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Soil Science Society of America Journal 64:2011-2017 (2000)
© 2000 Soil Science Society of America

DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Dynamics of Dissolved Organic Carbon and Methane Emissions in a Flooded Rice Soil

Yahai Lua, Reiner Wassmannb, Heinz-Ulrich Neuec and Changyong Huangd

a Laboratory of Soil Biology and Chemistry, Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya, 464-8601 Japan
b Fraunhofer Institute for Atmospheric Environmental Research, Garmisch-Partenkirchen, Germany
c UFZ-Center for Environmental Research, Department of Soil Sciences, Theodor-Lieser Strasse 4, D-06120 Halle, Germany
d College of Resources and Environmental Sciences, Zhejiang University, Zhejiang, China

lyahai{at}nuagr1.agr.nagoya-u.ac.jp


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
Limited information is available on the dynamics of dissolved organic C (DOC) and its relationship with CH4 emissions in flooded rice (Oryza sativa L.) soils as affected by rice cultivar. Greenhouse and laboratory experiments were conducted to determine root C release in culture solution, DOC and dissolved CH4 concentration in soil solution, and CH4 emission in a flooded soil planted with three rice cultivars. Soil solutions were sampled in the root zone (soil surrounding rice roots) and the non-root zone (soil outside the root zone). The release of root exudates increased in the order: IR65598 (new plant type) < IR72 (modern cultivar) < Dular (a traditional cultivar). Correspondingly, DOC concentrations in the root zone and CH4 emission rates increased. The dynamics of DOC and dissolved CH4 differed greatly between the root zone and the non-root zone. Dissolved organic C in the root zone increased with plant growth and reached maximum (13–24 mmol C L-1) between rice flowering and maturation (Week 11–13), whereas DOC in the non-root zone remained low (1–5 mmol C L-1) throughout the growing season. Similarly, dissolved CH4 concentrations in the root zone increased sooner and were greater (mean 138 µmol CH4 L-1) than those in the non-root zone (mean 97 µmol CH4 L-1). The seasonal patterns of CH4 emissions closely followed the dynamics of DOC concentrations in the root zone. The results suggest that (i) DOC pool in the root zone of rice plants is enriched by root-derived C; (ii) the rates of CH4 emissions are positively correlated with the dynamics of DOC in the root zone; (iii) the intercultivar difference in root C releases is responsible for the intercultivar difference in DOC production, and consequently in CH4 flux.

Abbreviations: DOC, dissolved organic C • GC, gas chromatograph • PI, panicle initiation • SOC, soil organic C


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
DISSOLVED ORGANIC C is a relatively mobile and labile form of soil organic C (SOC). In flooded soils, DOC may serve as C source for CH4 production, yet little is known on the dynamics of DOC and its influence on CH4 emissions in flooded rice. The release of organic C from plant roots is one of the important sources for C accumulation, transformation and emission from soils. On average, 30 to 60% of the net photosynthetic C is allocated to the roots, and as much as 40 to 90% of this fraction enters the soil in the form of rhizodeposition (Lynch and Whipps, 1990; Marschner, 1996). Root-derived organic C can contribute to various C pools and become an origin of CH4 emitted from flooded soils. Many studies attributed late season CH4 fluxes to the releases of root-derived C (Neue et al., 1997; Sass and Fisher, 1997). Schutz et al. (1989) speculated that the late season peak of CH4 emission observed during the reproductive stage of rice was the result of rapid increases in organic root exudates and litter. The CH4 emission rates were positively correlated with live root biomass in natural wetlands (Wilson et al., 1989) and rice fields (Sass et al., 1990). By using a 13C tracer approach, Minoda and Kimura (1994) found that within 3 to 5 h after assimilation of labeled 13CO2, part of photosynthesized 13C was transported to the rhizosphere, transformed to CH4, and emitted to the atmosphere. Similarly, Dannenberg and Conrad (1999) reported that {approx}3 to 6% of the assimilated radioactivity (14CO2) by rice plants at tillering stage could be emitted as 14CH4 within 16 d growth after labeling. Apparently, root and root exudates serve as the major C source for CH4 production during later stages of rice plants.

Root-derived materials include root exudates, mucilage, sloughed-off cells, and litter. The amounts and components of these materials are highly variable depending on growth stage, plant variety, and environmental factors such as mechanical impedance (Lynch, 1990). However, it is unknown if these factors translate into detectable differences in DOC production, and possible the CH4 emissions.

