Methane emission from rice fields as affected by land use change

Agriculture, Ecosystems and Environment 139 (2010) 742–748

Contents lists available at ScienceDirect

Agriculture, Ecosystems and Environment journal homepage: www.elsevier.com/locate/agee

Methane emission from rice fields as affected by land use change Moniruzzaman Khan Eusufzai a,∗ , Takeshi Tokida b,c , Masumi Okada d , Shu-ichi Sugiyama e , Guang Cheng Liu d , Miyuki Nakajima a , Ryoji Sameshima a a

Climate Change Research Team, National Agricultural Research Center for Tohoku Region (NARCT), 4 Akihira, Shimo-Kuriyagawa, Morioka, Iwate 020-0198, Japan National Institute for Agro-environmental Sciences, Tsukuba, Ibaraki 305-8604, Japan c Japan Society for the Promotion of Sciences, Tokyo 102-8471, Japan d Dept. of Agro-bioscience, Faculty of Agriculture, Iwate University, 3-18-8 Ueda, Morioka, Iwate 020-8550, Japan e Dept. of Biological Science, Faculty of Agriculture, Hirosaki University, 3 Bunkyo cho, Hirosaki-shi, Aomori-ken 036-8560, Japan b

a r t i c l e

i n f o

Article history: Received 29 August 2010 Received in revised form 27 October 2010 Accepted 1 November 2010 Available online 3 December 2010 Keywords: Rice paddy Land conversion Methane Electron donor Electron acceptor Methanogens

a b s t r a c t The purpose of this study was to evaluate how former upland cultivation history affects CH4 emission from rice paddies. We measured CH4 flux, methanogen population and in situ Fe(III) reduction in the rice paddies following three different lengths of time since upland crop (Soybean) cultivation. Results showed that CH4 emissions from long-term rice paddy (19 year’s continuous cultivation) were significantly higher than recently converted ones. Temporal dynamics of methanogens on rice roots also varied among the plots, and showed a good correlation with CH4 emission rates. Cumulative Fe(III) reduction acted as the dominant electron acceptor in all plots, accounting for 68–94% of the total electron consumption. Fe(II) concentration was highest in the 19-year plot and lowest in the 1-year plots, indicating lower electron availability in recently converted paddies necessary for Fe reduction and CH4 production. Anoxic laboratory soil incubation also suggested poor availability of electron donors in the recent paddies. Collectively, our results demonstrate that the conversion of upland to paddy rice cultivation significantly affected CH4 emission through changing availability of electron donors, redox status of soil Fe and activity of methanogens, which ultimately caused low CH4 emissions. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Human activities over the past several centuries have been identified as a leading cause of the increased atmospheric concentration of major greenhouses gases (GHGs): carbon dioxide (CO2 ), methane (CH4 ), nitrous oxide (N2 O) and halocarbons. Methane is the second most important greenhouse gas after CO2 , having a radiative forcing capacity of 25 times higher than CO2 (per mass basis, 100-year time-horizon) if indirect effects are taken into account (Forester et al., 2007; Hansen et al., 2005; Shindell et al., 2009) and roughly responsible for 18% of the total greenhouse effect (Forester et al., 2007). The global atmospheric concentration of CH4 has increased from its pre-industrial level of about 715 ppb to 1750 ppb in the early 1990s and thereafter remained almost constant (Dlugokencky et al., 1992; IPCC, 2007). However, the plateau might have been transitory and atmospheric CH4 level may increase again in the near future (Bousquet et al., 2006). In fact, the global mean atmospheric CH4 concentration has shown signs of renewed growth from the beginning of 2007 (Rigby et al., 2008; Dlugokencky et al., 2009).

∗ Corresponding author. E-mail address: mzk1973 [email protected] (M.K. Eusufzai). 0167-8809/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2010.11.003

Together with other greenhouse gases, the increased concentration of CH4 has been predicted to cause an average increase of the global mean surface temperature in the range of 2–4.5 ◦ C on a 100 years time scale (IPCC, 2007). Methane often known as “marsh gas” is produced in strictly anaerobic environment such as wetlands, sediments, paddy fields, sewage and landfills. Of these sources, paddy fields have long been identified as major contributor to anthropogenic CH4 emissions. At present, estimates of the total global rice paddy emissions range from 30 to 50 Mt yr−1 or 8–17% of the total anthropogenic CH4 flux to the atmosphere (Neue and Sass, 1998; Sass et al., 2002). This wide range of variability in the estimates may partly be driven by the uncertainty in the CH4 flux due to continuous change in land use or management practices (IPCC, 2001). In the past, efforts have been made to quantify changes in CH4 emission in response to land-use change primarily in natural or semi-natural ecosystems such as peatlands reverting to forest (Fowler et al., 1995; Malijanen et al., 2001; Ball et al., 2002) and upland grasslands becoming forest (Oleg Menyailo et al., 2008). These land-use changes typically result in net consumption of CH4 . However, little is known about the effect of land-use change on CH4 emissions in agricultural ecosystems such as the conversion from upland crops to rice paddy cultivation (Kumagai and Konno,

M.K. Eusufzai et al. / Agriculture, Ecosystems and Environment 139 (2010) 742–748

