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Effects of nitrogen fertiliser and wheat straw application on C[H.sub.4] and [N.sub.2]O emissions from a paddy rice field.

Introduction

Both C[H.sub.4] and [N.sub.2]O have been listed as controlled greenhouse gases following C[O.sub.2] in Kyoto Protocol. According to Climate Change 2001, C[H.sub.4] and [N.sub.2]O have 23 and 296 times higher global warming potential (GWP) than C[O.sub.2] on a 100-year horizon, respectively (IPCC 2001). Moreover, increasing atmospheric [N.sub.2]O is considered as an important factor in ozone depletion. It is estimated that doubling the concentration of [N.sub.2]O in the atmosphere would result in a 10% decrease in the ozone layer (Crutzen 1991).

Because of the unique nature of rice production, typically flooded soils and relatively high N input, the potential for significant emission of C[H.sub.4] during flooded periods and [N.sub.2]O emissions during non-flooded periods exists (Mosier et al. 2004). Rice fields have been identified as a major source of increasing atmospheric C[H.sub.4], accounting for about 15-20% (Sass and Fisher 1997) of global C[H.sub.4] emissions from all sources. Rice fields were also confirmed to be an important source of atmospheric [N.sub.2]O (Chen et al. 1997; Xu et al. 1997; Cai et al. 1999; Yu et al. 2004a), although the contribution was not as high as upland soil (Xing 1998). Therefore, it is essential to simultaneously measure C[H.sub.4] and [N.sub.2]O emissions from rice fields.

In China, the annual crop straw production is 7.94 x [10.sup.8] t and increases steadily at a yearly rate of 1.25 x [10.sup.7] t (Zhong et al. 2003). Returning crop straw back to field rather than burning it was highly recommended in China as a measure to increase the soil fertility in the long term and protect the gradually deteriorating environment. It is usually recommended that a certain amount of nitrogen fertiliser should be added to keep soil carbon-nitrogen balance while returning crop straw to the field. There is no doubt that the application of nitrogen fertiliser and straw has effects on C[H.sub.4] and [N.sub.2]O emissions. However, the reported results with regard to the effects of straw incorporation on [N.sub.2]O emission and urea application on C[H.sub.4] emission were conflicting. Huang et al. (1999) reported that the allelochemicals produced from decomposition of crop residue could inhibit soil [N.sub.2]O production. Cai et al. (2001) observed that evolved [N.sub.2]O was significantly less with straw addition than without straw amendment in the 70% water-holding capacity treatments, and Jiang et al. (2003) found that straw incorporation depressed [N.sub.2]O emission from a rice paddy field. In contrast, Avalakki et al. (1995) and Zou et al. (2001) reported that the addition of wheat straw to soil stimulated denitrification and subsequent [N.sub.2]O emissions. Baggs et al. (2000) and Huang et al. (2004) found that incorporation of crop residue enhanced [N.sub.2]O emissions. As for urea, Lindau et al. (1991) reported that C[H.sub.4] emission increased with the increase in application rate of urea, and Wang et al. (1992) found that the increase in soil pH following urea hydrolysis favoured C-H.sub.4] production. However, Cai et al. (1997, 2000) observed that urea application depressed C[H.sub.4] emission. Therefore, further studies on C[H.sub.4] and [N.sub.2]O emissions as affected by nitrogen fertiliser and straw application are still necessary.

In this study, we have conducted simultaneous measurements of C[H.sub.4] and [N.sub.2]O emissions from rice fields from 2003 to 2005 with 2 levels of wheat straw incorporation and 3 rates of nitrogen fertiliser application. The objectives of this study are to better understand the effects of straw incorporation on [N.sub.2]O emission and urea application on C[H.sub.4] emission.

Materials and methods

Field site and experimental treatments

The experiment was carried out in a paddy rice field located at Dapu town, Yixing city, Jiangsu province, China (31[degrees]17'N, 119[degrees]54'E) from 2003 to 2005. The characteristics of the experimental soil (0-0.20 m), Typic Epiaquepts (USDA Taxonomy 1975), were as follows: pH 6.23; total C 1.26%; total N 0.13%.

The rice cultivar used in this experiment was 'suyou 5356'. Rice seedlings at the 3-leaf stage were transplanted on 9 June 2003, 11 June 2004, and 9 June 2005. Detailed description of mineral fertiliser application for each year is shown in Table 1. Rice was harvested on 22 October 2003, 20 October 2004, and 18 October 2005.