The objective of our study was to investigate three hypotheses: (i) root-derived C governs the dynamics of DOC during the growing season of rice; (ii) the production and emission of CH4 in flooded rice soil is positively or negatively correlated with the dynamics of DOC pool, and (iii) rice cultivar and growth stage will influence DOC production and CH4 emission through varying C release from rice roots.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 Conclusions
 REFERENCES
 
Soil, Rice Cultivation, and Experimental Design
The Maahas clay soil (fine, mixed, Isophyperthermic Aquandic Epiaquall) was collected from the plow layer (15 cm) of a rice field of the International Rice Research Institute (IRRI), Los Baños, Philippines. Soil were air-dried and ground, passed through a 2-mm sieve, and thoroughly mixed. The roots were carefully removed by hand. The air-dried soil had a pH of 6.4 (1:1, soil/water), cation–exchange capacity of 37.3 cmol kg-1, available P of 10.0 mg kg-1, organic C of 15.7 g kg-1, total N of 1.74 g kg-1. It contained 66% clay, 28% silt, and 6% sand.

The selected rice cultivars comprised a modern cultivar (IR72), a new plant type that is not yet disseminated to farmers (IR65598), and a traditional cultivar (Dular). Plastic pots (22-cm i.d. and 25-cm height) were filled with 6 kg of Maahas clay. A nylon mesh bag (6-cm i.d. and 10-cm height) was placed in the center of each pot to separate the root zone and non-root zone (Fig. 1) . The nylon bag had mesh size of 24 µm, which allowed soil solution to pass through, while roots were unable to penetrate. Mesh bags were used to study the localized influence of rice roots on DOC; DOC concentrations inside the mesh bags were compared with root free soil outside the bags. The stretch of rice roots might be physically limited; however, the uptake of nutrients, and consequently the increase of root biomass, was not seriously restricted. Potassium chlorite was incorporated into soil at rate of 12.5 mg K kg-1 soil. Urea was applied through one basal application and one topdressing (4 wk after seeding), each at rate of 30 mg N kg-1 soil. Both the root zone and non-root zone was fertilized. A flood-water depth of 4 to 6 cm above the soil surface in each pot was established and manually maintained throughout the experiment. Three healthy seeds of one variety were directly planted in the center of nylon mesh bags on 1 Aug. 1997. After 1 wk later only one healthy plant was allowed to grow in each individual pot.



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  		Fig. 1 Potted plant showing the setup for sampling soil solution in root zone and non-root zone partitioned by a nylon mesh bag

 
Dissolved Methane and Dissolved Organic Carbon
Sampling and measurement of dissolved CH4 and DOC were conducted at Week 4, 5, 7, and 9 and then at weekly intervals for 16 wk, covering the complete season of rice plants. Two Rhizon samplers (Eijkelkamp, Giesbeek, the Netherlands) were vertically inserted into each pot at the depth of 1 to 15 cm under soil surface, one inside the mesh bag (root zone) and the other outside mesh bag (non-root zone) (Fig. 1). The samplers were used to collect soil solutions from the soil inside and outside the mesh bag. During sampling, an evacuated 10-mL vacutainer was connected to a polyvinyl chloride tube of each sampler through a rubber septum using a needle. After 5 mL of soil solution was evacuated, the vacutainer was detached from the plastic tube and the headspace of vacutainer was allowed to equilibrate with ambient air. The vacutainers were taken back to the laboratory and CH4 concentration in the headspace of each vacutainer was immediately analyzed using a gas chromatograph (GC). The gas chromatograph was equipped with a flame ionization detector and a Porapak N column (100/200 mesh, 2 cm length by 0.2 cm i.d.); N2 was the carrier gas, and the column and detector temperatures set at 60 and 150°C, respectively. The concentration of dissolved CH4 was calculated as follows.

where Ch is the CH4 concentration (µL L-1) in headspace, Ca is the CH4 concentration (µL L-1) in ambient air, Vh is the vacutainer headspace volume (mL), Vs is the soil solution volume (mL), P is the partition coefficient (0.03 mL air mL-1 water at the laboratory temperature), and Density of at the laboratory temperature.

