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Effect of antecedent soil water regime and rice straw application time on [CH.sub.4] emission from rice cultivation.

Introduction

Methane ([CH.sub.4]) is an important greenhouse gas and a key factor in tropospheric and stratospheric chemistry (Bouwman 1991). Rice fields are one of the major contributors to the increasing atmospheric [CH.sub.4] concentration (Dlugokencky et al. 1994). [CH.sub.4] is produced by the strictly anaerobic bacteria that are common in anoxic soils, such as rice fields (Cicerone and Oremland 1988). Since the first study of [CH.sub.4] emissions from rice fields in California (Cicerone and Shetter 1981), a number of studies have been conducted in various other parts of the world to study [CH.sub.4] emissions from rice fields and mitigation options for [CH.sub.4] production (Schutz et al. 1989; Sass et al. 1991; Cai et al. 1994; Yagi et al. 1994). Unfortunately, almost all the experimental treatments of the former studies focussed on the rice-growing period. More information is needed on whether some treatments, such as soil water regime and rice straw application time in the antecedent crop season, affect [CH.sub.4] emissions from rice fields, and if so, whether [CH.sub.4] emissions from rice fields can be reduced if land is managed in the previous crop season (winter crop season) in such a way that less [CH.sub.4] is emitted.

China is one of the major rice-producing countries, with 22.6% of the total rice-growing area and 36.3% of the total rice grain production in the world (Anon. 1991). Water regimes of rice paddy soils are different not only during the rice-growing period but also in the antecedent crop season in China. About 2.7-4.0 x [10.sup.6] ha of rice fields are continuously flooded and fallow in the winter crop season (Cai 1995); the other rice fields are dry fallow or planted with upland crop such as winter wheat. Cai (1997) summarised the data of [CH.sub.4] fluxes measured in China and found that [CH.sub.4] emissions from the rice fields drained in the winter crop season were much lower than those from the fields flooded annually. This indicates that water management in the winter crop season may be an important factor controlling [CH.sub.4] emission from rice cultivation. Due to the improved availability of coal gas, and more importantly, due to the need of sustainable agriculture, Chinese farmers now use more and more rice straw as organic manure rather than as daily fuel. Application of organic manure to rice fields is one of the key factors increasing [CH.sub.4] emission (Schutz et al. 1989; Yagi and Minami 1990). The effect of rice straw on [CH.sub.4] emissions was reduced greatly if the rice straw was composted before being applied to rice fields (Yagi and Minami 1990).

There may be other ways to decrease the effect of rice straw on [CH.sub.4] emissions from rice fields. Rice straw is applied to fields both before the winter crop season and before soil is flooded for rice transplanting. The timing of rice straw application may influence the effect of rice straw on [CH.sub.4] emissions from rice fields. To investigate the effect of antecedent soil water regime and rice straw application time on [CH.sub.4] emission from rice field, a pot experiment was conducted from October 1995 to October 1997 and the results are presented in this paper.