In 2003 and 2004 the local conventional practice of water management for rice cultivation was followed. After basal incorporation of the mineral N fertiliser into wet soil without standing water, rice fields were kept continuously flooded for about 35 days. Then a 1-week-long midseason aeration was imposed followed by intermittent irrigations until rice harvest. Water management in 2005 was similar to the former 2 years, except that a typhoon disrupted the midseason aeration, and after the topdressing of N fertiliser at panicle initiation, fields were kept continuously flooded for another 2 weeks.

Six treatments were adopted in this experiment (Table 2). Treatments were laid out in a split-plot, randomised complete block design with triplicates. Main plot treatments were 2 rates of wheat straw application and subplot treatments were 3 rates of N-fertiliser application. The plot area was 200 [m.sup.2](16 m by 12.5 m). Ten-cm chopped wheat straw was evenly spread on the field surface and ploughed into the topsoil to 0.12 m on 1 June of each year.

Sampling and measurements

The closed chamber method was used to determine C[H.sub.4] and [N.sub.2]O fluxes. Removable boardwalks (~2 m long) were installed early during field preparations to avoid disturbances of the soil system during the later measurements. The flux chambers (50 by 50 by 100 cm) were made of plexiglass and could be adjusted to the increasing plant height. All chambers were equipped with a fan inside to ensure complete gas mixing. Plastic bases for the chambers were installed in the fields 1 day before the first flux measurements to minimise disturbances. The chambers could be easily mounted on these bases without disturbances of the system, and were removed at the end of each set of flux measurements. The upper part of the plastic base also functioned as water trough (4 cm wide and 5 cm deep) to ensure airtight condition inside the chamber when flux measurements were done during the period of soil drying. Four gas samples from a chamber were collected using 18-mL vacuumed vials at an interval of 10min between 07:30 and 11:00 hours on every sampling day. All 18 plots were divided into 3 groups on replicate basis, with each group therefore covering all the 6 treatments. The gas samples were then collected group by group to minimise the effect of diel variations on temporal variations of C[H.sub.4] fluxes. Gas samples were collected at an interval of 2-3 days for the first 10 days after fertiliser application, at an interval of 5 days or so in the other period except the last 2 months office growth when an interval of 7 days was chosen. The gas concentrations were analysed by gas chromatograph (Shimadzu GC-14B) equipped with a flame ionisation detector for C[H.sub.4] analyses and electron capture detector for [N.sub.2]O analyses. C[H.sub.4] and [N.sub.2]O fluxes were determined from the slopes of the linearly increasing C[H.sub.4] and [N.sub.2]O mixing ratios and expressed in mg/[m.sup.2] x h (C[H.sub.4]) and [micro]g/[m.sup.2] x h ([N.sub.2]O-N). The mean C[H.sub.4] and [N.sub.2]O fluxes during the rice-growing period were the average of triplicate fluxes weighted by an interval of 2 measurements.

When C[H.sub.4] and [N.sub.2]O fluxes were monitored, the redox potentials (Eh) of rice soils were simultaneously measured by using Pt-tipped electrodes (Hirose Rika Co. Ltd, Japan) and an oxidation--reduction potential meter (Toa PRN-41). For the measurements of soil Eh, the electrodes were inserted into the soil at a depth of 0.10 m and maintained there throughout rice-growing period. All soil Eh measurements were made in triplicate. Effect of soil temperature and pH on Eh was not considered. Water layer depth in the field was measured manually with a ruler, and soil temperature at 0, 50, 100, and 150 mm depth were measured with a digital thermometer while gas sampling.

Results

C[H.sub.4] emissions

Temporal variations of C[H.sub.4] flux and soil Eh during the rice-growing period from 2003 to 2005 are shown in Fig. 1. In both 2003 and 2004 (Fig. 1a, b), the measured C[H.sub.4] fluxes varied from 0 to 63.9 mg/[m.sup.2] x h, and all 6 treatments showed a similar seasonal pattern of C[H.sub.4] flux variations. During the continuously flooded period, C[H.sub.4] flux gradually increased and reached the seasonal peak. Subsequently, C[H.sub.4] flux dropped rapidly to almost zero in a few days due to midseason aeration. Treatments with nitrogen fertiliser application emitted 55% or more of total seasonal C[H.sub.4] emissions within 40 days after rice transplantation. After midseason aeration, C[H.sub.4] fluxes increased and decreased corresponding to alteration of dry and wet water condition. The peaks of fluxes were lower than those before the midseason aeration. Treatments with nitrogen fertiliser application emitted more C[H.sub.4] during the continuously flooded period than treatments without nitrogen fertiliser application. During the late growth stage treatments with nitrogen fertiliser application emitted less C[H.sub.4].

