, 83 to 28% for urea 160 and 83 to 22% for the split urea application. 15N recovery from ammonium sulphate was greatest after the ®rst 2 intensive samplings, although recoveries from both urea treatments were good considering that N fertiliser application had occurred 45 and 73 d previously. Recoveries of 15N from the ammonium sulphate and urea 160 kg N haÿ1 treatments after the third sampling were similar whereas more 15N remained in the soil after the ®nal sampling for urea 160 kg N haÿ1 than for ammonium sulphate. 15N recoveries from the split urea treatment at the third sampling were higher than for the other treatments, but the percentage had decreased to a value below that of the urea 160 kg N haÿ1 treatment at the ®nal sampling. There were also signi®cant dierences between the recovery of bromide and mineral-N from the microplots with recovery again generally higher from the 80% WFPS treatment.
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K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
Table 4 The amounts of CH4, evolved or consumed, following application of fertiliser N to trash covered microplots in sugarcane rows. Treatments imposed were: (1) ammonium sulphate and urea applied at 160 kg N haÿ1, (2) urea applied at an initial rate of 80 kg N haÿ1 with a further 80 kg N haÿ1 added after the second intensive sampling, (3) water applied over 5 d at 4 times during each study period to achieve conditions of waterlogging and 80% WFPS and (4) natural moisture conditions between periods of intensive sampling. The negative signs indicate consumption of CH4. Values were obtained using an approximate integration to obtain the area under the curve Fertiliser
Resident time (d)
N rate (kg haÿ1)
Water treatment
CH4±C (g haÿ1)
Ammonium sulphate
104 104 143 143 143 143
160
WLa 80% WFPS WL 80% WFPS WL 80% WFPS
ÿ442 ÿ467 672 297 299 1005
Urea Urea a
160 2 80
WL=waterlogged.
4. Discussion The initial dierences in the total N2O emissions measured from the dierent N treatments were not unexpected. The quantity of N2O evolved during the process of nitri®cation is controlled by the soil N turnover rate (Davidson, 1993) whereas the nitrate available to the soil bacteria for conversion to the gaseous N forms controls the denitri®cation rate (Mosier et al., 1983). For both urea applications, nitri®cation had already occurred by 14 November, 1995 (Chapman et al., 1991), the diering rates of N2O evolution resulting from the dierent nitrate concentrations in the soil pro®le. Lower soil nitrate may also result in the conversion of some N2O to N2 (Blackmer and Bremner, 1978). For the ammonium sulphate treatment, nitri®cation was only beginning to occur with 86% of the mineral-N still in the ammonium form. This resulted in lower emissions of N2O. This disagrees with the ®ndings of Davidson et al. (1996) who found that 15 NH+ 4 was nitri®ed in both young and mature cane ®elds within 48 h of application. However, the 15N fertiliser was applied as a sprayed on solution whereas normal farm practice is to apply N fertiliser as a granular form. Hence, in the study of Davidson et al. (1996), the fertiliser N was in a form suitable for rapid nitri®cation by the soil bacteria. The heavy rainfall also in¯uenced total emission of N2O in early February. Diusion of N2O from ®ne textured soils into the atmosphere was delayed through longer retention of water in the smaller than average pores in a ®ne textured soil. Once drainage was complete, gaseous evolution occurred with the quantity of N2O observed dependent upon the amount of conversion of N2O to N2 in the soil pro®le (Smith and Arah, 1992). Matson et al. (1996) also found soil type to in¯uence N2O±N ¯uxes from Maui and Hawaiian sugarcane soils. However, the presence of a carbon source and the placement of fertiliser were greater in¯uences on the production of N2O from both areas.
The concentrations of 15N±N2O measured in the microplots formed a relatively small proportion of the total (14N+15N)±N2O evolved. This was also found at depth beneath a black earth (Weier et al., 1993) where the lower 15N±N2O concentrations were thought to be the result of immobilisation of the added N fertiliser. Although some immobilisation of added N fertiliser also occurred here, dilution of 15N±N2O with 14N± N2O, formed after nitri®cation of N mineralised from `native' and introduced soil organic matter, is another possibility. The addition of the water treatments to the formation microplots may have caused rapid NH+ 4 from mineralisation resulting in large pulses of N2O (Davidson et al., 1993). N2O production has been reported to be somewhat higher when the soil alternates between wet and dry cycles (Granli and Bockman, 1994). The similar rates of CH4 emission from the urea fertilised microplots, at both water contents, was unexpected as greater consumption rates generally occur from soils which retain their moisture, are intermittently ¯ooded and are therefore likely to possess methantrophic bacteria (Mosier et al., 1991; Nesbit and Breitenbeck, 1992). This was the case at this site in the previous year where high consumption rates were measured, apparently caused by increased atmospheric CH4 concentrations which resulted from bush®res in the hills surrounding the study area (Weier, 1996). However, CH4 consumption was found in microplots fertilised with ammonium sulphate, again at both water contents. Although this agrees with the ®ndings of Bronson and Mosier (1993), it contrasts with the results of Steudler et al. (1989)and Bronson and Mosier (1994). They all observed decreased CH4 consumption in the presence of NH+ 4 in laboratory and ®eld studies respectively. Total recovery of mineral-N after the ®rst 2 gas samplings was similar to that found in a glasshouse study but higher that that found in a ®eld study in northern NSW ( Weier et al., 1996). Immobilisation of
80
WL
80
WL
80
WL
b
WL=waterlogged. 80=80% water-®lled pore space. c n.d.=not determined.
a
25 Feb. 96
8 Jan. 96
17 Dec. 95
0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40
WLa
19 Nov. 95
80b
Depth (cm)
Treatment
Sampling Date 29.5 22.5 8.0 24.9 23.4 18.2 12.8 13.9 9.1 12.8 13.3 13.3 2.1 6.5 11.8 0.8 7.6 12.5 0.4 0.4 3.4 0.7 1.9 1.8
(2.9) (1.2) (2.9) (0.5) (1.3) (3.0) (2.6) (3.1) (2.2) (2.4) (1.6) (1.9) (0.9) (3.1) (0.9) (.04) (0.5) (1.8) (0.3) (0.2) (0.7) (0.4) (0.9) (0.5)
Bromide recovery (%)
4.4
4.2
20.9
20.4
39.4
35.8
66.5
60.0
Total Br recovery (%)
4.4
3.0
2.3
2.8
2.1
2.3
1.0
0.7
Trash N recovery (%)
n.d.
26.7 (0.2) 22.9 (5.6) nil 25.6 (0.1) 35.2 (2.7) nil 21.5 (3.0) 17.5 (1.6) 1.4 (0.4) 29.9 (0.7) 19.9 (2.2) 0.7 (0.2) 6.7 (3.3) 3.6 (0.6) 3.3 (2.0) 8.6 (1.4) 6.4 (2.0) 2.6 (0.9) n.d.c
Mineral N recovery (%)
n.d.
n.d.