Materials and methods

Soil and experimental design

The pot experiment was conducted in a greenhouse. The soil was collected from a rice field located in the experimental farm of Jurong Agricultural College, in the Jiangsu province, in 1995; the soil was air-dried and passed through a 5-mm sieve. It was derived from Xiashu loess and classified into Typic Haplaquepts according to soil taxonomy (Soil Survey Staff 1975). The soil (pH 6.3) contained 9.87 g/kg of organic carbon, and 1.18 g/kg of total nitrogen (N). Pots (inner diameter, 20 cm; height, 30 cm) were filled with 6 kg soil on 28 October 1995. There were 3 treatments, which differed in soil water regime or rice straw application time during the winter crop season: (i) dry fallow with rice straw being applied before the winter crop season (DFE: dry fallow, early); (ii) flooded fallow with rice straw being applied before the winter crop season (FFE: flooded fallow, early); and (iii) dry fallow with rice straw applied before soil was flooded for rice transplanting (DFL: dry fallow, late). Dry treatments mean the soils did not receive any water except from precipitation, whereas flooded means soil was covered with a water layer of at least 2 cm in the antecedent crop season. The soils of treatments DFE and DFL received water from rainfall (327.4 mm, November 1995-May 1996; 390.9 mm, November 1996-May 1997) 8 and 7 times in the 1995-96 and 1996-97 winter crop season, respectively (Agricultural Year Book of China 1996, 1997, 1998). The experiment was fully randomised with 3 replicates for each treatment. Rice straw (30 g, 1995; 35 g, 1996) was chopped into -10-mm segments and was mixed with the surface soil in treatments DFE and FFE on 28 October 1995 and 28 October 1996. Organic carbon contents of the straw were 413 g/kg in 1995 and 451 g/kg in 1996. Soils in all 9 pots were flooded on 1 June in 1996 and 1997, and the same amount of rice straw as that used for treatment DFE was incorporated with the surface soil in treatment DFL just before the soil was flooded for the rice crop. The pots thus prepared were placed in the open air outside a greenhouse. In case of heavy rain during rice-growing season, the pots were put inside the greenhouse. Rice was transplanted on 14 June and 17 June and harvested on 12 October and 7 October in 1996 and 1997, respectively. The rice stubble remained in the pot after rice harvest in 1996.

Water management of rice pots

In all the pots in 1996, soil was covered with a water layer of at least 2 cm, which was maintained during the period from flooding to rice harvest. Water management of rice pots in 1997 was similar to that in 1996, but a drainage was performed twice in the mid and late stages of the rice-growing period. The first drainage started on 28 July and all the rice pots were re-flooded on 30 July. The second drainage was performed on 2 September and lasted for 3 days.

Gas sampling and [CH.sub.4] measurement

Gas samples were collected on a schedule of 3-7 days from plexiglass chambers (51 cm by 51 cm by 100 cm) after the rice pots were placed on specially designed wooden tables during the period from when soil was flooded to rice harvest. Gas samples were regularly collected between 09 00 hours and 11 00 hours on every sampling day to minimise the effect of diurnal variations on temporal variations of [CH.sub.4] fluxes and soil redox potential (Eh). [CH.sub.4] concentration in gas samples was determined by gas chromatograph (Shimadzu GC-12A) equipped with a FID detector.

Measurement of soil Eh

When [CH.sub.4] flux was measured, soil Eh was also simultaneously determined by using Pt-tipped electrodes (Hirose Rika Co., Japan) and an ORP meter (Toa RM-1K) in 1996. For measurement of soil Eh, electrodes were inserted into the soil at a depth of 10 cm and maintained there throughout the flooding and rice-growing periods. All soil Eh measurements were made in triplicate.

Results and discussion

Temporal variations of [CH.sub.4] fluxes and soil temperatures

Temporal variations of [CH.sub.4] fluxes during the period from when soil was flooded to rice harvest in 1996 and 1997 are shown in Figs 1 and 2. The results indicate that the pattern of temporal variations of [CH.sub.4] fluxes of treatments FFE and DFL was quite different from that of treatment DFE. Fluxes were substantial several days after flooding and 4 emission peaks occurred during the period from soil flooding to rice harvest for treatments DFL and FFE; in contrast, fluxes were negligible until Day 55 (1996) or Day 50 (1997) after flooding and were still very low thereafter for treatment DFE.

[Figures 1-2 ILLUSTRATION OMITTED]

As shown in Figs 3 and 4, soil temperatures at a depth of 0, 5, and 10 cm had the same temporal variation pattern and fluctuated in ranges 17.6-36.1 [degrees] C and 18-34.5 [degrees] C during the period from soil flooding to rice harvest in 1996 and 1997, respectively. Schutz et al. (1990) measured [CH.sub.4] production under different soil temperatures. They found that the minimum, optimum, and maximum soil temperatures for [CH.sub.4] production were 15, 35, and 40 [degrees] C, respectively. Thus, in our experiment, soil temperature was not a limiting factor for [CH.sub.4] production.