As in the former 2 years, a generally similar temporal pattern of C[H.sub.4] flux variations was found in most treatments in 2005 (Fig. 1c). However, the temporal variation of C[H.sub.4] flux had its own characteristics in 2005 in the following ways: (i) a much higher seasonal maximum (163 mg/[m.sup.2] x h), (ii) considerable quantities of C[H.sub.4] emitted in the late stage (from 40 to 100 days after rice transplanting), and (iii) a general higher C[H.sub.4] flux during almost all the flee-growing period.

The seasonally mean C[H.sub.4] fluxes of different treatments from 2003 to 2005 are listed in Table 3. From 2003 to 2005, the mean C[H.sub.4] fluxes of treatments without straw incorporation ranged from 0.73 to 2.99 mg/[m.sup.2] x h, and those of treatments with straw incorporation varied from 3.94 to 27.37 mg/[m.sup.2] x h. Wheat straw incorporation increased C[H.sub.4] emissions 3-11 times, and 1-way ANOVA analysis showed that straw incorporation significantly enhanced C[H.sub.4] emissions (P < 0.05). The mean C[H.sub.4] fluxes of treatments with nitrogen applied at a rate of 200 kg N/ha were not only lower than those without nitrogen application, but also lower than treatments with a higher nitrogen application rate whether wheat straw was incorporated or not. Mean seasonal C[H.sub.4] fluxes of treatment SN0 were much lower in 2003 than in 2004. In 2004, compared with treatments without nitrogen application, the mean seasonal C[H.sub.4] fluxes of treatments N200 and N270 were decreased by 21% and 1%, respectively, and those of treatments SN200 and SN270 were further reduced by 40% and 19%, respectively.

In both 2003 and 2004 (Fig. 1d, e), soil Eh gradually dropped during the continuously flooded period and showed lowest potentials when C[H.sub.4] emission showed highest fluxes. Then, soil Eh increased and decreased according to middle-season aeration and intermittent irrigations. In 2005 (Fig. 1f), soil Eh remained below zero (mV) most of the time due to a typhoon-disrupted midseason aeration and an intentional 2-week continuous flooding after topdressing nitrogen fertiliser at panicle initiation. From 2003 to 2005, significant negative correlations between C[H.sub.4] flux and soil Eh were found during the rice-growing period as shown in Table 4.

[N.sub.2]O emissions

Temporal variations of [N.sub.2]O flux and water depth during the rice-growing period from 2003 to 2005 are shown in Fig. 2. All treatments had a similar temporal variation pattern of [N.sub.2]O flux during the whole rice-growing period in 2003 and 2004 (Fig. 2a, b). After the first week of flooding, [N.sub.2]O flux decreased from 32.46 to 0 [micro]g [N.sub.2]O-N/[m.sup.2] x h. During the midseason aeration, [N.sub.2]O flux first quickly increased and then rapidly decreased. The highest [N.sub.2]O emission peak with flux up to 516 [micro]g [N.sub.2]0-N/[m.sup.2] x h was observed when the soils became aerated and 3-6 days after fertiliser application at panicle initiation. For treatments N270 and SN270, 65-86% of total seasonal [N.sub.2]O emission took place within I l days after the second topdressing of N fertiliser at panicle initiation. Thereafter, [N.sub.2]O flux dropped rapidly to almost zero in a few days. During the last 2 months of rice growth, [N.sub.2]O fluxes fluctuated at very low level. [N.sub.2]O fluxes gradually increased while the fields fell dry before rice harvest.