17.6
13.6
50.5
40.4
60.8
49.6
Total mineral N recovery (%) 57.9 31.9 0.1 46.0 49.7 0.2 28.7 30.8 0.4 46.7 31.5 0.3 17.0 14.5 0.8 22.3 19.2 0.8 7.0 7.3 0.2 7.5 7.7 0.3
(0.8) (4.8) (0.01) (4.2) (6.0) (0.05) (3.5) (3.7) (0.1) (1.6) (1.9) (0.07) (3.1) (1.4) (0.4) (1.5) (2.4) (0.1) (0.1) (2.5) (0.02) (2.9) (1.2) (0.05)
Soil N recovery (%)
15.5
14.5
42.3
32.3
78.5
59.9
95.9
89.9
Total N recovery in soil (%)
19.9
17.5
44.6
35.1
80.6
62.2
96.9
90.6
Total 15N recovery in soil+trash (%)
Table 5 Percent recovery of bromide and 15N from soil and trash following the application of potassium bromide and 99% enriched ammonium sulphate on 13 November, 1995 to microplots placed in sugarcane rows at Ingham, north Queensland. Values in parentheses are standard errors
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941 1937
80
WL
80
WL
80
WL
b
WL=waterlogged. 80=80% water-®lled pore space. c n.d.=not determined.
a
25 Feb. 96
8 Jan. 96
17 Dec. 95
0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40
WLa
19 Nov. 95
80b
Depth (cm)
Treatment
Sampling date 26.2 23.2 8.3 33.8 25.9 10.7 17.7 14.4 8.0 19.6 15.4 4.5 6.0 14.1 18.2 9.0 17.9 14.5 0.4 1.8 4.4 1.3 1.4 6.4
(5.2) (1.9) (2.1) (3.2) (1.6) (2.8) (1.3) (1.1) (0.5) (0.5) (1.6) (1.2) (0.7) (1.0) (3.9) (2.1) (1.7) (4.2) (0.2) (0.3) (0.6) (0.4) (0.3) (1.8)
Bromide recovery (%)
9.1
6.6
41.4
38.3
39.5
40.1
70.4
57.7
Total Br recovery (%)
9.0
8.1
6.7
8.9
8.3
9.8
7.9
4.6
Trash N recovery (%)
n.d.
26.8 (3.6) 9.4 (0.4) nil 31.5 (2.9) 10.9 (1.4) nil 13.1 (2.2) 5.5 (0.8) nil 16.3 (3.4) 6.8 (1.1) nil 2.8 (1.7) 2.3 (0.7) nil 1.8 (0.9) 2.2 (0.9) nil n.d.c
Mineral N recovery (%)
n.d.
n.d.
4.0
5.1
23.1
18.6
42.4
36.2
Total mineral N recovery (%) 45.7 21.5 0.2 56.9 17.9 0.2 33.4 14.5 0.2 37.6 15.3 0.2 17.4 14.2 0.3 13.2 13.2 0.3 15.9 4.9 0.1 13.6 5.2 0.2
(3.2) (2.8) (0.05) (2.0) (1.9) (0.1) (3.6) (1.4) (0) (4.8) (0.8) (0.04) (1.6) (0.8) (0.07) (0.6) (2.0) (0.1) (4.8) (1.4) (0.03) (2.9) (0.6) (0.04)
Soil N recovery (%)
19.0
20.9
26.7
31.9
53.1
48.1
75.0
67.4
Total N recovery in soil (%)
28.0
29.0
33.4
40.8
61.4
57.9
82.9
72.0
Total 15N recovery in soil+trash (%)
Table 6 Percent recovery of bromide and 15N from soil and trash following the application of potassium bromide and 99% enriched urea on 5 October, 1995 to microplots placed in sugarcane rows at Ingham, north Queensland. Values in parentheses are standard errors
1938 K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
80
WL
80
WL
80
WL
b
WL=waterlogged. 80=80% water-®lled pore space. c n.d.=not determined.
a
25 Feb. 96
8 Jan. 96
17 Dec. 95
0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40
WLa
19 Nov 95
80b
Depth (cm)
Treatment
Sampling date 35.0 23.2 5.6 50.0 23.1 2.4 16.3 13.8 5.1 18.1 12.6 6.7 14.1 21.3 14.3 7.9 28.8 27.2 0.5 3.1 8.8 0.2 4.8 5.7
(3.7) (0.5) (1.2) (2.1) (0.4) (0.6) (3.7) (0.7) (0.2) (4.1) (1.6) (1.3) (4.6) (3.8) (8.3) (2.5) (5.2) (1.0) (0.2) (1.9) (2.5) (0.1) (2.0) (0.8)
Bromide recovery (%)
10.7
12.4
63.9
49.7
37.4
35.2
75.5
63.8
Total Br recovery (%)
6.8
8.4
6.5
6.0
11.1
7.0
9.8
7.9
Trash N recovery (%)
n.d.
23.6 (4.6) 7.5 (0.5) nil 37.7 (5.0) 5.5 (0.6) nil 10.7 (2.6) 4.5 (1.0) nil 22.2 (0.3) 4.1 (0.7) nil 7.5 (1.9) 7.2 (2.6) 2.2 (1.2) 11.3 (3.7) 11.1 (2.4) 7.1 (2.2) n.dc
Mineral N recovery (%)
n.d.
n.d.
29.5
16.9
26.3
15.2
43.2
31.1
Total mineral N recovery (%) 44.9 26.6 0.3 53.9 19.3 0.1 36.1 12.7 0.3 46.1 9.5 0.3 27.8 21.4 0.5 30.2 29.6 1.1 11.0 4.5 0.2 9.8 5.4 0.2
(1.3) (3.2) (0.06) (3.8) (2.3) (0.04) (4.2) (1.4) (0.03) (7.7) (2.2) (0.06) (5.9) (2.3) (0.3) (4.2) (3.4) (0.1) (1.3) (0.3) (0.02) (1.1) (0.2) (0.05)
Soil N recovery (%)
15.4
15.7
60.9
49.7
55.9
49.1
73.3
71.8
Total N recovery in soil (%)
22.2
24.1
67.4
55.7
67.0
56.1
83.1
79.7
Total 15N Recovery in soil+trash (%)
Table 7 Percent recovery of bromide and 15N from soil and trash following the application of potassium bromide and 99% enriched urea to microplots placed in sugarcane rows at Ingham, north Queensland. Half of the urea was applied on 5 October, 1995 with the 2nd application on 17 December, 1995. Values in parentheses are standard errors
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941 1939
1940
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
the added 15N occurred in that study as was the case here, with 1 to 11% found immobilised in the sugarcane trash placed on the soil surface and 19 to 41% found immobilised in the soil. The greatest values were found after the addition of urea, with between 8 and 18 kg N haÿ1 immobilised in the sugarcane trash and 36 to 65 kg N haÿ1 immobilised in the soil. This could also account for the dierences observed in the movement of nitrate and bromide down the microplot. The mineral-N percentages would suggest that very little movement of nitrate occurred out of the top 20 cm, whereas bromide concentrations would suggest some leaching of nitrate to 40 cm. Although the variation in N fertiliser type and application rate resulted in an initial decrease in N2O emissions, the changes did not result in reduced emissions during growth of the crop. The persistence of nitrate in the soil throughout the growing period presented opportunities for N2O emissions to occur once conditions were favourable. However, the quantities of N2O and CH4 measured emanating from the site in 1995±1996 were less than the quantities found in 1994 (Weier, 1996, 1998), with N2O emission down 46 to 90% and net consumption of CH4 less than 1.5%. This reduction in N2O evolution was due to the use of dierent calculation techniques. The 1994 estimates were obtained by extrapolating data from 10 d measurements to a year whereas the 1996 estimates were obtained by extrapolating data from measurements over 104 d (based on area under the curve) to a year. This exempli®es the need for more research similar to that conducted by Smith et al. (1994) where microplot measurements were compared with micrometeorological techniques and factors obtained that could be used to extrapolate from the microplot to the farm scale. Acknowledgements I thank Mr. C. McEwan and Mrs. M. Goode for their technical assistance, Mr. G. Morley for the use of his farm to conduct the study, the CSR technical ®eld sta for their assistance and Mr. M. Amato, CSIRO Land and Water, Adelaide for 15N±N2O analyses. The funding provided by the National Greenhouse Gas Inventory Committee was greatly appreciated. References Blackmer, A.M., Bremner, J.M., 1978. Inhibitory eect of nitrate on reduction of N2O to N2 by soil microorganisms. Soil Biology & Biochemistry 10, 187±191. Bouwman, A.F., 1990. Analysis of global nitrous oxide emissions from terrestrial natural and agro-ecosystems. In: Transactions 14th Congress Soil Science, Vol. 2, pp. 261±266.
