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Short-term effects of wheat straw incorporation into paddy field as affected by rice transplanting time.

Introduction

China has 13 million ha of rice--wheat cropping, mainly in the 4 provinces, namely Jiangsu, Anhui, Hubei, and Sichuan along the Yangtze River Valley (Zheng 2000). Incorporation of wheat straw in the following rice growth season was highly recommended in these areas. Recycling of crop residues is an important step towards saving natural resources and improving soil physical, chemical, and biological properties (Smil 1999; Mandal et al. 2004). Effects of incorporation of crop residues into paddy soil may be divided into long- and short-term ones. Long-term effects include: (i) increasing soil organic carbon storage which sequestrates atmospheric C[O.sub.2] (Aulakh et al. 2001; Mandal et al. 2004; Singh et al. 2004); (ii) building up soil fertility (Takahashi et al. 2003; Tirol-Padre et al. 2005); (iii) improving environmental benefits and sustaining agricultural development. Short-term effects include: (i) stimulating C[H.sub.4] emissions (Watanabe et al. 1995; Bronson et al. 1997; Cai 1997; Naser et al. 2007); (ii) immobilising available N (Tanaka et al. 1990; Jensen 1997); (iii) suppressing rice growth and causing accumulation of toxic materials, such as organic acids (Rao and Mikkelsen 1976, 1977; Lynch et al. 1980; Tanaka et al. 1990). Farmers are generally aware of the short-term impacts, in contrast to the long-term ones, and hence large amounts of crop straws are burned, either in the field or as household fuel (Duan 1995). Therefore, it is essential to work out technologies and practices for reducing the adverse short-term effects.

Methane is an important greenhouse gas, the second-largest (+0.48 W/[m.sup.2]) next to C[O.sub.2] among long-lived greenhouse gases in radiative forcing (IPCC 2007a). Methane concentration in the global atmosphere has increased from a pre-industrial value of about 0.715 ppm to 1.774 ppm in 2005 (IPCC 2007a). Rice fields are a significant source of atmospheric C[H.sub.4]. Annual global C[H.sub.4] emissions from rice fields are reported to be 31-112 Tg, about 5-19% of the total global C[H.sub.4] emissions (IPCC 2007b). Previous studies have reported that C[H.sub.4] emissions from rice fields are enhanced by organic amendment (Watanabe et al. 1995; Bronson et al. 1997; Cai 1997; Naser et al. 2007). The impact of timing of rice transplantation on C[H.sub.4] emissions has rarely been reported. Holzapfelpschorn et al. (1986) reported that the presence of rice plants stimulated emissions of C[H.sub.4] both in the laboratory and in the field. Accordingly, it is presumed that delayed transplantation of rice in paddy fields incorporated with wheat straws may reduce C[H.sub.4] emissions from the field, compared with normal transplantation.

It is well known that N available for rice to take up is an important factor in the regulation office growth and grain yield (Yoshida 1981; Rao and Mikkelsen 1976). Its content in the soil is affected by the amount of N mineralised or immobilised during the wheat straw decomposition process. Vigil and Kissel (1991) reported that the break point between net N immobilisation and mineralisation of residues was at a C/N ratio of 40. Jensen (1994) reported net immobilisation of soil N during the early stage of decomposition of pea residues with a C/N ratio of 16.8. Therefore, incorporation of wheat straw with high C/N ratio presumably stimulated net immobilisation of soil mineral N, resulting in lower N substrate for rice to take up. Rao and Mikkelsen (1976) reported that incubating straw in soil for 15-30 days before planting seedlings decreased N immobilisation and promoted plant growth. In fields with delayed transplantation, wheat straw decomposed anaerobically before the rice seedlings were transplanted, thus potentially decreasing N immobilisation, and hence improving rice growth and grain yield.

The objectives of this study are (i) to test the hypothesis that later transplantation of rice in soils incorporated with wheat straw will decrease C[H.sub.4] emissions and N immobilisation and increase rice growth and grain yields; and (ii) provide a scientific basis for abating short-term effects of wheat straw incorporation into paddy fields.

Materials and methods

Field site and experimental treatments

The field experiment was carried out in a paddy rice field at Dapu Town, Yixing City, Jiangsu Province, China (31[degrees]17'N, 119[degrees]54'E) in 2005. The soil was classified as Typic Epiaquepts (Soil Survey Staff 1975) and its properties were: initial pH 6.23, total C 1.26%, and total N 0.13%.