Considering the extremely high proportion of [N.sub.2]O emission just after the fertiliser application at panicle initiation, continuously flooded condition was maintained for 2 weeks after the second nitrogen fertiliser topdressing in 2005 to reduce [N.sub.2]O emission and nitrogen loss. In 2005, the smallest [N.sub.2]O fluxes were also observed after the panicle initiation fertiliser application. Keeping flooding at a depth of > 1.6 cm in 2005 reduced the peak flux to a much lower rate of 172 [micro]g N20-N/[m.sup.2] x h than those observed in 2003 and 2004 (Fig. 2c). [N.sub.2]O emissions within 11 days after the second topdressing accounted for 90% and 32% of total [N.sub.2]O emissions in treatments N270 and SN270, respectively, in 2005. Table 5 shows the comparison of the effect of water condition during those 11 days on mean C[H.sub.4], [N.sub.2]O, and GWP between 2004 and 2005. Lower [N.sub.2]O emissions but higher C[H.sub.4] emissions were observed in 2005, resulting in a greatly enhanced GWP.

Mean seasonal [N.sub.2]O fluxes of different treatments from 2003 to 2005 are also listed in Table 3. In both 2003 and 2004, nitrogen fertiliser application at rate of 200kg N/ha increased [N.sub.2]O emissions by 5-6 times, and nitrogen fertiliser application at 270kgN/ha increased [N.sub.2]O emissions 10-14 times compared with no nitrogen fertiliser application. In 2005, this stimulating effect was reduced to 2-4 times. From 2003 to 2005, the mean [N.sub.2]O fluxes of treatments without straw incorporation ranged from 1.46 to 16.96 [micro]g N20-N/[m.sup.2] x h, and those of treatments with straw incorporation varied from 0.67 to 14.02 [micro]g [N.sub.2]O-N/[m.sup.2] x h.

[FIGURE 1 OMITTED]

The trade-off relationship between C[H.sub.4] and [N.sub.2]O emissions

The conditions for C[H.sub.4] and [N.sub.2]O production in soil are different. Favourable conditions for C[H.sub.4] production are continuous flooding, whereas alternate soil drying and wetting is suitable for [N.sub.2]O production (Cai et al. 1997; Nishimura et al. 2004). In general, there was no or very low [N.sub.2]O emission during periods when a considerable amount of C[H.sub.4] emissions was observed (Figs 1, 2).The trade-off relationship between C[H.sub.4] and [N.sub.2]O emission is further illustrated with Fig. 3, which was consistent with some previous reports (Cai et al. 1997, 1999). Therefore, simultaneous measurements of C[H.sub.4] and [N.sub.2]O emissions were required to evaluate the combined GWP of paddy rice fields due to the trade-off relationship between C[H.sub.4] and [N.sub.2]O emissions during the rice-growing period.

To decrease the high [N.sub.2]O emission observed in 2003 and 2004 after topdressing of N fertiliser at panicle initiation, which is essential for adequate rice yields, fields were kept continuously flooded for 2 weeks after panicle initiation in 2005. Results showed that the amount of [N.sub.2]O emitted from rice fields in this period was decreased, but C[H.sub.4] emissions increased due to the trade-off relationship between C[H.sub.4] and [N.sub.2]O emissions. Considering the considerable contribution to GWP from C[H.sub.4] emissions, adjusting water management after topdressing of N fertiliser at panicle initiation to 2 weeks of flooding resulted in even higher GWP.

Global warming potential

The GWP of different treatments from 2003 to 2005 is summarised in Table 3. There were significant differences between treatments N0, N200, and N270 and treatments SN0, SN200, and SN270 with regard to GWP. Straw incorporation increased GWP 3-10 times. The GWP was affected by nitrogen fertiliser application rate. Compared with treatments of zero N application, N200 and SN200, with 200 kg N/ha urea application, had an average 25% lower GWP, while respective treatments with 270kgN/ha urea application showed no or smaller reductions.

Rice yields

The rice yields of different treatments are presented in Table 3. Rice yields were significantly affected by N fertilisation, while application of wheat straw had no effect on rice yield. In 2003 and 2004, rice yields slightly increased when urea application rate was increased from 200 to 270 kg N/ha, and the corresponding yield difference did not reach significance. Rice yields of treatments with urea applied at rates of 200 and 270kg N/ha were almost the same in 2005 (Table 3).