Bronson, K.F., Mosier, A.R., 1993. Nitrous oxide emissions and methane consumption in wheat and corn-cropped systems in north-eastern Colorado. In: Harper, L.A., Mosier, A.R., Duxbury, J.M., Rolston, D.E. (Eds.), Agricultural Ecosystems Eects on Trace Gases and Global Climate Change. American Society of Agronomy, Madison, WI, pp. 133±144. Bronson, K.F., Mosier, A.R., 1994. Suppression of methane consumption in aerobic soil by nitrogen fertilisers, nitri®cation inhibitors and urease inhibitors. Biology and Fertility of Soils 17, 263± 268. Brooks, P.D., Stark, J.M., McInteer, B.B., Preston, T., 1989. Diusion method to prepare soil extracts for automated nitrogen15 analysis. Soil Science Society of America Journal 53, 1707± 1711. Catchpoole, V.R., Weier, K.L., 1980. Water pretreatment helps during extraction of mineral-N from a clay soil. Communications of Soil Science and Plant Analysis 11, 327±333. Chapman, L.S., Haysom, M.B.C., Sagna, P.G., Freney, J.R., 1991. The eect of placement and irrigation on the eciency of use of 15 N labelled urea by sugar cane. In: Egan, B.T. (Ed.), Proceedings Australian Society of Sugar Cane Technologists. pp 44±52. Bundaberg, Qld. Crutzen, P.J., Andreae, M.O., 1990. Biomass burning in the tropics: impact on atmospheric chemistry and biogeochemical cycles. Science 250, 1669±1678. Davidson, E.A., 1993. Soil water content and the ratio of nitrous oxide to nitric oxide emitted from soil. In: Uremland, R.S. (Ed.), Biogeochemistry of Global Change. Chapman and Hall, New York, pp. 369±386. Davidson, E.A., Matson, P.A., Vitousek, P.M., Riley, R., Dinkin, K., Garcia-Mendez, G., Maass, J.M., 1993. Processes regulating soil emissions of NO and N2O in a seasonally dry tropical forest. Ecology 74, 130±139. Davidson, E.A., Matson, P.A., Brooks, P.D., 1996. Nitrous oxide emission controls and inorganic nitrogen dynamics in fertilised tropical agricultural soils. Soil Science Society of America Journal 60, 1145±1152. Eichner, M.J., 1990. Nitrous oxide emissions from fertilized soil: summary of available data. Journal of Environmental Quality 19, 272±280. Galbally, I.E., Fraser, P.J., Meyer, C.P., Grith, D.W.T., 1992. Biosphere± atmosphere exchange of trace gases over Australia. In: Giord, R.M., Barson, M.M. (Eds.), Australia's Renewable Resources: Sustainability and Global Change. P.J. Grills, Commonwealth Government Printer, Canberra, pp. 117±149 IGBP Workshop No. 14. Granli, T., Bockman, O.C., 1994. Soil water content. In: Wiig, M. (Ed.), Nitrous Oxide from Agriculture. Norwegian Journal of Agricultural Sciences (Suppl. 12), 34±42. Henzell, E.F., Vallis, I., Lindquist, J.K., 1968. Automatic colorimetric methods for the determination of nitrogen in digests and extracts of soil. Transactions 9th International Congress of Soil Science. Adelaide, pp. 513±520. Howden, S.M., McKeon, G.M., Galbally, I.E., 1991. Methane: other agricultural sources. In: Tullberg, J.N., Pulsford, J., Russell, J.S. (Eds.), The Global Greenhouse Eect and the Impact of Emission Policies on Rural Industry. AIAS, Melbourne, pp. 22± 28 AIAS Occasional Publication No. 66. Intergovernmental Panel on Climate Change, 1994. Radiative forcing of climate change. The 1994 report of the scienti®c assessment working group of IPCC. Summary for policymakers. World Meteorology Organisation, UN Environmental Program, Geneva, Switzerland. Marti V.C., Arozarena C.E., 1981. Automated determination of bromide in water. Proceedings No. 734. Pittsburgh, pp. 523±528. Matson, P.A., Billow, C., Hall, S., 1996. Fertilization practices and
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941 soil variations control nitrogen oxide emissions from tropical sugar cane. Journal of Geophysical Research 101, 18533±18545. Mosier, A.R., Parton, W.J., Hutchinson, G.L., 1983. Modelling nitrous oxide evolution from cropped and native soils. Ecological Bulletins 35, 229±241. Mosier, A.R., Guenzi, W.D., Schweizer, E.E., 1986. Soil losses of dinitrogen and nitrous oxide from irrigated crops in north-eastern Colorado. Soil Science Society of America Journal 50, 344±348. Mosier, A., Schimel, D., Valentine, D., Bronson, K., Parton, W., 1991. Methane and nitrous oxide ¯uxes in native, fertilized and cultivated grasslands. Nature 350, 330±332. Nesbit, S.P., Breitenbeck, G.A., 1992. A laboratory study of factors in¯uencing methane uptake by soils. Agriculture, Ecosystems and Environment 41, 39±54. Northcote, K.H., 1965. CSIRO Australian Division of Soils, Divisional Report No. 2/65. Rodhe, H., 1990. A comparison of the contributions of various gases to the greenhouse eect. Science 248, 1217±1219. Smith, K.A., Arah, J.R.M., 1992. Measurement and modelling of nitrous oxide emissions from soils. Ecological Bulletin 42, 116± 123. Smith, K.A., Clayton, H., Arah, J.R.M., Christensen, S., Ambus, P., Fowler, D., Hargreaves, K.J., Skiba, U., Harris, G.W., Wienhold, F.G., Klemedtsson, L., Galle, B., 1994. Micrometeorological and chamber methods for measurement of nitrous oxide ¯uxes between soils and the atmosphere: overview and conclusions. Journal of Geophysical Research 99, 16541± 16548. Stace, H.C.T., Hubble, G.D., Brewer, R., Northcote, K.H., Sleeman, J.R., Mulcahy, M.J., Hallsworth, E.G., 1968. A Handbook of Australian Soils. Rellim Technical Publications, Glenside, SA. Steudler, P.A., Bowden, R.D., Mellilo, J.M., Aber, J.D., 1989.