1998; Nishimura et al., 2004). Recently, Nishimura et al. (2004) have estimated the soil carbon budget from a single cropping paddy, single cropping upland rice field and a double cropping soybeanwheat rotation after converting rice paddy fields to upland crop cultivation. Their study demonstrated that CH4 fluxes from upland crops were significantly lower than those from rice paddy fields. Kumagai and Konno (1998) reported that CH4 emissions from a restored paddy after 8 years of upland farming was only 44% of the total emission from a continuous rice paddy. Although these studies considered the effects of land conversion on CH4 emissions, the mechanisms behind the observed changes remain largely unclear. Among the many soil properties involved in CH4 emission, microbially reducible Fe(III) content was found to be one of the most important components (Watanabe and Kimura, 1999; Huang et al., 2009). The reduction of Fe(III) has a quantitative relationship with CH4 production because of its natural abundance in soil as the dominant electron acceptor that competes for the common electron donors (mostly H2 or acetate) with CH4 production during the sequential reduction processes in anoxic paddy soil (Takai, 1961; Frenzel et al., 1999). Competition with Fe reduction strongly inhibits CH4 production because of differences in energy yield, as predicted by thermodynamic theory (Takai and Kimura, 1966). A change in land use might have marked effects on CH4 production due to a change in the soils reducing conditions, which are largely controlled by the redox status of Fe. Therefore, a simultaneous monitoring of soil Fe reduction would be highly informative for investigating the response of CH4 emission to land use change. The objective of our study was to evaluate how the conversion of upland soils into paddy rice cultivation affects CH4 emission. We hypothesized that change in CH4 emissions were associated with (i) soil properties (e.g. oxidation–reduction status), (ii) microbial population (e.g. methanogenic archaea) or (iii) substrate availability for microbial decomposition. To this end, we measured CH4 flux, concomitant Fe(III) reduction, changes in the methanogenic archaeal populations in paddy plots and CH4 production potential following different lengths of time since upland crop cultivation. 2. Materials and methods

743

2.2. Measurement of CH4 gas flux Methane emissions were measured using the closed chamber method with a rectangular, transparent, closed top, acrylic chamber (110 cm high with a basal area of 30 cm × 35 cm and 0.5 cm thick) during the rice cultivation period. In each plot, chambers were placed on clamps attached to metallic bars installed vertically at the time of transplanting. The clamps and bottom of the chamber were under the level of paddy water to ensure a gas tight seal. In each plot, the flux measurements were conducted in duplicates. The CH4 fluxes were measured on a weekly to bi-weekly basis from the day after transplanting to before harvesting. Two hills (i.e. six plants) were covered by each chamber. During the sampling period of 30 min, a gas sample of about 25-ml was drawn with a 35-ml plastic syringe at 0, 15 and 30 min after the chamber was emplaced and injected into 20-ml vacuum bottles with a rubber stopper and screw cap. The bottles were taken back to the laboratory, where the amount of CH4 was measured by a gas chromatograph (Shimadzu GC-14B, Kyoto, Japan) equipped with a flame ionization detector (FID). The operating temperatures for GC were adjusted to 80, 150, and 150 ◦ C for the column, injector and detector, respectively. Methane fluxes were calculated from the slope of the linear increase of gas concentration inside the chamber for the given time interval and expressed as per unit surface area. 2.3. Methane production potential Methane production potential was measured using an anaerobic laboratory incubation method. Before rice transplanting, triplicate soil samples were collected from each plot, air dried and passed through 1 mm sieve to exclude the visible rice residues and weeds. Approximately 10 g of air dried soil was incubated in 100ml serum bottles at 1:2 soil:water (soil to water, w/w) ratio. The head space was replaced with pure nitrogen gas and the samples were incubated at 30 ◦ C. All samples were run in triplicate for a period of 42 days. Methane production potential from the different paddy plots was estimated as the total amount of CH4 produced in the incubation bottle over 42 days of incubation (Cheng et al., 2000).

2.1. Description of experimental fields and cultivation history 2.4. Measurement of Fe(II) This experiment was conducted from May to September 2008 at the National Agricultural Research Center for Tohoku Region, Morioka, Japan (39◦ 45 N, 141◦ 08 E). A total of four plots were used, each comprising an area of 120 m2 . One plot has been under rice cultivation for the last 19 consecutive years (i.e.19 years as a paddy), since conversion from upland soybean (Glycine max L.). The 2-yearold paddy plot was converted from soybean to rice paddy just 1 year ago. Rice cultivation has just begun in the other two plots (1-year paddy) which were cultivated with soybean for the last 6 years. No organic matter (including rice straw) has been added to these plots. The soil is a volcanic ash soil (Andisol) and classified as a sandy loam with sand content of 45.2%. We used high yielding Japonica-type rice cultivar, Akitakomachi, which is common in this region. On May 29th, seedlings were transplanted to all four fields. Each hill contains three rice plants. Nitrogen was applied as control-release urea (Kumiai-42-Hifuku-Nyouso-LP70, Zen-Noh, Tokyo, Japan) at 60 kg-N ha−1 and ammonium sulfate at 20 kgN ha−1 before transplanting. Phosphorus (P) was supplied as fused magnesium phosphate at a rate of 130 kg-P ha−1 and 125 kg-K ha−1 was applied as potassium chloride and potassium silicate before flooding. All fields were continuously flooded during the cropping period.

The time course of Fe(III) reduction was monitored by measuring the in situ Fe(II) concentration as a cumulative product at 42, 56, 63, 70 and 92 days after transplanting (DAT) to cover the different plant growth stages. We collected the top 5 cm of soil using a cut-tip plastic syringe. Soil samples were collected from between the hills. Immediately after sampling, the plastic syringes were capped with a butyl rubber stopper to prevent redox reactions occurring prior to analysis. Soil Fe(II) was extracted and quantified following the method of Takai et al. (1958). About 2–3 g of wet soil was transferred to a 30 ml centrifuge tube and then 25 ml of pH 3.0 sodium acetate buffer solution was added. The sample was extracted by shaking for 5 min at room temperature, centrifuging at 2000 rpm for 5 min and filtered through a 0.4 ␮m filter paper (Advantec, Tokyo Roshi Kaisha Ltd., Japan). About 0.25–0.5 ml of supernatant was transferred into a 15 ml centrifuge tube (Iwaki Glass Corporation, Tokyo, Japan), mixed with 0.4 ml hydroxylamine hydrochloride and 0.4 ml of 0.2% o-phenanthroline. The supernatant was analyzed for Fe2+ calorimetrically with a UV/VIS spectrophotometer at 510 nm (Shimadzu UV mini-1240, Kyoto, Japan). The moisture content of the remaining wet soil sample was determined gravimetrically, and the Fe(II) content was calculated on a dry-soil basis.