Discussion

Urea has been widely used in rice paddy fields of China, and previous reports about the effect of urea application on CH4 emissions from rice fields were conflicting (Lindau et al. 1991; Wang et al. 1992; Cai et al. 1997, 2000). It is well known that there are 3 processes involved in emissions of C[H.sub.4] from rice paddy soil, i.e. production, oxidation, and transport of C[H.sub.4]. N fertilisation affects all the processes of C[H.sub.4] emissions from rice fields directly or indirectly. Schimel (2000) summarised the effects of ammonium-based fertilisers on C[H.sub.4] production and oxidation at different levels: (i) plant/ecosystem level, N[H.sub.4.sup.+] increases plant growth and then carbon supply to the C[H.sub.4] producers, stimulating C[H.sub.4] production in soil; (ii) microbial community level, N[H.sub.4.sup.+] stimulates growth and activity of methanotrophs and then C[H.sub.4] oxidation; and (iii) biochemical level, N[H.sub.4.sup.+] inhibits C[H.sub.4] consumption. Therefore, effects of urea application on C[H.sub.4] emissions from rice fields depend on its integrated effects on C[H.sub.4] production and oxidation. Since effects of urea application on C[H.sub.4] production, oxidation, and transport can be negative and positive and take place simultaneously, it is not surprising that the reported effects of urea application on C[H.sub.4] emissions from rice fields are contradictory. In this experiment, the effect of urea application on C[H.sub.4] emission seemed to be affected by application rate. Nitrogen fertiliser showed a negative effect on C[H.sub.4] emission when applied at a rate of 200 kg N/ha, but the effect decreased if the application rate was further increased to 270kgN/ha. During the continuously flooded periods before midseason aeration, treatments with nitrogen fertiliser application emitted more C[H.sub.4], which indicated that urea application probably had more effect on C[H.sub.4] emissions at ecological and biochemical levels than at microbial level at that time. Afterwards, lower C[H.sub.4] emissions were observed in these treatments. Previous studies have demonstrated that the number of methanotrophic bacteria increases with C[H.sub.4] concentration (Bender and Conrad 1995; Kightley et al. 1995; Cai and Yan 1996; Syamsul Arif et al. 1996). Since treatments with nitrogen fertiliser application had higher C[H.sub.4] fluxes before midseason aeration, C[H.sub.4] concentrations in soil solution of these treatments were likely also higher and high C[H.sub.4] concentration would probably lead to gradually increased number of methanotrophic bacteria and C[H.sub.4] oxidation capacity. As a result, C[H.sub.4] emissions in these treatments became lower after midseason aeration. Unfortunately we did not monitor C[H.sub.4] concentration and C[H.sub.4] oxidation capacity in this experiment. As a whole, the mean C[H.sub.4] fluxes of treatments with nitrogen fertiliser application were lower than those of treatments without nitrogen fertiliser application, which was consistent with some previous reports (Cai et al. 1997; Cai and Yan 1999). However, it was interesting that applying urea at a rate of 270 kg N/ha did not further bring down C[H.sub.4] emissions but otherwise led to higher C[H.sub.4] fluxes compared with urea application at a rate of 200 kg N/ha. It seems that the effects of N fertilisation on C[H.sub.4] emissions from rice fields are probably related to N application rates at all or part of the ecological, microbial, and biochemical levels. Further experiments with more N application rates and simultaneous measurements of C[H.sub.4] production, oxidation, and emission are necessary to help elucidate the underlying mechanisms.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

This experiment confirmed some previous reports that incorporation of organic materials would considerably increase C[H.sub.4] emission (Yagi and Minami 1990; Bronson et al. 1997; Cai 1997; Yu et al. 2004a). Soil organic matter acted not only as the substrate for C[H.sub.4] production, it also helped stimulate soil reduction and created a strict reductive condition for C[H.sub.4] production. The depressive effect of nitrogen fertiliser application on C[H.sub.4] emissions from rice fields became even more obvious when wheat straw was also incorporated. The C/N ratio of wheat straw was as high as 96.15 in this experiment. Soil inherent N[H.sub.4.sup.+] was severely immobilised due to decomposition of wheat straw, leading to depletion of available N and then affecting N effects on C[H.sub.4] emissions. The growth and activity of methanotrophic bacteria might be more sensitive to depletion of available N than plant growth, so the positive effect of N addition on C[H.sub.4] emissions was probably exceeded by the negative effect in the treatments with wheat straw compared with treatments without wheat straw.

A negative correlation of C[H.sub.4] flux and soil Eh had been reported in many previous studies (Kludze et al. 1993; Wang et al. 1993; Cai et al. 1998; Yu et al. 2004b). In 2003, the C[H.sup.4] flux of treatment SN0 was abnormally lower, which might be ascribed to a slow decrease in soil Eh of this treatment after rice field flooding (Fig. 1d).