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In¯uence of nitrogen fertilization on methane uptake in temperate forest soils. Nature (London) 341, 314±316. Stevens, R.J., Laughlin, R.J., Atkins, G.J., Prosser, S.J., 1993. Automated determination of nitrogen-15-labelled dinitrogen and nitrous oxide by mass spectrometry. Soil Science Society of America Journal 57, 981±988. Vallis, I., Keating, B.A., 1994. Uptake and loss of fertiliser and soil nitrogen in sugarcane crops. In: Egan, B.T. (Ed.), Proceedings of the Australian Society of Sugar cane Technologists. Townsville, Qld, pp. 105±113. Weier, K.L., 1996. Trace gas emissions from a trash blanketed sugarcane ®eld in tropical Australia. In: Wilson, J.R., Hogarth, D.M., Campbell, J.A., Garside, A.L. (Eds.), Sugarcane; Research towards Ecient and Sustainable Production. CSIRO Division of Tropical Crops and Pastures, Brisbane, pp. 271±272. Weier, K.L., 1998. Sugarcane ®elds; sources or sinks for greenhouse gas emissions? Australian Journal of Agricultural Research 49, 1± 9. Weier, K.L., MacRae, I.C., Myers, R.J.K., 1991. Seasonal variation in denitri®cation in a clay soil under a cultivated crop and a permanent pasture. Soil Biology & Biochemistry 23, 629±635. Weier, K.L., MacRae, I.C., Myers, R.K.J., 1993. Denitri®cation in a clay soil under pasture and annual crop: losses from 15N-labelled nitrate in the subsoil in the ®eld using C2H2 inhibition. Soil Biology & Biochemistry 25, 999±1004. Weier, K.L., McEwan, C.W., Vallis, I., Catchpoole, V.R., Myers, R.J.K., 1996. Potential for biological denitri®cation of fertiliser nitrogen in sugarcane soils. Australian Journal of Agricultural Research 47, 67±79. Weier, K.L., Rolston, D.E., Thorburn, P.J., 1998. The potential for N losses via denitri®cation beneath a green cane trash blanket. In: Hogarth, D.M. (Eds.), Proceedings of the Australian Society of Sugar Cane Technologists. Ballina, NSW, pp. 169±175.
www.elsevier.com/locate/soilbio
N2O and CH4 emission and CH4 consumption in a sugarcane soil after variation in nitrogen and water application K.L. Weier* CSIRO Tropical Agriculture, Cunningham Laboratory, 306 Carmody Road, St. Lucia, Brisbane, QLD 4067, Australia Accepted 22 June 1999
Abstract Nitrous oxide (N2O) and methane (CH4) exchanges between the soil and the atmosphere are in¯uenced by management practices currently used in agriculture. This study was conducted to determine whether N2O and CH4 emission and CH4 consumption in a trash blanketed sugarcane soil could be in¯uenced by changed N fertiliser practice and variation in soil water content. Microplots were placed in rows of sugarcane and urea and ammonium sulphate (both 99 atom % 15N excess) applied to the soil surface, urea as both a split application of 80 kg N haÿ1 and a full rate of 160 kg N haÿ1 and ammonium sulphate at 160 kg N haÿ1. Sugarcane trash (15 t haÿ1) was applied to the surface of all microplots. Water was then added to all microplots to achieve water contents of 80% water-®lled pore space (WFPS) in half the plots and saturation in the other half. Initially, the application of urea at 160 kg N haÿ1 to microplots at 80% WFPS resulted in greater emission of (14N+15N)±N2O than was recorded from the split urea and the ammonium sulphate treatments at both moisture regimes. A mean value of 47 g N2O±N haÿ1 dÿ1 was measured for this treatment compared with 18.5 and 15.2 g and 4.9 and 7.7 g N2O±N haÿ1 dÿ1 for the split urea and ammonium sulphate waterlogged and 80% WFPS treatments respectively. Greater emissions of (14N+15N)±N2O were measured following 132 mm rainfall in February, with mean values of 214 and 143 g N2O±N haÿ1 dÿ1 being recorded for the waterlogged treatments of the split urea and ammonium sulphate treatments respectively. A decrease in the 15N±N2O emission from the split urea treatment was the only dierence recorded when comparing the initial (14N+15N)±N2O and 15N±N2O emissions from all treatments. 14N±N2O appeared to be the major gaseous N product from this treatment initially. Total emission of 15N±N2O ranged from 105 to 453 g N2O±N haÿ1, which represented between 25.3 and 23.4% of the total amount of N2O evolved. CH4 emission occurred from all microplots fertilised with urea whereas CH4 consumption was measured in plots fertilised with ammonium sulphate only. CH4 emission ranged from 297 to 1005 g CH4±C haÿ1 while CH4 consumption ranged from 442 to 467 g CH4±C haÿ1. The application of ammonium sulphate and using a split application of urea initially decreased N2O emissions but the delay in nitri®cation obtained from changing the type and rate of application of these fertilisers resulted in greater N2O emissions later in the season when the soil moisture regime had changed. CH4 emission and consumption were not aected by these management changes. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Nitrous oxide; Methane; Emission; Consumption; Microplot; Fertiliser; Residue; Sugarcane
1. Introduction Soils in agricultural regions can be an important source of atmospheric nitrous oxide (N2O) and maybe sinks for atmospheric methane when they become aerated (Bouwman, 1990). Application of nitrogen fertili* Fax: +61-617-3214-2288. E-mail address: [email protected] (K.L. Weier)
sers to soils increase N2O emissions (Eichner, 1990) while ammonium and nitrate based fertilisers have been found to suppress and stimulate CH4 consumption, respectively (Nesbit and Breitenbeck, 1992; Bronson and Mosier, 1994). The concentration of the 2 gases in the atmosphere is increasing at an annual rate of 0.8 and 0.25% respectively (Intergovernmental Panel on Climate Change, 1994). Both gases are estimated to be collectively responsible for about 20% of anticipated global warming (Rodhe, 1990) with
0038-0717/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 0 7 1 7 ( 9 9 ) 0 0 1 1 1 - X
1932
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
Table 1 Characteristics of the soil underlying the sugarcane crop at the Ingham study site Site
Ingham a
Depth (cm)
0±5 5±20
BDa (Mg mÿ3)
1.18 1.48
Total N (%)
0.11 0.11
Total C (%)
1.47 1.55
Mineral-N (mg kgÿ1)
Textural Analysis (%)
NO3±N
NH4±N
sand
silt
clay
0.04 0.10
0.18 0.22
30 32
29 27
40 41
BD=bulk density.
Australia's contribution to this global increase about 958 kt N2O±N yrÿ1 and 6.5 Mt CH4 yrÿ1 respectively (Galbally et al., 1992). The net contribution of Australian agriculture to these totals has been estimated at about 140 and 2049 kt yrÿ1 respectively (Howden et al., 1991; Galbally et al., 1992). The contribution from the Australian sugar industry to these estimates has been largely unknown. Values for N2O emissions have been calculated from denitri®cation losses using an approximate N budget for the crop (Vallis and Keating, 1994) However, this required some knowledge of the proportion of gaseous N loss that was occurring as N2O and N2 and this has only recently been documented (Weier et al., 1996; 1998). For CH4, values both in Australia and globally, have only been calculated from the burning of sugarcane with CH4 emission occurring during the smouldering phase (Crutzen and Andreae, 1990). However, there are no known recorded measurements of CH4 production from waterlogged sugarcane ®elds or from areas where trash blanketing is the normal management practice. In the only study so far that has attempted to document N2O emission and CH4 emission and consumption from sugarcane areas, Weier (1998) estimated that, for the 1994 season, a total of 10.7 kt N2O±N yrÿ1 was emitted from bare soils, trash blanketed soils and after burning. For CH4, 6.7 kt CH4±C yrÿ1 was emitted after burning and 34 kt CH4±C yrÿ1 was consumed by the trash blanket. Net uptake was therefore occurring in sugarcane ®elds. My objective was to determine the eect of dierent N fertilisers and dierent rates of N application on N2O and CH4 ¯uxes from trash blanketed sugarcane soils. These eects were investigated at 2 soil moisture contents.