[Figures 3-4 ILLUSTRATION OMITTED]

The first [CH.sub.4] emission peak of treatments DFL and FFE appeared before rice transplanting, presumably due to decomposition of some organic constituents in rice straw, which were most easily decomposed (DFL), or due to the effect of soil temperature (FFE) (Figs 1-4). The other 3 peaks of treatments DFL and FFE were observed on Days 21, 57, and 106 (1996) or on Days 27, 55, and 83-89 (1997) after flooding, corresponding to recovering (rice roots partly damaged by transplanting recover during the stage), tillering, and ripening stages of rice growth (Figs. 1 and 2). Drainage was performed twice for all rice pots in the mid and late stages of rice growth in 1997. [CH.sub.4] fluxes of all 3 treatments rapidly decreased to zero after mid drainage. After the rice pots were reflooded, [CH.sub.4] fluxes recovered to some extent. [CH.sub.4] fluxes droped to zero after final drainage and did not recover after reflooding for the remaining 33 days of the experiment (Fig. 2).

Temporal variations of soil Eh

Antecedent soil water regime and rice straw application time affected not only the temporal variation pattern of [CH.sub.4] fluxes but also the pattern of soil Eh change after flooding (Fig. 5). The soil Eh of treatment FFE was very low and in the active range of methanogenic bacteria throughout the period of flooding. The soil Eh of both treatments DFE and DFL was very high just after flooding, but the decreasing rate of the soil Eh of treatment DFL after flooding was quite different from that of treatment DFE. The soil Eh of treatment DFL decreased rapidly to reach the active range of methanogenic bacteria 13 days after flooding, whereas, it was not until Day 65 after flooding that soil Eh of treatment DFE dropped to the same level.

[Figure 5 ILLUSTRATION OMITTED]

The progress of soil reduction is controlled by the relative abundance of electron donors and electron acceptors in the soil. In the absence of [O.sub.2], the main electron acceptors are [Fe.sup.3+], [Mn.sup.4+], [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and readily decomposable organic matter is a main electron donor (Yagi et al. 1994). Laboratory anaerobic incubations have demonstrated that there is a good relationship between [CH.sub.4] production and soil organic carbon content (Crozier et al. 1995). The addition of rice straw plays a similar role to that of organic matter in flooded soil. Although the soil was dry for treatment DFL in the winter crop season, the addition of rice straw before flooding provided the soil with extra electron donors, energies, and carbon sources. This caused a faster reduction in the Eh of treatment DFL after flooding and the temporal variation pattern of that soil Eh was very close to that of treatments FFE (Fig. 5). On the other hand, the addition of rice straw before the winter crop season (DFE) allowed the rice straw to decompose during the whole winter crop season under aerobic conditions, and the effect of rice straw on decreasing the soil Eh in rice fields was drastically reduced.

Effects of soil Eh and temperature on [CH.sub.4] fluxes

Soil Eh is one of the most important factors influencing [CH.sub.4] production in soil. It is well known that [CH.sub.4] production occurs when soil suspension Eh is below -150 mV (Masschelyn et al. 1993; Wang et al. 1993). As shown in Figs 1 and 5, differences in the temporal variation patterns of soil Eh due to different soil-water regimes and rice straw application times in the antecedent crop season were the main reason for the difference in temporal variation patterns of [CH.sub.4] fluxes. With regard to the relationship between temporal variation of [CH.sub.4] fluxes and temporal variation of soil Eh of the treatments, regression analysis showed that there were no significant correlations between [CH.sub.4] fluxes and soil Eh during the period from flooding to rice harvest for treatments FFE (r = 0.207) and DFL (r = 0.244), using the equation y = ax + b to simulate experimental data (Figs 6 and 7). This indicated that soil Eh was not a critical factor controlling [CH.sub.4] fluxes of treatments FFE and DFL. In striking contrast to the treatments FFE and DFL, there was significant correlation between [CH.sub.4] flux and soil Eh for treatment DFE (r = 0.788) (Fig. 8). The data presented in Fig. 8 confirm those of Wang et al. (1993) that [CH.sub.4] production increases exponentially when the soil Eh decreases from -150 to -250 mV and mineralisable carbon is not limiting. Carbon probably became limiting when the Eh became sufficiently low in FFE and DFL, resulting in a lack of correlations (r = 0.207 and 0.244, respectively).