[N.sub.2]O is produced in soils by nitrification, denitrification, and chemo-denitrification. Previous studies reported that nitrogen fertiliser application enhanced [N.sub.2]O emissions (Mulvaney et al. 1997; Dobbie et al. 1999; Xiong et al. 2002). In the present experiment, [N.sub.2]O emissions increased significantly with the increase in nitrogen fertiliser application rate, which was consistent with the previous reports. It is generally accepted that [N.sub.2]O emission is very low when a rice field is submerged continuously (Cai et al. 1997; Xu et al. 1997; Yan et al. 2000). Our results showed that [N.sub.2]O fluxes during the continuously flooded period before midseason aeration were mostly negligible, with the exception of few times of low emission, which was in line with the previous reports (Fig. 2). Drainage practice usually stimulates [N.sub.2]O emission because it forms soil conditions favourable to both nitrification and denitrification. In 2003 and 2004, the conspicuously high [N.sub.2]O emission peak occuring just after the second topdressing of N fertiliser at panicle initiation should be mainly due to the combined effects of soil drainage, supply of abundant nitrogen by panicle initiation topdressing, and relatively high soil temperature (data not shown). This extremely high [N.sub.2]O emission lasted only for a short time and [N.sub.2]O emissions remained in a negligible level afterwards excepting some low emissions that mainly occurred during the final drainage period (Fig. 2). Even if rice fields were under draining condition in the very late stage of rice growth, [N.sub.2]O emissions were still very low at that time, which might be due to relatively low soil temperature and more importantly the lack of available nitrogen caused by plant uptake.

As mentioned above, the previous reports about the effect of straw incorporation on [N.sub.2]O emissions from rice fields were conflicting (Avalakki et al. 1995; Huang et al. 1999, 2004; Baggs et al. 2000; Cai et al. 2001; Zou et al. 2001; Jiang et al. 2003). In this experiment, wheat straw incorporation showed an inhibitory effect on [N.sub.2]O emissions. Returning wheat straw with higher C:N ratio to rice fields presumably stimulated net immobilisation of plant-available N, resulting in lower N substrate for nitrification and denitrification. Moreover, intensified anaerobic conditions during the period of straw decomposition might weaken nitrification, and consequently reduce [N.sub.2]O emission. Cai et al. (2001) found that straw addition affected [N.sub.2]O evolution, depending on water regime. Baggs et al. (2000) and Huang et al. (2004) reported a stimulatory effect of crop residue on [N.sub.2]O emission, but they also observed that [N.sub.2]O emissions increased temporarily after incorporation of crop residues (most of the emission occurred during the first 2 weeks, returning to background levels after 30-40 days), and amendments of plant residues with higher C/N ratio could enhance N immobilisation and thus reduce N losses of added urea fertiliser.

Based on the results of our 3-year experiment, the rate of 200 kg N/ha is strongly recommended for urea application from both economical and environmental perspectives in the region represented by the field site. Compared with the rate of 270 kg N/ha, applying urea at rate of 200 kg N/ha can reduce not only the cost to buy urea but also C[H.sub.4] and [N.sub.2]O emissions and consequently GWP (Table 3). Rice yield is one of the most important factors to be taken into account in appraisals of any mitigation strategies of C[H.sub.4] and [N.sub.2]O emissions from rice cultivation. Compared with rate of 200 kg N/ha, the gain of rice yield to apply urea at 70 kg N/ha more was very limited (Table 3). This indicated that we would not risk loss of rice yield when achieved economical and environmental benefits to apply urea at a rate of 200 instead of 270 kg N/ha.

Conclusions

Clearly, C[H.sub.4] and [N.sub.2]O emissions from rice fields were influenced by nitrogen fertilser and wheat straw application, in contrast to stimulation or inhibition by nitrogen fertilisation on C[H.sub.4] emission in previous reports, our results showed the effect of nitrogen fertiliser application on C[H.sub.4] emission seemed to be affected by application rate. Nitrogen fertiliser showed a negative effect on C[H.sub.4] emission when applied at a rate of 200 kg N/ha, but the effect decreased if the application rate was further increased to 270 kg N/ha. [N.sub.2]O emission from rice paddies was enhanced by nitrogen fertiliser application, but slightly reduced by wheat straw incorporation. A trade-off relationship between C[H.sub.4] and [N.sub.2]O emissions was found during the rice growing period. The total GWP of C[H.sub.4] and [N.sub.2]O was lowest when nitrogen fertiliser was applied at a rate of 200 kg N/ha. Rice yields were significantly affected by N fertilisation, but the gain in rice yield by increasing the urea application rate from 200 to 270 kgN/ha was very limited. Based on all the results, the rate of 200 kg N/ha is strongly recommended for urea application in the specific region from both economical and environmental perspectives in terms office grain yield and the cumulative GWP of C[H.sub.4] and [N.sub.2]O emitted from rice fields.