2. Materials and methods 2.1. Study site The experimental site was established on a sugarcane farm, 21 km west of Ingham (18.7S, 146.2E), north Queensland, following harvesting of the crop at the
end of September. The climate of the area is tropical with a mean annual rainfall of 2275 mm. Site selection was based on soil type and the use of a trash blanket as a management practice. The soil type was a bleached grey clay (Stace et al., 1968), of principal pro®le form Ug 3.2 (Northcote, 1965), some characteristics of which are given in Table 1. 2.2. Field procedure Microplots were formed by pushing PVC cylinders (23.5 cm i.d. by 25 cm long) into the soil to a depth of 20 cm between the stools in the rows of sugarcane. The experimental design was a factorial with 2 N fertilisers at 3 rates, 2 soil water contents, 4 intensive sampling periods and 3 replications, making a total of 72 microplots. On 5 October, 1995, urea (99 atom % 15 N excess) was broadcast on the soil surface of 48 microplots, with 24 receiving 160 kg N haÿ1 and 24 receiving 80 kg N haÿ1. A further 80 kg N haÿ1 was applied to the latter microplots on 17 December, 1995, following the second intensive sampling period. Sugarcane trash (15 t haÿ1) was applied to the soil surface of the 48 microplots. On 13 November, 1995, ammonium sulphate (99 atom % 15N excess) was broadcast on the soil surface of the remaining 24 microplots at a rate of 160 kg N haÿ1. Sugarcane trash (15 t haÿ1) was immediately placed on the soil surface of these microplots. Potassium bromide (200 kg Brÿ haÿ1) was added to the soil surface of all microplots, at the same time as the N fertiliser, as a guide to the extent of solute leaching of nitrate below the end of the microplot. Water was added to all 72 microplots following the 14.00 h gas sampling on d 1 and similarly on d 2, 3 and 4. Thirty-six microplots received enough water to achieve saturation and 36 received sucient water to acquire a water-®lled pore space (WFPS) of 80% to the depth of the microplot base. In each case, 24 of the 36 were urea treated and 12 ammonium sulphate treated. Water-®lled pore space is synonymous with relative saturation and was calculated as WFPS=[(gravimetric water content soil bulk density)/total soil porosity], where soil porosity=[1ÿsoil bulk density/2.65] and 2.65 equals the assumed particle density of soil (Mg mÿ3). Rainfall
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
1933
15
N±(N2O+N2) by isotope mass spectrometry (Mosier et al., 1986). Total (14N+15N)±N2O and 15N±N2O emission and CH4 consumption and emission from the microplots over the 104 d study was calculated by applying the Trapezoidal Rule (Origin 4.0, Microcal Software Inc., MA) which uses an approximate integration to calculate the area under the curve. After each intensive 5 d sampling, 18 soil cores (6 from each fertiliser N treatment) were destructively sampled and returned to the laboratory for analysis. Total N and C in the soil and trash were measured by dry combustion using a Europa ANA mass spectrometer. Brÿ was determined colorimetrically after extraction of the soil with 10 mM MgSO4 (Marti and Arozarena, 1981). Mineral N was determined after extraction of the soil with 2 M KCl (Catchpoole and Weier, 1980) and NO3±N and NH4±N measured colorimetrically (Henzell et al., 1968). 15N±(NO3+NH4) was measured by diusion using the procedure of Brooks et al. (1989). 3. Results 3.1. Nitrous oxide (unlabelled and labelled)
Fig. 1. (14N+15N)±N2O emission over 104 d from microplots placed in sugarcane rows within a green-cane, trash blanketed ®eld. Microplots were fertilised with: (a) ammonium sulphate at 160 kg N haÿ1 (b) urea at 80 kg N haÿ1 on 5 Oct. 95 and again on 17 Dec. 95 and (c) urea at 160 kg N haÿ1. Rainfall over the sampling period is shown at the top of the graph. Vertical bars show standard errors of the mean [WL=waterlogged; 80=80% WFPS].
at the experimental site was recorded over the duration of the study. Soil temperature was recorded at each gas sampling using thermometers inserted 5 cm below the soil surface. Gas sampling commenced at 06.00 h on 14 November, 1995 (d 1). Covers were placed on the microplots for 1 h and gas samples taken from the headspace after 0 and 60 min. This procedure was repeated every 8 h for 5 d. Further 5 d periods of intensive gas sampling occurred from 12±16 December, 1995; 3±7 January, 1996 and 20±24 February, 1996, during which water was applied as above. During the weeks between intensive gas samplings, gas samples were collected at 06.00 h on Tuesday and Friday of each intervening week. However, no water treatments were reimposed on the plots for the bi-weekly samplings between intensive measurements. The gas samples were analysed for N2O and CH4 by g.c. (Mosier et al., 1991; Weier et al., 1991), 15N±N2O by Europa Trace Gas Analyzer (Stevens et al., 1993) and
Initial (14N+15N)±N2O emission, over the ®rst 5 d of intensive gas sampling, was greatest from microplots at 80% WFPS and fertilised with urea at 160 kg N haÿ1 (Fig. 1). Values ranged from 9 to 142 g N2O±N haÿ1 dÿ1, with a mean value of 47 210 g N2O±N haÿ1 dÿ1. For the urea treatment, with a split application of fertiliser, average values of 18.5 2 10.5 and 15.2 2 8.0 g N2O±N haÿ1 dÿ1 were obtained for the waterlogged and 80% WFPS treatments, respectively. However, at the ®nal sampling in this period, ¯uxes of 164 and 127 g N2O±N haÿ1 dÿ1 were observed from the respective water treatments. For the microplots fertilised with ammonium sulphate, values from the waterlogged and 80% WFPS treatments ranged from 0.8 to 11.2 (mean 4.9 20.8) and 2.8 to 15.3 (mean 7.7 21.2) g N2O±N haÿ1 dÿ1, respectively. Except for a ¯ux of 248 g N2O±N haÿ1 dÿ1 at the end of November from the 80% WFPS treatment receiving the split N application, a period of low N2O evolution then occurred from all microplots. In early January, 1996, when water application for the third intensive gas sampling was supplemented by rainfall, a maximum ¯ux of 144 g N2O±N haÿ1 dÿ1 was observed from the 80% WFPS urea treatment receiving 160 kg N haÿ1. Fluxes of 150 and 110 g N2O±N haÿ1 dÿ1 were also measured from the waterlogged and 80% WFPS split urea treatments respectively. However, the greatest emissions occurred from waterlogged microplots following 132 mm rainfall at the beginning of February 1996. Evolution of N2O did
1934
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
Table 2 The amounts of (14N+15N)±N2O and 15N±N2O evolved following application of fertiliser N to trash covered microplots in sugarcane rows. Treatments imposed were: (1) ammonium sulphate and urea applied at 160 kg N haÿ1, (2) urea applied at an initial rate of 80 kg N haÿ1 with a further 80 kg N haÿ1 added after the second intensive sampling, (3) water applied over 5 d at 4 times during each study period to achieve conditions of waterlogging and 80% WFPS and (4) natural moisture conditions between periods of intensive sampling. Values were obtained using an approximate integration to obtain the area under the curve Fertiliser
Resident time (d)
N rate (kg haÿ1)
Water treatment
15
(14N+15N)±N2O (g haÿ1)
Ammonium sulphate
104 104 143 143 143 143
160
WLa 80% WFPS WL 80% WFPS WL 80% WFPS
154 241 105 362 453 118
1082 951 415 1143 1934 1562
Urea Urea a
160 2 80
N±N2O (g haÿ1)
WL=waterlogged.