M.K. Eusufzai et al. / Agriculture, Ecosystems and Environment 139 (2010) 742–748

2.6. Stoichiometric analysis A stoichiometric analysis was conducted to quantitatively evaluate the importance of Fe reduction as an alternative terminal electron-accepting reaction that competes against CH4 production. The number of electrons transferred for Fe(III) reduction and CH4 production was calculated based on measured Fe(II) concentrations and CH4 flux using the following half reaction equations which have already verified experimentally (Roden and Wetzel, 1996): CO2 + 8H+ + 8e− → CH4 + 2H2 O −

Fe(III) + e → Fe(II)

(1) (2)

Electron demand by CH4 production was estimated from CH4 production rate, which we assumed was identical to the observed CH4 flux. We acknowledge that this assumption may underestimate the gross CH4 production as some portion of CH4 produced would be consumed by oxidation by methanotrophs. In situ measurement of CH4 oxidation rates have suggested that CH4 oxidation might be significant (∼40%) at the beginning of the growing season (Krüger et al., 2001), although it is of only minor importance (<7%) during later periods (Groot et al., 2003). Units are converted into a per area basis by taking bulk density (Mg m−3 ) and plow depth (ca. 10 cm) into account. 2.7. Statistical analysis Land-conversion history was analyzed by treating year(s)after-conversion as a fixed effect. The 1-year-old plots had two replications; however, no replication was available for the 2- and 19-year experimental units, thereby our statistical results should be interpreted under consideration of few and missing replications. For statistical computation the PROC MIXED of SAS v9.2 (SAS Institute Inc.) with the restricted maximum likelihood (REML) method was used (Littell et al., 2006). The Kenward–Roger method was used

30

100

25

80

20

60

15

40

10

20 0

5 7 14 21 28 35 42 49 56 63 70 77 84 91 98 105

Days after transplanting Mean daily Precipitation(mm)

Water temperature

Air temperature

Soil temperature

Fig. 1. Climate conditions of the study area during the rice cultivation period. Air temperatures, soil temperatures and precipitation are the daily average for the growing period. Soil temperature was measured at 0–10 cm depth.

to estimate the denominator degree of freedom (Spilke et al., 2005; Kenward and Roger, 1997). 3. Results 3.1. Study area and environmental factors The study area had a relatively short and cool summer during the rice growing season. Average daily air temperature was 22.3 ◦ C and showed considerable seasonal variation during the cropping season (Fig. 1). The average daily soil temperature (0–10 cm depth) was approximately 1.3 ◦ C higher than the air temperature during the rice growth period and did not differ among different year’s paddy fields (data not shown). The seasonal temperature of flooding water was 23.2 ◦ C and was identical for all plots. Maximum precipitation occurred around 49–77 DAT which corresponded to the mid-growth stage of rice (Fig. 1). 3.2. CH4 flux The amount and pattern of CH4 emission varied significantly among fields with different cultivation history. Emissions from the long-term paddy plot were greater than those from the recently converted 1- and 2-year plots (Fig. 2). The average CH4 fluxes 10.0

CH4 flux (mg C m-2 hr-1)

The abundance of methanogens was quantified by real-time PCR using a methanogen specific ␣-subunit of the methyl-coenzyme reductase (mcrA) gene. Soil was sampled from the midpoint between the hills using a 100 ml soil core sampler (5 cm in diameter) at the tillering (DAT 34), flowering (DAT 74) and grain filling (DAT 93) stages of plant growth. Total DNA was extracted from root free soil (ca. 0.66 g) collected from the mid-section of the core samplers and was subsequently used for DNA extraction. DNA extraction was performed with the UltrCclean 15 DNA Purification kit (MO BIO Labs, Solana Beach, CA, USA) according to the manufacturer’s instructions and stored at −20 ◦ C before analysis. Fresh root contained in the sampled soils were separated and their DNA was also extracted by CTAB method. Real time-PCR was performed using the nucleic acid strain SYBR Green I (Takara Co., Tokyo) and a thermal cycler, DNA Engine PTC200 (Biorad, USA) according to the manufacturer’s instructions. The primer set used was ME1 (GCMATGCARATHGGWATGTC) and ME2 (TCATKGCRTAGTTDGGRTAGT) (Hales et al., 1996). The operating conditions for ME1 and ME2 were 30 cycles of 94 ◦ C for 40 s, 53 ◦ C for 40 s, 72 ◦ C for 1 min and a final extension step at 72 ◦ C for 7 min. The DNA amount of purified PCR products of mcrA gene was quantified by spectrophotometer (NonoDrop ND-1000, Thermo Scientific, Wilmington, USA). The DNA amount of each sample was estimated by a calibration curve obtained by a series of dilution solution of the known PCR products. The copy numbers of the mcrA genes were calculated by dividing the DNA amount by molecular weight of the target PCR product (760 bp).

Daily mean temperature (ºC)

2.5. Methanogens by real time PCR

Daily mean precipitation (mm)

744

1-year 2-year

8.0

*

19-year 6.0

*

4.0

*

**

*

*

ns

2.0

* ns

0.0 7

21

35

49

63

77

91

105

Days after transplanting Fig. 2. Seasonal changes in CH4 fluxes in different year’s paddy plots after converting from upland crops to rice paddy cultivation. Vertical bars indicate 95% confidence interval (n = 2). Asterisks (*, **) are probability levels of significance at 5% and 1%; ns, not significant. Arrow indicates heading day.