Acknowledgments

This work was funded by the National Natural Science Foundation of China (Grant No. 40621001 and 40671094). The authors express their sincere gratitude to the responsible editors and anonymous reviewers for their corrections and suggestions to the manuscript.

Manuscript received 21 March 2007, accepted 12 July 2007

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J. Ma (A,C,) X. L. Li (A,C), H. Xu (A), Y. Han (A), Z. C. Cai (A), and K. Yagi (B)

(A) State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China.

(B) National Institute of Agro-Environmental Sciences, 3-1-1, Kannondai, Tsukuba 305, Japan.

(C) Graduate University of Chinese Academy of Sciences, Beijing 100049, China.

(D) Corresponding author. Email: hxu@issas.ac.cn
Table 1. Details of nitrogen fertiliser application for rice
cultivation from 2003 to 2005

Compound fertiliser [N(15), P(10), K(15)] and ammonium bicarbonate
were combined as basal fertiliser for A and B, while
Ca[([H.sub.2]P[O.sub.4]).sub.2] and potassium chloride containing
the same amount of phosphorus and potassium as A and B were used as
basal fertiliser for C. Urea was applied as supplemental fertiliser

Nitrogen fertiliser at: Application rate (kg N/ha)

 200 (A) 270 (B) 0 (C)

Basal fertiliser 76 132 0
Tillering fertiliser 55 69 0
Panicle initiation 69 69 0
 fertiliser

Nitrogen fertiliser at: Application date

Basal fertiliser 8 June 2003, 10 June 2004,
 8 June 2005
Tillering fertiliser 18 June 2003, 20 June 2004,
 18 June 2005
Panicle initiation 1 August 2003, 31 July 2004,
 fertiliser 22 July 2005

Table 2. Experimental treatments

Treatments Nitrogen fertiliser Wheat straw
 application (kg N/ha) application (kg/ha)

N0 0 0
N200 200 0
N270 270 (A) 0
SNO 0 3.75 x [10.sup.3]
SN200 200 3.75 x [10.sup.3]
SN270 270 3.75 x [10.sup.3]

(A) Urea is conventionally applied at a rate of 270 kg N/ha by
local farmers.

Table 3. Mean C[H.sub.4] flux, mean [N.sub.2]O flux, global warming
potential (GWP), and grain yield during rice-growing period of 2003,
2004, and 2005

GWP was calculated on the basis of mass factors of 23 for C[H.sub.4]
and 296 for [N.sub.2]0 of 100-year time horizon. Mean [+ or -]
standard deviation. Values within each column for each year followed
by the same letter are not significantly different (P > 0.05)

Treatments Mean C[H.sub.4] flux Mean [N.sub.2]O flux
 (mg C[H.sub.4]/[m.sup.2] x h) ([micro]g [N.sub.2]O-N/
 [m.sup.2] x h)

 2003

N0 1.26 [+ or -] 0.71a 1.47 [+ or -] 1.05a
N200 0.73 [+ or -] 0.66b 9.05 [+ or -] 8.13b
N270 1.06 [+ or -] 0.39ab 16.96 [+ or -] 17.57c
SNO 3.94 [+ or -] 1.70c 1.28 [+ or -] 1.17a
SN200 7.06 [+ or -] 3.89d 5.93 [+ or -] 4.63b
SN270 7.35 [+ or -] 6.67d 13.69 [+ or -] 11.45c

 2004

NO 1.63 [+ or -] 0.39a 1.46 [+ or -] 0.94a
N200 1.28 [+ or -] 1.17a 9.41 [+ or -] 7.81 b
N270 1.61 [+ or -] 0.99a 14.15 [+ or -] 21.59c
SNO 7.90 [+ or -] 3.55b 1.03 [+ or -] 0.91a
SN200 4.72 [+ or -] 1.0oc 5.63 [+ or -] 2.65d
SN270 6.37 [+ or -] 2.576c 14.02 [+ or -] 18.31c