not occur immediately after rainfall ceased, but 7 d later for all treatments. The largest values were recorded from microplots, which received a second application of urea in December, 1995, with a peak emission of 317 g N2O±N haÿ1 dÿ1 and a mean value of
214 2 89 g N2O±N haÿ1 dÿ1. For microplots fertilised with ammonium sulphate, the mean value was 143 2 162 g N2O±N haÿ1 dÿ1 even though a peak emission of 408 g N2O±N haÿ1 dÿ1 was recorded. Much lower emissions were recorded for microplots, which were fertilised with urea at 160 kg N haÿ1. Total emission of (14N+15N)±N2O from the microplots over the study period ranged from 415 g N2O±N haÿ1 for the waterlogged urea 160 kg N haÿ1 treatment to 1934 g N2O±N haÿ1 for the waterlogged, split urea treatment (Table 2). 3.2. N-labelled gases 15
Fig. 2. 15N±N2O emission over 104 d from microplots placed in sugarcane rows within a green-cane, trash blanketed ®eld. Microplots were fertilised with: (a) ammonium sulphate at 160 kg N haÿ1 (b) urea at 80 kg N haÿ1 on 5 Oct 95 and again on 17 Dec 95 and (c) urea at 160 kg N haÿ1. Rainfall over the sampling period is shown at the top of the graph. Vertical bars show standard errors of the mean [WL=waterlogged; 80=80% WFPS].
N±N2O emission from the microplots virtually followed the same pattern observed for total emission of (14N+15N)±N2O, with the only dierence occurring in initial N2O evolution from the split urea application (Fig. 2). Here, the initial N2O ¯uxes, at both water contents, had disappeared which indicated that all measured N2O ¯uxes had occurred as 14N±N2O. Total emission of 15N±N2O from all microplots over the 104 d study ranged from 105 to 453 g N2O±N haÿ1 which comprised 25.3 and 23.4% of the total N2O emissions (Table 2). The in¯uence of soil moisture on the gaseous loss process can be seen from the quantities of (14N+15N)±N2O, 15N±N2O and 15N± (N2O+N2) evolved from the microplots at 06.00 h on 13 February, 1996 (Table 3). 15N±N2O emission was not large, comprising between 3.6 and 36.2% of the total (14N+15N)±N2O evolved and between 0.05 and 14.2% of the 15N±(N2O+N2) evolved. This suggests that, under these conditions, 14N±N2O was the major component of N2O being evolved and that 15N±N2, produced following denitri®cation, was the major gaseous N product. 3.3. Methane emission and consumption Methane emission occurred from the urea 160 kg N haÿ1 treatment during the ®rst and second intensive
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
1935
Table 3 The quantities of (14N+15N)±N2O, 15N±N2O and 15N±(N2O+N2) evolved from the N fertilised, trash covered microplots at 06.00 h on Tuesday, 13 February, 1996 following 132 mm of rainfall in the preceding week. Values are the means of 3 replications with standard errors in parentheses Fertiliser
N rate (kg haÿ1)
Water treatment
(14N+15N)±N2O (g N haÿ1 dÿ1)
15
N±N2O (g N haÿ1 dÿ1)
15
Ammonium sulphate
160
Urea
160
Urea
2 80
WLa 80% WFPS WL 80% WFPS WL 80% WFPS
143 (132) 58 (40) 5 (2) 11 (3) 214 (73) 13 (4)
26 (25) 21 (14) 1 (0.6) 0.4 (0.1) 61 (25) 3 (2)
183 569 1024 764 1334 1185
a
N±(N2O+N2) (g N haÿ1 dÿ1) (74) (344) (381) (460) (209) (287)
WL=waterlogged.
gas samplings following water application to the microplots and in the intervening period between these samplings (Fig. 3). Total emissions during this period were 651 g CH4±C haÿ1 for the waterlogged treatment and 438 g CH4±C haÿ1 for the 80% WFPS treatment. Over the remainder of the study, only limited periods occurred when either consumption or emission of CH4 was occurring. For the ammonium sulphate and split
urea treatments, some consumption and emission occurred in the former whereas CH4 emission occurred mainly from the latter. Consumption of CH4 occurred during the study period in microplots fertilised with ammonium sulphate, with values ranging from 442 to 467 g CH4±C haÿ1 (Table 4). For the other 2 treatments, CH4 was evolved from the microplots with concentrations between 297 and 1005 g CH4±C haÿ1. There was no signi®cant eect of water treatments on CH4 consumption in the microplots fertilised with ammonium sulphate. However, large, but inconsistent dierences in CH4 emissions were found between water treatments for microplots fertilised with urea. 3.4.
Fig. 3. CH4 consumption and emission over 104 d from microplots placed in sugarcane rows within a green-cane, trash blanketed ®eld. Microplots were fertilised with: (a) ammonium sulphate at 160 kg N haÿ1 (b) urea at 80 kg N haÿ1 on 5 Oct. 95 and again on 17 Dec. 95 and (c) urea at 160 kg N haÿ1. Rainfall over the sampling period is shown at the top of the graph. Vertical bars show standard errors of the mean [WL=waterlogged; 80=80% WFPS; se=standard error].
15
N recovery
Total recovery of 15N from all microplots for all treatments decreased over time, with recoveries generally higher from microplots, which were brought to a moisture content of 80% WFPS (Tables 5±7). Values over the 4 samplings for this treatment ranged from 97 to 20% for ammonium sulphate, 83 to 28% for urea 160 and 83 to 22% for the split urea application. 15N recovery from ammonium sulphate was greatest after the ®rst 2 intensive samplings, although recoveries from both urea treatments were good considering that N fertiliser application had occurred 45 and 73 d previously. Recoveries of 15N from the ammonium sulphate and urea 160 kg N haÿ1 treatments after the third sampling were similar whereas more 15N remained in the soil after the ®nal sampling for urea 160 kg N haÿ1 than for ammonium sulphate. 15N recoveries from the split urea treatment at the third sampling were higher than for the other treatments, but the percentage had decreased to a value below that of the urea 160 kg N haÿ1 treatment at the ®nal sampling. There were also signi®cant dierences between the recovery of bromide and mineral-N from the microplots with recovery again generally higher from the 80% WFPS treatment.