[Figures 6-8 ILLUSTRATION OMITTED]

The activity of methanogenic bacteria, the decomposition rate of soil organic matter, and [CH.sub.4] production and transport rate all increased with soil temperature. It was generally accepted that there was a significant correlation between the diurnal changes in [CH.sub.4] fluxes and the diurnal changes in soil temperatures (Schutz et al. 1990; Yagi et al. 1994). However, there were conflicting results with respect to the relationship between seasonal variations in [CH.sub.4] fluxes and soil temperature (Cicerone et al. 1983; Schutz et al. 1989; Lindau et al. 1991). Our results showed no significant correlation between [CH.sub.4] fluxes and soil temperatures at a depth of 0, 5, and 10 cm during the period from flooding to rice harvest for treatment DFE in 1996 and 1997 (Table 1); however, we observed a significant correlation for treatments DFL and FFE in 1996, but not in 1997 (Table 1).

Table 1. Correlation coefficients of linear regression between [CH.sub.4] fluxes and soil temperatures during the period from flooding to rice harvest

DF, soil was fallow and did not receive any water except rain water; FF, soil was fallow and maintained a water layer of > 2 cm; E, rice straw was applied just before the winter crop season; L, rice straw was applied just before the soil was flooded for rice transplanting
Treatment Depth 0 cm Depth 5 cm Depth 10 cm
 1996 1997 1996 1997 1996 1997

DFE 0.056 0.071 0.093 0.065 0.088 0.041
DFL 0.449(*) 0.071 0.414(*) 0.069 0.402(*) 0.053
FFE 0.466(*) 0.069 0.459(*) 0.066 0.453(*) 0.058


(*) P < 0.05.

It is well known that a number of agro-environmental factors, including soil properties, plant activities, cultivation practices, climatic factors, etc., act together and influence [CH.sub.4] emissions from rice fields (Yagi et al. 1994). For treatments FFE and DFL in 1996, soil Eh was already in the active range of methanogenic bacteria almost every day after flooding and was not a limiting factor for [CH.sub.4] production (Fig. 5), so [CH.sub.4] flux may be mainly controlled by other factors such as rice plant activities and soil temperature. Thus we did not find significant correlation between [CH.sub.4] fluxes and soil Eh, but observed significant correlation between [CH.sub.4] fluxes and soil temperature. In 1997, the temporal variation pattern of [CH.sub.4] fluxes was strongly affected by drainage performed twice in the mid and late stages of rice growth, which may be the main reason why there was no significant correlation between [CH.sub.4] fluxes and soil temperature for treatments DFL and FFE. As to treatment DFE, soil Eh decreased slowly after flooding. It took approximately 51 days after flooding for soil Eh to decrease below zero (Fig. 5). On the other hand, soil temperatures fluctuated in a suitable range for [CH.sub.4] production (Figs 3 and 4). Therefore, soil Eh was more likely than soil temperature to be the controlling factor for [CH.sub.4] flux of treatment DFE. Thus we observed no significant correlation between [CH.sub.4] fluxes and soil temperatures in 1996 and 1997, but found significant correlation between [CH.sub.4] fluxes and soil Eh for treatment DFE (Fig. 8).

Water regime effects

Water management in the rice-growing season has a strong influence on [CH.sub.4] emission from rice fields (Sass et al. 1992). Our results showed that soil water regime in the winter crop season also played a very important role in [CH.sub.4] emissions from rice fields. The mean [CH.sub.4] flux of the treatment with continuously flooded fallow in the winter crop season (FFE) was 6.06 and 5.15 times higher than that of treatment with dry fallow (DFE) in 1996 and 1997, respectively (Table 2). Cai et al. (1998) also found that [CH.sub.4] flux from rice fields continuously flooded in the previous crop season was 2.81 times higher than that with winter wheat as previous crop.