M.K. Eusufzai et al. / Agriculture, Ecosystems and Environment 139 (2010) 742–748

745

Table 1 Changes in the mcrA gene copies of the methanogens in paddy soils at tillering (34 DAT), flowering (74 DAT) and grainfilling (93 DAT) stages of the cropping period. Values are mean ± S.E. of triplicate harvests (n = 3). Years after conversion

Methanogens (×106 copies g−1 soil) Days after transplanting

1 2 19

34

74

93

0.08 ± 0.08 0.21 ± 0.02 0.32 ± 0.03

0.09 ± 0.02 0.04 ± 0.01 0.16 ± 0.02

0.08 ± 0.01 0.07 ± 0.01 0.17 ± 0.02

Table 2 Copy number of the mcrA methanogens in rice roots at tillering (34 DAT), flowering (74 DAT) and grainfilling (93 DAT) stages of the cropping period. Values are mean ± S.E. of triplicate harvests (n = 3). Fig. 3. Methane production potential of the different paddy soils (0–5 cm) collected before rice transplanting. Bars indicate standard deviation among the mean values (n = 3).

Years after conversion

during the rice growth period were 0.20 ± 0.18, 0.81 ± 0.32 and 2.65 ± 0.36 mg C m−2 h−1 for 1-, 2- and 19-year plots, respectively. The CH4 emission from 19- and 2-year paddy plot showed distinct seasonal variations with a peak around flowering (70–77 DAT) then declined sharply and at the end tend to be stabilized before harvesting. However, such a clear seasonal trend was not observed in the 1-year plots. Instead of a definite peak, a rapid flush of CH4 emission was observed at the very early stage of rice growth (28 DAT) and thereafter dropped to a near zero value which persisted throughout entire growth period. Integrating CH4 emission over the rice growth period revealed that the total CH4 emissions from the 19-year plot were greater than the 1- and 2-year plots and that the differences were statistically significant (p < 0.05). Total seasonal CH4 emissions from 1and 2-year plots were 0.35 and 1.72 g m−2 , respectively which corresponded to only 6 and 28% of the total emission observed in the 19-year plot (6.10 g m−2 ).

1 2 19

Methanogens (×106 copies g−1 root) Days after transplanting 34

3.3. Methane production potential (MPP) Cumulative CH4 production measured in the laboratory incubation represented the CH4 production potential (MPP) of the different years’ paddy soils. In 42 days of incubation, the MPP of the 19-year paddy soils was larger by several orders of magnitude than for the 1- and 2-year fields (Fig. 3). The average MPP of the

Fe(II) concentration ( µmol g-1dw soil )

200

2 year

ns

19 year

100

ns ns

ns *

50

42

49

ND 2.53 ± 0.23 15.64 ± 4.64

ND = not determined.

1-, 2- and 19-year paddy soils were 0.03 ± 0.01, 0.07 ± 0.04 and 3.06 ± 0.72 mg C kg−1 dw soil−1 day−1 , respectively, at 30 ◦ C.

3.4. Fe(II) concentration Fe(III) reduction followed a similar trend as for CH4 flux, i.e. highest in the 19-year plot followed by the 2-year and was the lowest in the 1-year plots (Fig. 4). The concentration of Fe(II) in 19-year plot was higher than in the 1- and 2-year plots. After 92 DAT, the rate of Fe reduction in 1-, 2- and 19-year paddy plots were 0.64, 0.76 and 1.48 ␮mol g−1 soil d−1 , respectively. The amount of Fe(II) varied with the rice growth, especially for 19- and 2-year paddy plots. Fe(II) concentration gradually increased in the early growth period (0–42 DAT), steadied around heading to flowering (56–70 DAT) and again sharply increased at the later stage (70–92 DAT) of rice growth. However, Fe(II) concentrations in the 1-year paddy plots temporally ceased until 70 DAT and thereafter gradually increased.

**

150

35

93

3.5. Population dynamics of the methanogens

1 year

0

74

0.47 ± 0.17 0.21 ± 0.17 0.48 ± 0.10 0.32 ± 0.02 0.73 ± 0.35 22.06 ± 4.46

56

63

70

77

84

91

98

105

Days after transplanting Fig. 4. Temporal changes in Fe(II) concentrations in the different years paddy soils during the rice growth season. Vertical bars indicate 95% confidence interval (n = 2). ns, not significant; *p < 0.05; **p < 0.01.

The abundance of methanogenic archaea, analyzed by the copies of mcrA genes, both in the soil and on the rice roots in the different year’s paddy plots at tillering, flowering and grain filling stages of rice growth are shown in Tables 1 and 2, respectively. Methanogenic archaeal population in different year’s paddy soils either remained constant or decreased over the rice growth period. However, at any stage of plant growth, methanogens in 19-year soils were greater than 1- and 2-year paddy plots (Table 1). Methanogens on the rice roots were also higher in the 19-year plot when compared with 1- and 2-year plots, amounting to more than 107 copies g−1 roots (Table 2). The abundance of methanogens on the rice roots was pronounced around flowering to grainfilling stage in the 19- and 2-year paddy plots. However, root methanogens in the 1-year paddy plots showed a decreasing trend until flowering stage (74 DAT) and amounted to 106 copies g−1 roots.

M.K. Eusufzai et al. / Agriculture, Ecosystems and Environment 139 (2010) 742–748