Two levels of wheat straw (0 and 3.75 t/ha) and 2 rice transplanting times (normal and delayed) were adopted in this experiment (Table 1). Four treatments were laid out in a randomised block design in triplicate. The plot area was 50[m.sup.2] (8m by 6.25m). Ten-cm chopped wheat straw was evenly incorporated into the 0.12-m topsoil on 1 June. All the plots were first flooded on 7 June, and the crops therein were all harvested on 18 October.

An application rate of 270kg N/ha was adopted in this experiment. Compound fertiliser [N(15)-P(10)-K(15)] at a rate of 375kg/ha and ammonium bicarbonate at a rate of 475kg/ha were applied together as basal fertiliser incorporated into the soil on 8 June in Treatments U and U +S and 30 June in Treatments L and L+S. Urea at a rate of 150 kg/ha was broadcast as topdressing at the tillering stage on 18 June in Treatments U and U+S and 10 July in Treatments L and L+S, and at the panicle initiation stage on 22 July in Treatments U and U+S and 3 August in Treatments L and L+S.

Local conventional practices of water management for rice cultivation were followed. After the first flooding, the plots were submerged continuously for 32 days in the normal transplanting treatments and for 46 days in the delayed treatments. Then a 1-week-long midseason aeration was imposed followed by intermittent irrigations until the crop was ready for harvest. The midseason aeration in all the treatments was disrupted by heavy rain.

Sampling and measurements

After the first flooding, C[H.sub.4] flux was monitored using the static chamber technique. The flux chamber (0.5 by 0.5 by 1 m) covered 6 hills each in the paddy field. Plastic bases (0.5 by 0.5 by 0.2 m) for the chambers were permanently installed at all plots 1 day before the first flux measurement and remained there until the crop was harvested. Removable wooden gangways (2 m long) were set up well before land preparation to avoid soil disturbance during gas sampling. Gas sampling was arranged at an interval of 2-7 days throughout the rice season. Four gas samples from each chamber were collected using 18-mL vacuumed vials at an interval of 10min between 08:00 to 11:00 hours on every sampling day. Methane concentration was analysed using a gas chromatograph equipped with a flame ionisation detector (Shimadzu GC-12A, Japan). Methane flux was determined from the slope of the linear regression and expressed in mg/[m.sup.2].h (C[H.sub.4]). Total C[H.sub.4] emission was calculated using the following expression:

T C[H.sub.4] = [n.summation.over (i=1)]([F.sub.i] x 24 x [D.sub.i])

where [F.sub.i] stands for C[H.sub.4] flux in the ith sampling interval (mg/[m.sub.2].h); [D.sub.i] the number of days in the ith sampling interval (d); n the number of sampling intervals; 24 the number of hours per day.

When C[H.sub.4] flux was monitored, soil redox potential (Eh) was simultaneously measured, using Pt-tipped electrodes (Hirose Rika Co. Ltd, Japan) and an oxidation-reduction potential meter with a reference electrode (Toa PRN-41). For measuring soil Eh, electrodes were inserted into the soil at a depth of 0.10 m and kept there throughout the rice-growing period.

After gas sampling, soil samples were taken from the 0-0.15m soil layer at 3locations in each replicate plot and mixed evenly within replicates. Soil water content was determined gravimetrically by oven-drying samples at 105[degrees]C for > 8 h. Fresh soil samples (20 g) were extracted with 100 mL 2 M KCl. N[H.sub.4.sup.+]-N and N[O.sub.3.sup.-]-N contents in the extracted solution were determined using a continuous-flow automatic analyser (Skalar, Holland). Soil mineral N concentration was calculated by N[H.sub.4.sup.+]-N and N[O.sub.3.sup.-]-N contents in the extracted solution and corresponding soil water content. The mean concentration of soil mineral N during the observation period was the average of triplicate contents weighted by an interval of 2 measurements.

Samples of whole plant were taken from a 0.2-[m.sup.2] area in each plot at 4 rice growth stages, maximum tillering (7 July for Treatments U and U+S and 23 July for Treatments L and L+S), shooting (21 July for Treatments U and U+S and 3 August for Treatments L and L+S), heading (27 August for Treatments U and U+S and 20 August for Treatments L and L+S), and ripening (16 October for Treatments U and U+S and 15 October for Treatments L and L+S). Dry matter was determined by oven-drying at 80[degrees]C for 72h. Rice grain yields of all treatments were determined at harvest.