1936
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
Table 4 The amounts of CH4, evolved or consumed, following application of fertiliser N to trash covered microplots in sugarcane rows. Treatments imposed were: (1) ammonium sulphate and urea applied at 160 kg N haÿ1, (2) urea applied at an initial rate of 80 kg N haÿ1 with a further 80 kg N haÿ1 added after the second intensive sampling, (3) water applied over 5 d at 4 times during each study period to achieve conditions of waterlogging and 80% WFPS and (4) natural moisture conditions between periods of intensive sampling. The negative signs indicate consumption of CH4. Values were obtained using an approximate integration to obtain the area under the curve Fertiliser
Resident time (d)
N rate (kg haÿ1)
Water treatment
CH4±C (g haÿ1)
Ammonium sulphate
104 104 143 143 143 143
160
WLa 80% WFPS WL 80% WFPS WL 80% WFPS
ÿ442 ÿ467 672 297 299 1005
Urea Urea a
160 2 80
WL=waterlogged.
4. Discussion The initial dierences in the total N2O emissions measured from the dierent N treatments were not unexpected. The quantity of N2O evolved during the process of nitri®cation is controlled by the soil N turnover rate (Davidson, 1993) whereas the nitrate available to the soil bacteria for conversion to the gaseous N forms controls the denitri®cation rate (Mosier et al., 1983). For both urea applications, nitri®cation had already occurred by 14 November, 1995 (Chapman et al., 1991), the diering rates of N2O evolution resulting from the dierent nitrate concentrations in the soil pro®le. Lower soil nitrate may also result in the conversion of some N2O to N2 (Blackmer and Bremner, 1978). For the ammonium sulphate treatment, nitri®cation was only beginning to occur with 86% of the mineral-N still in the ammonium form. This resulted in lower emissions of N2O. This disagrees with the ®ndings of Davidson et al. (1996) who found that 15 NH+ 4 was nitri®ed in both young and mature cane ®elds within 48 h of application. However, the 15N fertiliser was applied as a sprayed on solution whereas normal farm practice is to apply N fertiliser as a granular form. Hence, in the study of Davidson et al. (1996), the fertiliser N was in a form suitable for rapid nitri®cation by the soil bacteria. The heavy rainfall also in¯uenced total emission of N2O in early February. Diusion of N2O from ®ne textured soils into the atmosphere was delayed through longer retention of water in the smaller than average pores in a ®ne textured soil. Once drainage was complete, gaseous evolution occurred with the quantity of N2O observed dependent upon the amount of conversion of N2O to N2 in the soil pro®le (Smith and Arah, 1992). Matson et al. (1996) also found soil type to in¯uence N2O±N ¯uxes from Maui and Hawaiian sugarcane soils. However, the presence of a carbon source and the placement of fertiliser were greater in¯uences on the production of N2O from both areas.
The concentrations of 15N±N2O measured in the microplots formed a relatively small proportion of the total (14N+15N)±N2O evolved. This was also found at depth beneath a black earth (Weier et al., 1993) where the lower 15N±N2O concentrations were thought to be the result of immobilisation of the added N fertiliser. Although some immobilisation of added N fertiliser also occurred here, dilution of 15N±N2O with 14N± N2O, formed after nitri®cation of N mineralised from `native' and introduced soil organic matter, is another possibility. The addition of the water treatments to the formation microplots may have caused rapid NH+ 4 from mineralisation resulting in large pulses of N2O (Davidson et al., 1993). N2O production has been reported to be somewhat higher when the soil alternates between wet and dry cycles (Granli and Bockman, 1994). The similar rates of CH4 emission from the urea fertilised microplots, at both water contents, was unexpected as greater consumption rates generally occur from soils which retain their moisture, are intermittently ¯ooded and are therefore likely to possess methantrophic bacteria (Mosier et al., 1991; Nesbit and Breitenbeck, 1992). This was the case at this site in the previous year where high consumption rates were measured, apparently caused by increased atmospheric CH4 concentrations which resulted from bush®res in the hills surrounding the study area (Weier, 1996). However, CH4 consumption was found in microplots fertilised with ammonium sulphate, again at both water contents. Although this agrees with the ®ndings of Bronson and Mosier (1993), it contrasts with the results of Steudler et al. (1989)and Bronson and Mosier (1994). They all observed decreased CH4 consumption in the presence of NH+ 4 in laboratory and ®eld studies respectively. Total recovery of mineral-N after the ®rst 2 gas samplings was similar to that found in a glasshouse study but higher that that found in a ®eld study in northern NSW ( Weier et al., 1996). Immobilisation of
80
WL
80
WL
80
WL
b
WL=waterlogged. 80=80% water-®lled pore space. c n.d.=not determined.
a
25 Feb. 96
8 Jan. 96
17 Dec. 95
0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40
WLa
19 Nov. 95
80b
Depth (cm)
Treatment
Sampling Date 29.5 22.5 8.0 24.9 23.4 18.2 12.8 13.9 9.1 12.8 13.3 13.3 2.1 6.5 11.8 0.8 7.6 12.5 0.4 0.4 3.4 0.7 1.9 1.8
(2.9) (1.2) (2.9) (0.5) (1.3) (3.0) (2.6) (3.1) (2.2) (2.4) (1.6) (1.9) (0.9) (3.1) (0.9) (.04) (0.5) (1.8) (0.3) (0.2) (0.7) (0.4) (0.9) (0.5)
Bromide recovery (%)
4.4
4.2
20.9
20.4
39.4
35.8
66.5
60.0
Total Br recovery (%)
4.4
3.0
2.3
2.8
2.1
2.3
1.0
0.7
Trash N recovery (%)
n.d.
26.7 (0.2) 22.9 (5.6) nil 25.6 (0.1) 35.2 (2.7) nil 21.5 (3.0) 17.5 (1.6) 1.4 (0.4) 29.9 (0.7) 19.9 (2.2) 0.7 (0.2) 6.7 (3.3) 3.6 (0.6) 3.3 (2.0) 8.6 (1.4) 6.4 (2.0) 2.6 (0.9) n.d.c
Mineral N recovery (%)
n.d.
n.d.
17.6
13.6
50.5
40.4
60.8
49.6
Total mineral N recovery (%) 57.9 31.9 0.1 46.0 49.7 0.2 28.7 30.8 0.4 46.7 31.5 0.3 17.0 14.5 0.8 22.3 19.2 0.8 7.0 7.3 0.2 7.5 7.7 0.3
(0.8) (4.8) (0.01) (4.2) (6.0) (0.05) (3.5) (3.7) (0.1) (1.6) (1.9) (0.07) (3.1) (1.4) (0.4) (1.5) (2.4) (0.1) (0.1) (2.5) (0.02) (2.9) (1.2) (0.05)
Soil N recovery (%)
15.5
14.5
42.3
32.3
78.5
59.9
95.9
89.9
Total N recovery in soil (%)
19.9
17.5
44.6
35.1
80.6
62.2
96.9
90.6
Total 15N recovery in soil+trash (%)
Table 5 Percent recovery of bromide and 15N from soil and trash following the application of potassium bromide and 99% enriched ammonium sulphate on 13 November, 1995 to microplots placed in sugarcane rows at Ingham, north Queensland. Values in parentheses are standard errors
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941 1937
80
WL
80
WL
80
WL
b
WL=waterlogged. 80=80% water-®lled pore space. c n.d.=not determined.
a
25 Feb. 96
8 Jan. 96
17 Dec. 95
0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40
WLa
19 Nov. 95
80b
Depth (cm)
Treatment
Sampling date 26.2 23.2 8.3 33.8 25.9 10.7 17.7 14.4 8.0 19.6 15.4 4.5 6.0 14.1 18.2 9.0 17.9 14.5 0.4 1.8 4.4 1.3 1.4 6.4
(5.2) (1.9) (2.1) (3.2) (1.6) (2.8) (1.3) (1.1) (0.5) (0.5) (1.6) (1.2) (0.7) (1.0) (3.9) (2.1) (1.7) (4.2) (0.2) (0.3) (0.6) (0.4) (0.3) (1.8)
Bromide recovery (%)
9.1
6.6
41.4
38.3
39.5
40.1
70.4
57.7
Total Br recovery (%)
9.0
8.1
6.7
8.9
8.3
9.8
7.9
4.6
Trash N recovery (%)
n.d.