4. Discussion 4.1. The CH4 flux and production potential The seasonal variation of CH4 fluxes (Fig. 2) exhibited a more or less similar pattern to those reported in many previous studies, i.e. maximum around heading to mid-grain filling stage and then decreasing at maturity (Nouchi et al., 1994; Wassmann et al., 2000; Inubushi et al., 2003). The high emission during the reproductive stage has often been related to an increase in availability of substrate for CH4 production from dead leaves, slough off cells and root exudates in the form of rhizodeposition (Minoda et al., 1996; Watanabe et al., 1999) as well as a higher conductivity of CH4 through the rice pant due large amount of biomass being present and well developed aerenchyma system (Nouchi et al., 1990; Watanabe et al., 1999; Aulakh et al., 2002). However, an early peak in 1-year paddy fields might be associated with a rapid flush of microbial biomass upon flooding and the degradation of antecedent soil organic matter (Jia et al., 2002) which diminishes over the course of time. The magnitude of the CH4 emissions from the 19-year plot was much higher than 1- and 2-year plots. High CH4 emissions from 19-year plot were probably caused by the increased amount of soil organic carbon due to continuous rice cultivation for 19 consecutive years compared with the short-term rice paddy (i.e. 1- and 2-year plots). This observation was also supported by Witt et al. (2000), who found increased soil carbon due to rice-rice double cropping for 2 years in a paddy field in the Philippines. Nishimura et al. (2004) also found a positive soil carbon budget in continuous paddy rice fields by analyzing the soil carbon content. The positive soil carbon budget was attributed to net increases of carbon supply due to photosynthetic CO2 absorption by aquatic weeds and algae compared with a net loss by CO2 emission from paddy fields. Organic carbon content is an important parameter in CH4 production in paddy soils; however, the rate of production is highly dynamic and strongly coupled to other soil factors such as the presence of alternative electron acceptors and abundance of microorganisms in methanogenic system (Lovely and Phillips, 1986; Hou et al., 2000). In the following section of this paper, we will discuss how these factors could explain the observed differences in CH4 emission rate among different year’s paddy plots. Although CH4 emission from paddy rice is a plant mediated process, net CH4 emission is determined by the balance between CH4 production and consumption (Bouwman, 1990; Conrad, 1989; Inubushi et al., 2003). Methane production is primarily a biologically driven process under strictly anaerobic conditions, where the supply of organic substrate is considered to be the major limiting factor. It is generally assumed that under anaerobic conditions, soils containing a high amount of organic compounds have higher production potential. The MPP of different-year paddy soils convincingly confirmed the above theoretical considerations (Fig. 3) and followed the same order of CH4 emission rates. The higher MPP in the 19-year plot reflects the greater soil carbon accumulation during long term rice cultivation than in the short duration paddies. We realize that rhizodeposition may serve as an important precursor of CH4 , especially in the later stage of plant growth (Watanabe et al., 1999). However, soil organic carbon could be an important substrate for CH4 in the early part of the season (Watanabe et al., 1999) and also for Fe(III) reduction (Cheng et al., 2007), which is critically important for the initiation of CH4 production (see below). 4.2. Cumulative Fe(III) reduction in relation to CH4 emission In anaerobic paddy soils, microbial degradation of organic matter is achieved via sequential reduction of inorganic electron acceptors such as, O2 , nitrate, Mn(IV), Fe(III), sulfate and CO2 (Peters

Number of electron transferred (mol m-2)

746

10 9

CH4

8

Fe

7 6 5 4 3 2 1 0 1

2 0-42

19

1

2

19

42-70

1

2

19

70-92

Days after transplanting Fig. 5. Amount of electron accepted by Fe(III) and CH4 at tillering (0–42 DAT), flowering (42–70 DAT) and mid-grain filling stages (70–92 DAT) of rice growth. The calculations are based on the measurement of Fe(II) concentrations and CH4 flux.

and Conrad, 1996; Watanabe, 1984). Thermodynamically, CH4 production (either by CO2 reduction with H2 or transmethylation of acetate) would be the terminal electron-accepting process because of the smallest free energy change (Patrick and Reddy, 1978; Peters and Conrad, 1996). Thus, the production of CH4 requires consistently low reducing conditions, which is determined by a balance between availability of electron donors (ED) and electron acceptors (EA) (Takai, 1970). It is well known that in wetland sediments such as paddy soils, Fe, in the form of Fe(III), is the main EA and readily decomposable organic matter serves as the principal ED (Takai, 1961). In rice fields plant residues (rice stubble, straw and weeds), amended organic matters (e.g. manures) and intrinsic soil organic matter are the sources of ED. As the rice grows, the supply of ED from the plant itself (rhizodeposition) would become an important source for CH4 production. The varying amounts of Fe(II) in the different-year paddy plots (Fig. 4) are most likely due to the difference in abundance of ED as a consequence of the cropping history of preceding years. Continuous paddy rice cultivation enhanced the supply of organic residues into the soil through stubble rice residues, root exudation and leaf litter, fueling Fe(III) reduction upon flooding. Indeed a direct correlation was found between the Fe(III) reduction rate constant as well as the initial Fe(III) reduction rate and the abundance and mineralization rate of labile organic carbon in wetland sediments (Roden and Wetzel, 2002). However, lower rates of Fe(III) reduction in 1- and 2-year plots could be inferred as being a result of a low ED supply which was in turn affected by the length of non-rice period. The above explanation is supported by previous studies where non-rice growing periods significantly reduced soil organic C content through enhanced organic C decomposition by oxidation, which then leads to retardation of the sequential reduction processes (Xu et al., 2003; Peters and Conrad, 1996). Therefore, for precise estimation of CH4 emission from paddy fields, quantitative evaluation of labile organic C is necessary, which serves as an ED for both Fe(III) reduction and CH4 production. Currently we are unable to provide quantitative evidence of ED supply among the different paddy fields. However, an estimation of electron flow towards Fe(III) reduction and CH4 production could support the above proposition. We calculated the electron equivalent to Fe reduction and CH4 production in each year plots using the half reaction equation of (1) and (2) (Fig. 5) with an assumption of the initial concentration of Fe(II) being zero (␮mol g−1 ) for all plots. The amount of electron transfer due to Fe(III) reduction accounted