Immediately after gas sampling, the soil solution in each vacutainer was centrifuged at 12880 g for 10 min. The supernatant aliquot was used for measurement of DOC concentration by a protocol according to Nelson and Sommers (1996) with a slight modification. A 2-mL aliquot was mixed with 3.0 mL of deionized H2O, 5.0 mL of 0.0175 M K2Cr2O7, 10.0 mL of 98% H2SO4, and 5.0 mL of 88% H3PO4 in a tube and digested for 30 min at 150°C. Upon cooling, the solutions were transferred to 150-mL Erlenmeyer flasks and titrated with 0.005 M Fe(NH4)2(SO4)2 · 6 H2O in 0.4 M H2SO4 solution. Sucrose was used as a standard.

Methane Emission
The methane emission rates were measured using a closed chamber technique (Lu et al., 1999) at the same sampling date of DOC and dissolved CH4 measurements. The gas collection chamber was 100 cm tall with a 26.5-cm i.d. with a fan installed inside the chamber and a rubber septum fixed in the chamber wall. During gas sampling, the pots were placed in a large trough filled with water, and the gas collection chamber was placed over each pot; water in the trough was used to seal the bottom of chamber. Gas sampling was conducted during 1000 to 1100 h, because this time approximates the daily average flux rate under the conditions of this experiment (Buendia et al., 1997). Five milliliters of gases was withdrawn with an air-tight syringe through rubber septum at 0, 10, 20, and 30 min after chamber enclosure. The CH4 concentration in gas samples was analyzed by GC. Rates of CH4 emission were determined from the linear regression of the temporal increase in chamber CH4 concentration. Data were discarded if r2 values were .

Root Exudates and Methane Production
Root exudates of the same three varieties as used in pot experiment were collected with hydroponics cultivation of rice plants (Lu et al., 1999). Briefly, three rice cultivars were grown in the greenhouse in culture solution containing following nutrient components (Yoshida et al., 1976): 40 mg L-1 N as NH4NO3; 40 mg L-1 K as K2SO4; 10 mg L-1 P as NaH2PO4H2O; 40 mg L-1 Ca as CaCl2; 40 mg L-1 Mg as MgSO4; and traces of Mn, Mo, B, Zn, Cu, and Fe (in the form of Fe-EDTA). Root exudates were sampled at Week 9 after planting, corresponding with growth stage of panicle initiation (PI). During the sampling, plants were transferred to 500-mL Erlenmeyer flasks with the roots hanging in sterilized deionized water. After 4 h, the solutions were collected, filtered and subsequently concentrated 20-fold by freeze-drying in a lyophilizer. Water-soluble C in root exudates was determined by the same procedure as for DOC measurement.

An experiment of anaerobic incubation was conducted in the laboratory to test the response of CH4 production to addition of the root exudates. Ten grams of air-dried Maahas soil were weighed into each of the 16 100-mL beakers and 16 mL of deionized water was added. The beakers were tightly covered with rubber stoppers equipped with gas inlet–outlet (Gaunt et al., 1997). A magnetic bar was kept inside each beaker for homogenizing the soil slurry. After thorough flushing with N2 and stirring, the beakers were preincubated at 30°C for 2 wk to ensure the development of anaerobic conditions. Then 4 mL of exudate solutions from each cultivar was added to four replicate beakers. Four control beakers were added with 4 mL of deionized water each. Methane production was measured at 2, 4, 7, and 9 d after exudate addition. At the sampling date, the soil slurry was stirred and flushed with N2 for 3 min and then were incubated at 30°C for exactly 12 h. The beaker was stirred again and a 1-mL gas sample was withdrawn from the headspace for analysis of CH4 concentration. The rate of CH4 production was determined from the 12-h accumulation of CH4 in the headspace within the beaker.

The factors in this study were sampling location (the root zone and the non-root zone) and rice cultivar (IR72, IR65598, and Dular). Both pot and incubation experiments were carried out with four replications and arranged in a completely randomized design. Statistical analysis of experimental data was accomplished using the SAS program (SAS, 1996). The paired t test was used to test differences between root zone and non-root zone for the measured variables at individual growth stage of each cultivar. The Duncan Multiple Range Test was used to test cultivar differences in the measured variables.