26.8 (3.6) 9.4 (0.4) nil 31.5 (2.9) 10.9 (1.4) nil 13.1 (2.2) 5.5 (0.8) nil 16.3 (3.4) 6.8 (1.1) nil 2.8 (1.7) 2.3 (0.7) nil 1.8 (0.9) 2.2 (0.9) nil n.d.c
Mineral N recovery (%)
n.d.
n.d.
4.0
5.1
23.1
18.6
42.4
36.2
Total mineral N recovery (%) 45.7 21.5 0.2 56.9 17.9 0.2 33.4 14.5 0.2 37.6 15.3 0.2 17.4 14.2 0.3 13.2 13.2 0.3 15.9 4.9 0.1 13.6 5.2 0.2
(3.2) (2.8) (0.05) (2.0) (1.9) (0.1) (3.6) (1.4) (0) (4.8) (0.8) (0.04) (1.6) (0.8) (0.07) (0.6) (2.0) (0.1) (4.8) (1.4) (0.03) (2.9) (0.6) (0.04)
Soil N recovery (%)
19.0
20.9
26.7
31.9
53.1
48.1
75.0
67.4
Total N recovery in soil (%)
28.0
29.0
33.4
40.8
61.4
57.9
82.9
72.0
Total 15N recovery in soil+trash (%)
Table 6 Percent recovery of bromide and 15N from soil and trash following the application of potassium bromide and 99% enriched urea on 5 October, 1995 to microplots placed in sugarcane rows at Ingham, north Queensland. Values in parentheses are standard errors
1938 K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
80
WL
80
WL
80
WL
b
WL=waterlogged. 80=80% water-®lled pore space. c n.d.=not determined.
a
25 Feb. 96
8 Jan. 96
17 Dec. 95
0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40 0±5 5±20 20±40
WLa
19 Nov 95
80b
Depth (cm)
Treatment
Sampling date 35.0 23.2 5.6 50.0 23.1 2.4 16.3 13.8 5.1 18.1 12.6 6.7 14.1 21.3 14.3 7.9 28.8 27.2 0.5 3.1 8.8 0.2 4.8 5.7
(3.7) (0.5) (1.2) (2.1) (0.4) (0.6) (3.7) (0.7) (0.2) (4.1) (1.6) (1.3) (4.6) (3.8) (8.3) (2.5) (5.2) (1.0) (0.2) (1.9) (2.5) (0.1) (2.0) (0.8)
Bromide recovery (%)
10.7
12.4
63.9
49.7
37.4
35.2
75.5
63.8
Total Br recovery (%)
6.8
8.4
6.5
6.0
11.1
7.0
9.8
7.9
Trash N recovery (%)
n.d.
23.6 (4.6) 7.5 (0.5) nil 37.7 (5.0) 5.5 (0.6) nil 10.7 (2.6) 4.5 (1.0) nil 22.2 (0.3) 4.1 (0.7) nil 7.5 (1.9) 7.2 (2.6) 2.2 (1.2) 11.3 (3.7) 11.1 (2.4) 7.1 (2.2) n.dc
Mineral N recovery (%)
n.d.
n.d.
29.5
16.9
26.3
15.2
43.2
31.1
Total mineral N recovery (%) 44.9 26.6 0.3 53.9 19.3 0.1 36.1 12.7 0.3 46.1 9.5 0.3 27.8 21.4 0.5 30.2 29.6 1.1 11.0 4.5 0.2 9.8 5.4 0.2
(1.3) (3.2) (0.06) (3.8) (2.3) (0.04) (4.2) (1.4) (0.03) (7.7) (2.2) (0.06) (5.9) (2.3) (0.3) (4.2) (3.4) (0.1) (1.3) (0.3) (0.02) (1.1) (0.2) (0.05)
Soil N recovery (%)
15.4
15.7
60.9
49.7
55.9
49.1
73.3
71.8
Total N recovery in soil (%)
22.2
24.1
67.4
55.7
67.0
56.1
83.1
79.7
Total 15N Recovery in soil+trash (%)
Table 7 Percent recovery of bromide and 15N from soil and trash following the application of potassium bromide and 99% enriched urea to microplots placed in sugarcane rows at Ingham, north Queensland. Half of the urea was applied on 5 October, 1995 with the 2nd application on 17 December, 1995. Values in parentheses are standard errors
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941 1939
1940
K.L. Weier / Soil Biology and Biochemistry 31 (1999) 1931±1941
the added 15N occurred in that study as was the case here, with 1 to 11% found immobilised in the sugarcane trash placed on the soil surface and 19 to 41% found immobilised in the soil. The greatest values were found after the addition of urea, with between 8 and 18 kg N haÿ1 immobilised in the sugarcane trash and 36 to 65 kg N haÿ1 immobilised in the soil. This could also account for the dierences observed in the movement of nitrate and bromide down the microplot. The mineral-N percentages would suggest that very little movement of nitrate occurred out of the top 20 cm, whereas bromide concentrations would suggest some leaching of nitrate to 40 cm. Although the variation in N fertiliser type and application rate resulted in an initial decrease in N2O emissions, the changes did not result in reduced emissions during growth of the crop. The persistence of nitrate in the soil throughout the growing period presented opportunities for N2O emissions to occur once conditions were favourable. However, the quantities of N2O and CH4 measured emanating from the site in 1995±1996 were less than the quantities found in 1994 (Weier, 1996, 1998), with N2O emission down 46 to 90% and net consumption of CH4 less than 1.5%. This reduction in N2O evolution was due to the use of dierent calculation techniques. The 1994 estimates were obtained by extrapolating data from 10 d measurements to a year whereas the 1996 estimates were obtained by extrapolating data from measurements over 104 d (based on area under the curve) to a year. This exempli®es the need for more research similar to that conducted by Smith et al. (1994) where microplot measurements were compared with micrometeorological techniques and factors obtained that could be used to extrapolate from the microplot to the farm scale. Acknowledgements I thank Mr. C. McEwan and Mrs. M. Goode for their technical assistance, Mr. G. Morley for the use of his farm to conduct the study, the CSR technical ®eld sta for their assistance and Mr. M. Amato, CSIRO Land and Water, Adelaide for 15N±N2O analyses. The funding provided by the National Greenhouse Gas Inventory Committee was greatly appreciated. References Blackmer, A.M., Bremner, J.M., 1978. Inhibitory eect of nitrate on reduction of N2O to N2 by soil microorganisms. Soil Biology & Biochemistry 10, 187±191. Bouwman, A.F., 1990. Analysis of global nitrous oxide emissions from terrestrial natural and agro-ecosystems. In: Transactions 14th Congress Soil Science, Vol. 2, pp. 261±266.
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