M.K. Eusufzai et al. / Agriculture, Ecosystems and Environment 139 (2010) 742–748

for 68–94% of the total electron equivalent (Fe reduction plus CH4 production), suggesting Fe(III) acted as the principal electron acceptor and Fe(III) reduction successfully outcompeted methanogenesis, which is consistent with previous studies (Achtnich et al., 1995; Roden and Wetzel, 1996; Yao et al., 1999). This successful competition by Fe(III) against methanogenesis appeared to be more obvious where the supply of ED was insufficient (e.g. 1- and 2-year plots). This may help to explain, why CH4 emission from 1- and 2-year plots were substantially lower when compared with the 19-year plot. 4.3. Population dynamics of methanogenic archaea The population dynamics of methanogens observed in the different paddy fields for both soil and roots (Tables 1 and 2) are within a similar order of magnitude for Japanese paddy soils as reported by Asakawa and Hayano (1995). However, the populations of methanogens in the soil and roots showed a different temporal pattern. The number of methanogens in the soil showed a constant or decreasing trend, which is similar to other reported studies (Asakawa and Hayano, 1995; Mayer and Conrad, 1990; Fetzer et al., 1993). In contrast, the root populations of methanogenic archaea varied dynamically over the course of rice growth and corresponded well to CH4 emission rates (Fig. 2). This dynamic nature of the methanogens may not have been observed in many previous studies, where a culture based technique was used to count methanogenic populations in paddy soils (Asakawa and Hayano, 1995; Mayer and Conrad, 1990). It is possible that culture based enumeration studies (e.g. MPN counting) underestimate the actual number of methanogens, because of a very long incubation time (>40 weeks) required to count the acetate-utilizing bacteria (Chin et al., 1999). Our data revealed that the greater number of methanogens in the 19- and 2-year plots (Tables 1 and 2) was associated with the development of highly reductive conditions (Fig. 4). Methanogens produce CH4 mostly from either acetate or H2 /CO2 as simple substrate derived from the decomposition of organic matter. Thus, the abundance of methanogens in paddy soils depends on the availability of readily decomposable organic carbon (Wang and Adachi, 2000) and soil redox conditions, because methanogenesis is active under highly reductive conditions (Conrad and Klose, 2006). However, insufficient reductive conditions and substrate limitation in the 1-year paddy fields probably restricted their proliferation, because under carbon-limited condition, Fe-reducing bacteria (FeRB) effectively suppressed the activity of methanogens as FeRB has lower threshold for H2 than do methanogens (Roden and Wetzel, 2003). Unfortunately we did not measure the population changes in FeRB in this experiment. However, it is reasonable to assume that both the Fe redox cycling (i.e. Fe(III)/Fe(II)) and the availability of organic substrates triggered the activity of methanogens, as they can act as a signal for initiating methanogenesis in paddy soils (Unden et al., 1994; Peters and Conrad, 1996). 5. Conclusion Our results showed that the conversion from upland crops to rice paddy cultivation significantly affected CH4 emission and the rate of emission was associated with the conversion history. In general, the continuously cultivated rice paddy (i.e. 19-year plot) resulted in greater CH4 emissions than in short term paddy fields (i.e. 1and 2-year plots). The high emissions from the continuous rice cultivation can likely be attributed to increased Fe(II) concentrations and populations of methanogenic archaea due to the greater abundance of organic carbon. However, length of non-paddy quan-

747

titatively suppressed the availability of electron donors as revealed by low Fe(III) reduction and methanogenic archaeal populations which resulted in low CH4 emission. Our results suggested that rotating with upland cropping practices may be a promising option to reduce CH4 emission from rice paddy cultivation, which should be better evaluated along with the changes in soil organic carbon. Acknowledgements The NARCT received the research funding from the Ministry of Agriculture, Fisheries and Forestry (MAFF), Government of Japan. We thank Mr. Hirofumi Nakamura, Yoshinori Okawara, Kunihiko Tamura, Eisaku Kumagai, Kazuhiro Hayasaka, Akio Yoshida, Yuichi Otsubo, Yasuo Omori and Hisashi Hashimoto of NARCT for their support in the experiment and managing the experimental field. References Achtnich, C., Bak, F., Conrad, R., 1995. Competition for electron donors among nitrate reducers, ferric iron reducers, sulfate reducers and methanogens in anoxic paddy soil. Biology and Fertility of Soils 19, 65–72. Asakawa, S., Hayano, K., 1995. Population of methanogenic bacteria in paddy soil under double cropping conditions (rice–wheat). Biology and Fertility of Soils 20, 113–117. Aulakh, M.S., Wassmann, R., Rennenberg, H., 2002. Methane transport capacity of twenty-two rice cultivars from five major Asian rice-growing countries. Agriculture, Ecosystem and Environment 91, 59–71. Ball, B.C., Mctaggart, I.P., Watson, C.A., 2002. Influence of organic ley-arable management and afforestation in sandy loam to clay loam soils on fluxes of N2 O and CH4 in Scotland. Agriculture, Ecosystems and Environment 90, 305–317. Bousquet, P., Ciais, J.B., Miller, E.J., 2006. Contribution of anthropogenic and natural sources to atmospheric methane variability. Nature 443, 439–443. Bouwman, A.F., 1990. Exchange of greenhouse gases between terrestrial ecosystems and the atmosphere. In: Bouwman, A.F. (Ed.), Soils and the Greenhouse Effect. John Wiley and Sons, Chichester, UK, pp. 61–127. Cheng, W., Chander, K., Inubushi, K., 2000. Effect of elevated CO2 and temperature on methane production and emission from submerged soil microcosms. Nutrient Cycling in Agroeco-systems 58, 339–347. Cheng, W., Yagi, K., Akiyama, H., Nishimura, S., Sudo, S., Fumoto, T., Hasegawa, T., Hartley, A.E., Megonigal, J.P., 2007. An empirical model of soil chemical properties that regulate methane production in Japanese rice paddy soils. Journal of Environmental Quality 36, 1920–1925. Chin, K.J., Lukow, S., Conrad, R., 1999. Effect of temperature on structure and function of the methanogenic archaeal community in an anoxic rice field soil. Applied Environmental Microbiology 65, 2341–2349. Conrad, R., 1989. Control of methane production in terrestrial ecosystems. In: Andreae, M.O., Schimel, D.S. (Eds.), Exchange of trace gases between terrestrial ecosystems and the atmosphere. Dahlem Konferenzen. Willey, Chichester, UK, pp. 39–58. Conrad, R., Klose, M., 2006. Dynamics of methanogenic archaeal community in anoxic rice soil upon addition of straw. European Journal of Soil Science 57 (4), 476–484. Dlugokencky, E.J., Houweling, S., Bruhwiler, L., Masarie, K.A., Lang, P.M., Miller, J.B., Tans, P.P., 1992. Atmospheric methane levels off: temporary pause or a new steady-state? Geophysical Research Letters 30, doi:10.1029/2003GL018126. Dlugokencky, E.J., Bruhwiler, L., White, J.W.C., Emmons, L.K., Novelli, P.C., Montzka, S.A., Masarie, K.A., Lang, P.M., Crotwell, A.M., Miller, J.B., Gatti, L.V., 2009. Observational constraints on recent increases in the atmospheric CH4 burden. Geophysical Research Letters 36, L18803. Fetzer, S., Bak, F., Conrad, R., 1993. Sensitivity of methanogenic bacteria from paddy soil to oxygen and desiccation. FEMS Microbiology and Ecology 12, 107–115. Forester, P., Ramaswamy, V., Ataxo, P., Berntsen, T., Betts, R., Fahey, D.W., Haywood, J., Lean, J., Lowe, D.C., Myhre, G., Nganga, J., Prinn, R., Raga, G., Schulz, M., Van Dorland, R., 2007. Changes in atmospheric constituents and in radiative forcing. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller, H.L. (Eds.), Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom/New York, NY, pp. 129–234. Fowler, D., Hargreaves, K.J., MacDonald, J.A., Gardiner, B., 1995. Methane and CO2 exchange over petland and the effects of afforestation. Forestry 68, 327–334. Frenzel, P., Bosse, U., Jansen, P.H., 1999. Rice roots and methanogenesis in a paddy soil: ferric iron as an alternative electron acceptor in the rooted soil. Soil Biology and Biochemistry 31, 421–430. Groot, T.T., van Bodegom, P.M., Harren, F.J.M., Meijer, H.A.J., 2003. Quantification of methane oxidation in the rice rhizosphere using C-13-labelled methane. Biogeochemistry 64, 355–372. Hales, B.A., Edward, C., Ritchie, D.A., Hall, G., Pickup, R.W., Saunders, J.R., 1996. Isolation and identification of methanogen-specific DNA from blanket bog peat by PCR amplification and sequence analysis. Applied Environmental Microbiology 62 (2), 668–675.