 2005

NO 2.99 [+ or -] 1.48a 1.67 [+ or -] 0.35a
N200 2.21 [+ or -] 1.14a 3.47 [+ or -] 1.52b
N270 2.60 [+ or -] 0.84a 6.05 [+ or -] 5.22c
SNO 22.65 [+ or -] 19.046 0.67 [+ or -] 0.89d
SN200 21.77 [+ or -] 10.02b 1.81 [+ or -] 1.31a
SN270 27.37 [+ or -] 21.916 2.43 [+ or -] 2.19a

Treatments GWP Grain yield
 (mmol C[O.sub.2]/ (t/ha)
 [m.sup.2] x h)

 2003

N0 1.83 [+ or -] 1.03a 6.27 [+ or -] 0.50a
N200 1.15 [+ or -] 1.03b 7.42 [+ or -] 0.27b
N270 1.70 [+ or -] 0.75a 7.69 [+ or -] 0.46b
SNO 5.68 [+ or -] 2.46c 6.67 [+ or -] 0.38a
SN200 10.21 [+ or -] 5.64d 7.38 [+ or -] 0.22b
SN270 10.71 [+ or -] 9.71d 8.08 [+ or -] 0.56b

 2004

NO 2.36 [+ or -] 0.57a 5.56 [+ or -] 0.52a
N200 1.94 [+ or -] 1.76a 7.18 [+ or -] 0.31b
N270 2.46 [+ or -] 1.65a 7.24 [+ or -] 0.48b
SNO 11.37 [+ or -] 5.11b 6.13 [+ or -] 0.43a
SN200 6.84 [+ or -] 1.47c 7.35 [+ or -] 0.25b
SN270 9.31 [+ or -] 3.896 7.49 [+ or -] 0.45b

 2005

NO 4.32 [+ or -] 2.13a 5.77 [+ or -] 0.54a
N200 3.21 [+ or -] 1.656 7.04 [+ or -] 0.45b
N270 3.80 [+ or -] 1.26ab 6.68 [+ or -] 0.50ab
SNO 32.57 [+ or -] 27.38c 6.46 [+ or -] 0.48ab
SN200 31.31 [+ or -] 14.42c 7.03 [+ or -] 0.28b
SN270 39.37 [+ or -] 31.52c 6.61 [+ or -] 0.34ab

Table 4. Coefficients of correlation between C[H.sub.4] flux and soil
Eh from 2003 to 2005

** P < 0.01; * P <0.05

Treatments 2003 (n = 20) 2004 (n = 27) 2005 (n = 32)

N0 -0.509 * -0.437 * -0.401 *
N200 -0.454 * -0.402 * -0.322 *
N270 -0.594 ** -0.307 -0.357 *
SNO -0.455 * -0.445 * -0.063
SN200 -0.501 * -0.407 * -0.239
SN270 -0.692 ** -0.419 * -0.348 *

Table 5. Mean C[H.sub.4] flux, mean [N.sub.2]O flux, and global
warming potential (GWP) during 11 days after panicle initiation
fertiliser incorporation in 2004 and 2005

GWP was calculated on the basis of mass factors of 23 for
C[H.sub.4] and 296 for [N.sub.2]O of 100-year time horizon.
[N.sub.2]O emission mainly occurred 11 days after panicle
initiation fertilisation

Treatments Mean C[H.sub.4] Mean [N.sub.2]O
 flux flux
 (mg/[m.sup.2] x h) ([micro]g/[m.sup.2] x h)

 2004 2005 2004 2005

N0 1.32 1.93 4.45 3.06
N200 0.97 2.18 87.23 20.78
N270 0.73 0.41 127.09 61.99
SN0 8.13 9.66 0.00 6.32
SN200 3.78 6.05 28.30 10.15
SN270 5.55 10.32 142.16 8.78

Treatments GWP
 (mmol C[0.sub.2]/
 [m.sup.2] x h)

 2004 2005

N0 1.93 2.80
N200 1.99 3.27
N270 1.90 1.01
SN0 11.69 13.93
SN200 5.63 8.77
SN270 8.93 14.89
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Article Details
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Author:Ma, J.; Li, X.L.; Xu, H.; Han, Y.; Cai, Z.C.; Yagi, K.
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:8AUST
Date:Aug 1, 2007
Words:7234
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