Statistical analysis of the data was performed in triplicate by 1-way ANOVA and l.s.d. (least significant difference) statistical test in SPSS 10.0 for Windows. Standard deviation of the mean was calculated using Microsoft Excel 2003 software for Windows. The relationship between C[H.sub.4] flux and soil Eh was analysed by linear regression in SPSS 10.0 for Windows.

Results

Methane emissions

Treatment U+S was higher than Treatment U in C[H.sub.4] flux (Fig. 1a), and about 15 times in seasonal flux peak (163 mg/[m.sup.2].h). One-way ANOVA analysis showed that wheat straw incorporation significantly enhanced C[H.sub.4] emissions (P=0.025, Table 2). In Treatment U+S, a large standard deviation was found between triplicates (Table 2), which indicated that the experimental field was not homogenous. After the first flooding of rice field, soil Eh in Treatment U+S decreased faster than that in Treatment U (Fig. 1b). Correspondingly, C[H.sub.4] flux in Treatment U+S was detected on day 9 after the first flooding, whereas it was not observed in Treatment U until day 22 (Fig. 1a). Significant negative correlations between C[H.sub.4] flux and soil Eh were found during the observation period ([alpha] = 0.95, R = -0.357 to -0.348, P = 0.038-0.044).

[FIGURE 1 OMITTED]

For the 2 delayed transplant treatments, higher C[H.sub.4] flux was observed in Treatment L+S throughout the observation period (Fig. 1a). Seasonally, C[H.sub.4] flux peaked up to 110mg/[m.sup.2].h in Treatment L+S and 12 mg/[m.sup.2].h in Treatment L. The C[H.sub.4] flux in Treatment L+S was ~12-fold of that in Treatment L (P = 0.022). Before delayed transplantation was done, the field had been flooded for 24 days. Methane flux during this period averaged 44.85 mg/[m.sup.2].h in Treatment L+S, whereas it was not detectable in Treatment L. Soil Eh in Treatment L+S decreased more quickly than that in Treatment L after the first flooding event (Fig. 1b). Significant negative correlations between C[H.sub.4] flux and soil Eh were also found in the 2 treatments ([alpha] = 0.95, R = -0.379 to-0.345, P = 0.027-0.046).

No significant difference was observed between Treatments U+S and L+S in total C[H.sub.4] emission(P=0.992, Table 2). In Treatment U+S, the paddy field was kept flooded continuously for 32 days before midseason aeration, and about 61% of the total C[H.sub.4] emission occurred during this period. In Treatment L +S, ~63% of total C[H.sub.4] emission emitted within 46 days before midseason aeration. Methane flux averaged 65.37mg/[m.sup.2].h in Treatment U+S before midseason aeration, about 1.4 times as much as that in Treatment L+S (45.08 mg/[m.sup.2].h).

Soil mineral N

In Treatments U and U+S, soil N[H.sub.4.sup.+]-N concentration increased rapidly after basal fertiliser application, remained at a high level for about half a month, and then gradually decreased (Fig. 2a). After midseason aeration, soil N[H.sub.4.sup.+]-N concentration remained < 15 mg/kg soil until fertiliser application at panicle initiation rapidly increased the soil N[H.sub.4.sup.+]-N concentration up to 49 mg/kg soil, which then dropped back to 12 mg/kg soil within 6 days (Fig. 2a). After basal application and topdressing at tillering, the soil N[H.sub.4.sup.+]-N content was much higher in Treatment U than in Treatment U+S. The mean soil NH4+-N concentration for the season was 33.65 mg/kg soil in Treatment U and 28.73 mg/kg soil in Treatment U+S. Incorporation of wheat straw resulted in net N immobilisation in soils (P = 0.047). Soil N[O.sub.3.sup.-]-N concentration remained at low levels during the entire rice-growing period in Treatments U and U+S, showing no difference between the 2 treatments (P = 0.537, Fig. 2b).