748

M.K. Eusufzai et al. / Agriculture, Ecosystems and Environment 139 (2010) 742–748

Hansen, J., Sato, M., Ruedy, R., Nazarenko, L., Lacis, A., Schmidt, G.A., Russell, G., Aleinov, I., Bauer, M., Bauer, S., Bell, N., Cairns, B., Canuto, V., Chandler, M., Cheng, Y., Del Genio, A., Faluvegi, G., Fleming, E., Friend, A., Hall, T., Jackman, C., Kelley, M., Kiang, N., Koch, D., Lean, J., Lerner, J., Lo, K., Menon, S., Miller, R., Minnis, P., Novakov, T., Oinas, V., Perlwitz, J., Perlwitz, J., Rind, D., Romanou, A., Shindell, D., Stone, P., Sun, S., Tausnev, N., Thresher, D., Wielicki, B., Wong, T., Yao, M., Zhang, S., 2005. Efficacy of climate forcings. Journal of Geophysical Research 110, D18104. Hou, A.X., Chen, G.X., Wang, Z.P., Van Cleemput, O., Patrick Jr., W.H., 2000. Methane and nitrous oxide emissions from rice field in relation to soil redox and microbiological processes. Soil Science Society of America Journal 64, 2180–2186. Huang, B., Yu, K., Gambrell, R.P., 2009. Effects of ferric iron reduction and regeneration on nitrous oxide and methane emissions in a rice soil. Chemosphere 74, 481–486. Inubushi, K., Cheng, W., Aonuma, S., Hoque, M.M., Kobayashi, K., Miura, S., Kim, H.Y., Okada, M., 2003. Effects of free-air CO2 enrichment (FACE) on CH4 emission from a rice paddy field. Global Change Biology 9, 1458–1464. Intergovernmental Panel on Climate Change (IPCC), 2001. Climate Change 2001: The Scientific Basis. Cambridge University Press, Cambridge. Intergovernmental Panel on Climate Change (IPCC), 2007. The Physical Science Basis: Summery for Policymakers, http://www.ipcc.ch/SPM2feb07.pdf. Jia, Z.J., Cai, Z.C., Xu, H., Tsuruta, H., 2002. Effects of rice cultivars on methane fluxes in a paddy soil. Nutrient Cycling in Agroecosystems 64, 87–94. Kenward, M.G., Roger, J.H., 1997. Small sample inference for fixed effects from restricted maximum likelihood. Biometrics 53 (3), 983–997. Krüger, M., Frenzel, P., Conrad, R., 2001. Microbial processes influencing methane emission from rice fields. Global Change Biology 7, 49–63. Kumagai, K., Konno, Y., 1998. Methane emission from rice fields after upland farming. Japanese Journal of Soil Science and Plant Nutrition 69, 333–339 (in Japanese with English Abstract). Littell, R.C., Milliken, G.A., Stroup, W.W., Wolfinger, R.D., Schabenberger, O., 2006. SAS® for Mixed Models, 2nd edition. SAS Institute Inc., Cary, NC. Lovely, D.R., Phillips, E.J.P., 1986. Availability of ferric iron for microbial reduction in bottom sediments of the freshwater tidal Potomac River. Applied and Environmental Microbiology 52, 751–757. Malijanen, M., Hytönen, J., Martikainen, P.J., 2001. Fluxes of N2 O, CH4 and CO2 on afforested boreal agricultural soils. Plant and Soil 231, 113–121. Mayer, H.P., Conrad, R., 1990. Factors influencing the population of methanogenic bacteria and the initiation of methane production upon flooding of paddy soils. FEMS Microbiology and Ecology 73, 103–112. Minoda, T., Kimura, M., Wada, E., 1996. Photosynthates as dominant source of CH4 and CO2 in soil water and CH4 emitted to the atmosphere from paddy fields. Journal of Geophysical Research 101, 21091–21097. Neue, H.U., Sass, R.L., 1998. The budget of methane from rice fields. IG Activities Newsletter 12, 3–11. Nishimura, S., Yonenmura, S., Sawamoto, T., Shirato, Y., Akiyama, H., Sudo, S., Yagi, K., 2004. Effect of land use change from paddy rice cultivation to upland crop cultivation on soil carbon budget of a cropland in Japan. Agriculture, Ecosystems and Environment 125, 9–20. Nouchi, I., Mariko, S., Aoki, K., 1990. Mechanism of methane transportation from the rhizosphere to the atmosphere through rice plants. Plant Physiology 94, 59–66. Nouchi, I., Hosono, T., Aoki, K., Minami, K., 1994. Seasonal variation in methane flux from rice paddies associated with methane concentration in soil water, rice biomass and temperature, and its modeling. Plant and Soil 161, 195–208. Oleg Menyailo, V., Bruce Hungate, A., Wolf-Rainer, A., Conrad, R., 2008. Changing land use reduces soil CH4 uptake by altering biomass and activity but not composition of high-affinity methanotrophs. Global Change Biology 14, 2405–2419. Patrick Jr., W.H., Reddy, C.N., 1978. Chemical changes in rice soils. In: Institute, I.R.R. (Ed.), Soils and Rice. IRRI, Los Banos, Philippines, pp. 361–379. Peters, V., Conrad, R., 1996. Sequential reduction processes and initiation of CH4 production upon flooding of oxic upland soils. Soil Biology and Biochemistry 28, 371–382.