Table 2. Mean [CH.sub.4] fluxes (mg/[m.sup.2].h [+ or -] s.d.) (n = 3) and soil Eh (mV) [+ or -] s.d. (n = 3) of different treatments during the period from flooding to rice harvest

DF, soil was fallow and did not receive any water except rain water; FF, soil was fallow and maintained a water layer of 2 cm; E, rice straw was applied just before the winter crop season; L, rice straw was applied just before the soil was flooded for rice transplanting
Treatment Mean [CH.sub.4] fluxes

 1996 1997

DFE 4.06 [+ or -] 0.62 3.15 [+ or -] 0.74
DFL 19.73 [+ or -] 0.83 35.20 [+ or -] 12.18
FFE 24.59 [+ or -] 2.96 16.21 [+ or -] 1.05

Treatment Mean soil Eh
 1996

DFE -3.47 [+ or -] 23.02
DFL -161.95 [+ or -] 35.79
FFE -184.82 [+ or -] 33.00


The mean soil Eh of different treatment during the period from flooding to rice harvest in 1996 is also shown in Table 2. The mean Eh of treatment FFE was as low as -184.82 mV, whereas the mean soil Eh of treatment DFE was much higher than what is suitable for [CH.sub.4] production. This suggests that the maintenance of soil Eh at a very low level was one of the main reasons why the mean [CH.sub.4] flux of treatment FFE was much higher than that of treatment DFE.

The existence of active methanogenic bacteria is a prerequisite for [CH.sub.4] production. Methanogenic bacteria are active only under very strict anaerobic conditions and are highly sensitive to oxygen (Ming 1993). The methanogenic population in soils and its activities may be reduced if the soil is drained and exposed to atmospheric oxygen. In this experiment, the effect of soil water regime on the methanogenic bacteria population in the winter crop season was not investigated. However, this might be one of the reasons that the soil flooded in the winter crop season emitted much more [CH.sub.4] than soils that did not receive any other water except rain water.

An early field measurement showed that [CH.sub.4] emissions from the rice fields in China were very high and represented the highest record of mean [CH.sub.4] flux throughout the rice-growing period in the world (Khalil et al. 1991). However, most of the mean [CH.sub.4] fluxes from rice fields in China measured thereafter were much lower than this earlier measurement (Cai 1997). It now can be explained why [CH.sub.4] fluxes of the early measurement were much higher than those measured thereafter in China, for [CH.sub.4] fluxes of the early measurement were recorded in a rice field flooded in the antecedent crop season (Khalil et al. 1991). In China, the annual water management of rice fields can be roughly divided into 3 kinds: (i) intermittent irrigation, (ii) continuous flood during the rice-growing period but dry in the winter crop season; and (iii) annual flood (Cai 1997). The annually flooded rice fields, which are mainly distributed in south-west China, accounted for 8-12% of the total rice-cultivating area (Cai 1997). If the irrigation and drainage facilities for the annually flooded rice fields could be improved substantially and the flooding water could be drained completely in the winter crop season, then total [CH.sub.4] emission from Chinese rice fields would be significantly reduced.

Effect of rice straw application

Methane fluxes from rice fields were strongly enhanced by incorporation of green manure or rice straw (Yagi and Minami 1990; Denier and Neue 1995). In this experiment, the timing of rice straw application also significantly influenced [CH.sub.4] fluxes from rice fields. As shown in Table 2, the application of rice straw just before soil was flooded for rice growth (DFL) significantly enhanced [CH.sub.4] flux by 385.96% in 1996 and 1017.46% in 1997 compared with rice straw application before the winter crop season (DFE). The addition of rice straw before flooding not only provided more substrate for methanogenic bacteria, but also stimulated soil reduction by increasing the ratio of electron donors to electron acceptors. The mean soil Eh of treatment DFL during the period from flooding to rice harvest was much lower than that of treatment DFE (Table 2). This can reasonably explain why the mean [CH.sub.4] flux of treatment DFL was much higher than that of treatment DFE (Table 2). The rate of rice straw application and the organic carbon content of rice straw were both higher in 1997 than in 1996, which may be the main reason why the [CH.sub.4] flux of treatment DFL in 1997 was much higher than that in 1996 (Table 2).