In Treatments L and L+S, soil N[H.sub.4.sup.+]-N concentration increased gradually after the first flooding of the rice field (Fig. 2a). A rapid increase in soil N[H.sub.4.sup.+]-N concentration was observed after basal fertiliser application, which kept the soil N[H.sub.4.sup.+]-N concentration high for a month. During this period, the soil N[H.sub.4.sup.+]-N content was greater in Treatment L+S than in Treatment L (Fig. 2a). Soil N[H.sub.4.sup.+]-N concentration rose in response to topdressing at panicle initiation, but fell to below 15mg/kg soil in the following months. During the entire observation period, the mean soil N[H.sub.4.sup.+]-N concentration in Treatment L+S was 36.58mg/kg soil, about 1.4 times as much as that in Treatment L (25.42 mg/kg soil). Significant mineralisation of N occurred in Treatment L+S (P = 0.001). During the observation period, soil N[O.sub.3.sup.-]-N concentration remained low and at similar levels in Treatments L and L+S (P = 0.114, Fig. 2b).

Dry matter accumulation and grain yield

At all 4 growth stages, dry matter production tended to be higher in Treatment U than in Treatment U+S (P = 0.157-0.942), but tended to be lower in Treatment L than in Treatment L+S (P = 0.340-1.000, Fig. 3). Grain yield did not differ between Treatment U and Treatment U+S (P = 0.853), and tended to be higher in Treatment L+S than in Treatment L (P = 0.299, Table 2).

Discussion

Effect of timing of rice transplanting on C[H.sub.4] emissions from paddy fields incorporated with wheat straw

An effect of wheat straw incorporation stimulating C[H.sub.4] emissions was observed in this study (Table 2), which supported previous reports (Watanabe et al. 1995; Bronson et al. 1997; Cai 1997; Naser et al. 2007). Easily degradable crop residues were a primary source of C[H.sub.4] production. Decomposition of wheat straw in an anaerobic environment provided substantial methanogenic substrates. Besides, organic matter input accelerated decrease in soil Eh (Fig. 1b), which favoured C[H.sub.4] production. Negative correlation between C[H.sub.4] flux and soil Eh has been reported in previous studies (Kludze et al. 1993; Wang et al. 1993; Yu and Patrick 2004).

Before delayed rice transplantation was performed, wheat straw underwent anaerobic decomposition for 24 days, which provided considerable substrates and optimal conditions for C[H.sub.4] production (Fig. 1b). Methane could be transferred from soil to atmosphere through ebullition of gas bubbles, and diffused through rice aerenchyma or through flooding water (Schultz et al. 1989; Neue 1993). Previous studies reported that ebullition played an important role in C[H.sub.4] emission from unvegetated soil (Holzapfelpschorn et al. 1986; Wassmann et al. 1996, 2000). Although rice plants were absent during the first 24 days of flooding, substantial C[H.sub.4] could be emitted from unplanted paddy fields by ebullition as a result of anaerobic decomposition of wheat straw (Fig. 1a).

In this study, delayed transplanting was originally adopted as a means to reduce the effect of wheat straw incorporation stimulating C[H.sub.4] emissions. Unfortunately, the total C[H.sub.4] emissions from the delayed transplanting treatment were almost the same as from normal transplanting (Table 2), especially before midseason aeration. Water management was crucial for C[H.sub.4] emissions from rice field. Midseason aeration was extensively adopted in rice cultivation in China, and its effect on reducing C[H.sub.4] emission has been widely reported (Cai et al. 1997; Lu et al. 2000). The results showed that in fields incorporated with wheat straw, most C[H.sub.4] was emitted before midseason aeration. Although C[H.sub.4] flux during this period was lower in the delayed transplanting treatment than in the normal transplanting treatment, it was counter-balanced by the former's longer duration of the corresponding period. As a result, total amounts of C[H.sub.4] emissions were comparable between normal and delayed transplanting treatments. Delayed transplanting had no impact on reducing the effect of wheat straw incorporation stimulating C[H.sub.4] emission under the intermittent irrigation pattern.