Rigby, M., Prinn, R.G., Fraser, P.J., Simmonds, P.G., Langenfelds, R.L., Huang, J., Cunnold, D.M., Steele, L.P., Krummel, P.B., Weiss, R.F., O’Doherty, S., Salameh, P.K., Wang, H.J., Harth, C.M., Muhle, J., Porter, L.W., 2008. Renewed growth of atmospheric methane. Geophysical Research Letters 35, 22, doi:10.1029/2008GL036037, eid:L22805. Roden, E.E., Wetzel, R.G., 1996. Organic carbon oxidation and suppression of methane production by microbial Fe(III) oxide reduction in vegetated and unvegetated freshwater wetland sediments. Limnology and Oceanography 41, 1733–1748. Roden, E.E., Wetzel, R.G., 2002. Kinetics of microbial Fe(III) oxide reduction in freshwater wetland sediments. Limnology and Oceanography 47 (1), 198– 211. Roden, E.E., Wetzel, R.G., 2003. Competition between Fe(III)-reducing and methanogenic bacteria for acetate in iron rich fresh water sediment. Microbial Ecology 45, 252–258. Sass, R.L., Mosier, A., Zheng, X.H., 2002. Introduction and summary: Proceedings of the International Workshop on Greenhouse Gas Emissions from Rice Fields in Aisa. Nutrient Cycling in Agroecosystems 64 (1–2), ix–xv. Shindell, D.T., Faluvegi, G., Koch, D.M., Schmidt, G.A., Unger, N., Bauer, S.E., 2009. Improved attribution of climate forcing to emissions. Science 326, 716–718. Spilke, J., Piepho, H.P., Hu, X., 2005. Analysis of unbalanced data by mixed linear models using the mixed procedure of the SAS System. Journal of Agronomy and Crop Science 191 (1), 47–54. Takai, Y., 1961. Reduction and microbial metabolism in paddy soils (2). Nogyo-Gijitsu 16 (2), 51–53 (in Japanese). Takai, Y., 1970. The mechanism of methane fermentation in flooded paddy soil. Soil Science and Plant Nutrition 16, 238–244. Takai, Y., Kimura, T., 1966. The mechanism of reduction in waterlogged paddy soil. Folia Microbiologia 11, 304–313. Takai, Y., Kamura, T., Adachi, I., 1958. On dynamic behavior of iron compound in paddy soils (Part 2). Japanese Journal of Soil Science and Plant Nutrition 29, 216–220 (in Japanese). Unden, G., Becker, S., Bongaerts, J., Schirawski, J., Six, S., 1994. Oxygen regulated gene expression in facultatively anaerobic bacteria. Antonie van Leeuwenhoek 66, 3–22. Wang, B., Adachi, K., 2000. Differences among rice cultivars in root exudation, methane oxidation, and populations of methanogenic and methanotropic bacteria in relation to methane emission. Nutrient Cycling in Agroecosystems 58, 349–356. Wassmann, R., Neue, H.U., Lantin, R.S., Buendia, L.V., Rennenberg, H., 2000. Characterization of methane emissions from rice field in Aisa. I. Comparison among field sites in five countries. Nutrient Cycling in Agroecosystems 58, 1–12. Watanabe, I., 1984. Anaerobic decomposition of organic matter in flooded rice soils. In: Institute, I.R.R. (Ed.), Organic Matter and Rice. IRRI, Los Banos, Philippines, pp. 237–238. Watanabe, A., Kimura, M., 1999. Influence of chemical properties of soils on methane emissions from rice paddies. Communications in Soil Science and Plant Analysis 30, 2449–2463. Watanabe, A., Takeda, T., Kimura, M., 1999. Evaluation of origins of CH4 carbon emitted from rice paddies. Journal of Geophysical Research 104, 23623– 23629. Witt, C., Cassman, K.G., Olk, D.C., Biker, U., Libon, S.P., Samson, M.I., Ottow, J.G.G., 2000. Crop rotation and residue management effects on carbon sequestration, nitrogen cycling and productivity of irrigated rice systems. Plant and Soil 225, 263–278. XU, H., Cai, Z.C., Tsuruta, H., 2003. Soil moisture between rice-growing season affects methane emission, production and oxidation. Soil Science Society of America Journal 67, 1147–1157. Yao, H., Conrad, R., Wassmann, R., Neue, H.U., 1999. Effect of soil characteristics on sequential reduction and methane production in sixteen rice paddy soils from China, the Philippines and Italy. Biogeochemistry 47, 269–295.