To mitigate [CH.sub.4] emissions from rice fields, organic amendments should be minimised. However, this may conflict with soil fertility aspects as well as sustainable agricultural development. In this experiment, if the rice straw was applied before the winter crop season, [CH.sub.4] fluxes during the following flooding and rice-growing period were very low (Table 2). As a result of undergoing aerobic decomposition in the winter crop season, the effect of rice straw on [CH.sub.4] emission from rice fields decreased greatly. Thus, rice straw application not only sustains soil fertility and meets the needs of sustainable agriculture, it also prevents large amounts of [CH.sub.4] being emitted into the atmosphere, if a suitable application time is chosen.

Effect on rice yield

Rice yield is one of the most important factors to be taken into account in appraisals of any mitigation strategies of [CH.sub.4] emission from rice cultivation. Although soil water regime and rice straw application time in the antecedent crop season greatly affected [CH.sub.4] emissions from rice soils in pots, they had no significant effect on rice yields (Table 3). This indicated that we would not risk decreasing rice yield when we tried to reduce [CH.sub.4] emission from rice cultivation by means of draining water in the winter crop season or applying rice straw before the winter crop season.

Table 3. Rice yields (g/pot [+ or -] s.d.) (n = 3) of treatments with different water regimes and rice straw application times in the antecedent crop season in 1996 and 1997

DF, soil was fallow and did not receive any water except rain water; FF, soil was fallow and maintained a water layer of 2 cm; E, rice straw was applied just before the winter crop season; L, rice straw was applied just before the soil was flooded for rice transplanting
Treatment Rice yields
 1996 1997

DFE 37.97 [+ or -] 3.38 39.45 [+ or -] 2.41
DFL 39.67 [+ or -] 3.73 43.03 [+ or -] 4.38
FFE 44.00 [+ or -] 0.88 41.30 [+ or -] 5.19


Conclusions

Water regime and timing of rice straw application in the antecedent crop season significantly affected [CH.sub.4] emission and soil Eh during the wetland rice cropping season (Table 2). Differences in soil Eh and its temporal variation patterns caused by water regime and timing of rice straw application in the previous crop season were the main reasons why [CH.sub.4] fluxes and their temporal variation patterns during the period from flooding to rice harvest were not alike.

Water management in the antecedent crop season is one of the key factors influencing [CH.sub.4] emissions from rice paddy soils. Compared with the management of flooded fallow in the previous crop season, which is widely adopted in south-west China, dry fallow or planting winter wheat, which widely exists in rice-growing areas in China, could significantly reduce [CH.sub.4] emissions during the wetland rice cropping season.

Rice straw application is also one of the key factors controlling [CH.sub.4] emission from rice paddy soils. However, if the rice straw were applied just before the previous crop season, its effect on [CH.sub.4] emissions from rice paddy soils would decrease greatly. Thus, to meet the needs of both sustaining soil fertility and reducing [CH.sub.4] emission from rice cultivation, we recommend that, as much as possible, rice straw be applied before the winter crop season.

Acknowledgments

Financial support was provided by the National Natural Science Foundation of China (49371039), and the Laboratory of Material Cycling in Pedosphere, Institute of Soil Science, Chinese Academy of Sciences.

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Manuscript received 16 April 1999, accepted 15 October 1999

H. Xu(A), Z. C. Cai(A), X. P. Li(A), and H. Tsuruta(B)

(A) 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.
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Author:Xu, H.; Cai, Z. C.; Li, X. P.; Tsuruta, H.
Publication:Australian Journal of Soil Research
Geographic Code:8AUST
Date:Jan 1, 2000
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