[FIGURE 2 OMITTED]

Effect of timing of rice transplanting on soil mineral N, rice growth, and grain yield in paddy fields incorporated with wheat straw

In fields where rice seedlings were transplanted at normal time, wheat straw incorporation resulted in decrease in soil N[H.sub.4.sup+]-N concentration (Fig. 2a), which clearly indicated the net N immobilisation effect of wheat straw. In this study, the C/N ratio of wheat straw was as high as 96.15, beyond the critical value of the C/N ratio for N mineralisation and immobilisation (Black 1968; Vigil and Kissel 1991). When these wheat straws with high C/N ratio were incorporated into the field, the mineral N applied on the normal date was severely immobilised by soil microorganisms, leading to depletion of available N and then suppression of the growth of rice plants. Consequently, rice growth, as represented by dry matter accumulation, was slightly inhibited by wheat straw incorporation (Fig. 3). The yield in the normal transplanting treatment was not affected by wheat straw incorporation in this study (Table 2), which was mainly attributed to the input of chemical N fertiliser at a high level.

In fields where rice transplantation was delayed, a higher concentration of soil N[H.sub.4.sup+]-N was detected in the field incorporated with wheat straw (Fig. 2a), which demonstrated the net N mineralisation effect of wheat straw after 24 days of anaerobic decomposition. High soil temperature in summer (30[degrees]C) and flooding aided decomposition of wheat straw, thus leading to mineralisation of considerable straw-C during the initial 24 days of the flooding period. Substantial C[H.sub.4] emission from unplanted field incorporated with wheat straw (Fig. 1a) supported this argument. Methane is produced as the terminal step of the anaerobic decomposition of organic matter in flooded soils (Neue 1993). Under flooding conditions, C[H.sub.4] originated primarily from decomposition of straw and not, or only little, from soil organic matter (Devevre and Horwath 2000). After 24 days of anaerobic decomposition, the decrease in straw-C not only limited incorporation of applied N into microbial tissues, but also induced mineralisation of organic N, leading to significant N mineralisation in a rice field pre-incorporated with wheat straw (Fig. 2a). Previous studies reported a definite beneficial effect of incubating the rice straw for 15-30 days on inorganic N (Rao and Mikkelsen 1976). In fields where rice transplantation was delayed, dry matter accumulation in rice at all 4 growth stages (Fig. 3) and grain yield (Table 2) tended to be enhanced by wheat straw pre-incorporation, which may be attributed to the net N mineralisation.

[FIGURE 3 OMITTED]

Conclusion

The findings of this study suggested that the practice of delayed transplanting will partially alleviate the short-term effects of wheat straw incorporation into rice fields. It will not reduce the effect of wheat straw application stimulating C[H.sub.4] emissions, but will significantly stimulate mineralisation of N in wheat straw incorporated fields, compared with normal transplantation on net N immobilisation. Accordingly, delayed transplantation of rice in wheat straw incorporated fields may tend to promote growth of rice plant and increase grain yield.

Acknowledgments

This work was funded by the National Natural Science Foundation of China (grant Nos. 40621001 and 40671094). This research was also supported by 'Global environment research fund' S-2-3a of the Ministry of the Environment, Japan.

Manuscript received 7 August 2007, accepted 5 March 2008

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J. Ma (A,C), H. Xu (A,D), 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, 3-1-3 Kannondai, Tsukuba 305-8604, Japan.

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

(D) Corresponding author. Email: hxu@issas.ac.cn
Table 1. Experimental treatments

 Wheat straw Rice transplanting
Treatments incorporation (dha) time Rice cultivar

U 0 9 June suyou5356
U+S 3.75 9 June suyou5356
L 0 1 July N04
L+S 3.75 1 July N04

Table 2. Total C[H.sub.4] emissions and grain yields in a rice field
experiment at Dapu, China

Mean [+ or -] standard deviation. U, Normal transplanting time; L,
late transplanting; S, wheat straw incorporation. Values within each
column followed by the same letter are not significantly different
(P>0.05)

 Total C[H.sub.4] emission Grain yield
Treatment (kg C[H.sub.4]/ha) (t/ha)

U 79.6 [+ or -] 25.4a 6.68 [+ or -] 10.50a
U+S 827.7 [+ or -] 662.6b 6.61 [+ or -] 0.34a
L 66.2 [+ or -] 31.2a 6.45 [+ or -] 0.30a
L+S 824.6 [+ or -] 20.96 6.84 [+ or -] 0.31a
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Author:Ma, J.; Xu, H.; Han, Y.; Cai, Z.C.; Yagi, K.
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:9CHIN
Date:May 1, 2008
Words:4820
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