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 Conclusions
 REFERENCES
 
Plant Growth
Plant growth parameters of three rice cultivars showed that Dular and IR72 produced higher straw dry matter than IR65598 (Table 1) . IR72 produced more tillers and panicles and had higher grain yield than Dular and IR65598. Dular had relatively higher root dry weight than IR72 and IR65598, although the differences were not significant at P < 0.05. Total primary biomass increased in the order: IR65598 < Dular < IR72.


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  		Table 1 Plant growth parameters, dissolved organic C (DOC) and dissolved CH4 (means for the season) and cumulative CH4 emission of three cultivars planted in a Maahas clay soil

 
Dissolved Organic Carbon
The measurement of DOC was initiated at Week 4 after seeding (early tillering stage). The seasonal pattern of DOC differed significantly between root zone and non-root zone within each cultivar (Fig. 2) . In the root zone soil, the DOC concentrations increased with plant growth throughout the growing season for the three cultivars and ranged from 4.0 to 24.2 mmol C L-1. The rapid increase began at Week 7 (maximal tillering) and the maximal concentrations occurred at Weeks 10 to 13 (flowering to maturity); DOC concentration decreased at the end of the season. In the non-root zone soil, DOC concentrations remained low (1.3–4.6 mmol C L-1) throughout the growing season (Fig. 2). The mean DOC concentrations were three to six times higher in the root zone than those in the non-root zone (Table 1).



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  		Fig. 2 Dissolved organic C (DOC) concentrations in root and non-root zone of flooded soil planted with three rice cultivars. Asterisks indicate significant difference between root zone and non-root zone by t test at P < 0.05

 
Among three cultivars, mean DOC concentrations of Dular and IR72 are higher than IR65598 (Table 1). During Weeks 10 to 12 (flowering stage), Dular also had higher DOC concentrations than IR72 (P < 0.05). There was no difference in the concentrations of DOC in the non-root zone among the three cultivars.

Dissolved Methane
The dissolved CH4 in both root zone and non-root zone soil increased with plant growth, and ranged from 0.3 to 330 µmol CH4 L-1 among cultivars (Fig. 3) . In the root zone soil, CH4 increased at Week 7, whereas in non-root zone soil the increase did not occur until Week 9 (PI). Maximal CH4 concentrations in root zone soil occurred during Weeks 13 to 15. The concentrations of dissolved CH4 in the non-root zone also increased to levels that were similar to those in the root zone, but a lag phase of 1 to 3 wk was visible throughout the growth season. During PI to maturity (from Week 9 to 13), the dissolved CH4 was significantly higher in the root zone than in the non-root zone soil. Among cultivars, IR72 produced higher mean dissolved CH4 concentrations than IR65598 (Table 1), while no differences were observed between Dular and the other two cultivars.



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  		Fig. 3 Dissolved CH4 concentrations in root and non-root zone of flooded soil planted with three rice cultivars. Asterisks indicate significant differences between root zone and non-root zone by t test at P < 0.05

 
Comparison of concentrations between dissolved CH4 and DOC showed that C in dissolved CH4 pool is only a small proportion of DOC pool (Table 2) .


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  		Table 2 Dissolved CH4 as proportion of dissolved organic C (DOC) in the root zone of a flooded soil planted to three rice cultivars

 
Methane Emissions
Methane emission rates ranged from zero to 1.53 mmol plant-1 d-1 throughout the growing season (Fig. 4) . Emission rates began to rapidly increase at Week 7, and reached maximum during Weeks 11 to 13. Emission rates decreased toward the end of season. There were no differences in emission rates among three cultivars during the first three and the last two sampling dates (Fig. 4). From Week 9 to 11, however, differences were evident; that is, Dular emitted more CH4 than IR65598 (P < 0.05). At Week 11, Dular emitted more CH4 than IR72 (P < 0.05). At Week 11 and 12, IR72 emitted more CH4 than IR65598 (P < 0.05). The cumulative CH4 emissions in the growing season were 35.2, 29.0, and 19.4 mmol CH4 plant-1 for Dular, IR72, and IR65598, respectively.



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  		Fig. 4 Methane emission rates from flooded soil planted with three rice cultivars. Bars represent standard errors. For clarity, only positive bar values are shown

 
The correlation among CH4 emission rates, dissolved CH4, and DOC concentrations was illustrated by linear regression analysis of the pooling data for the three cultivars (Table 3) . Methane emission rate and dissolved CH4 concentrations in both the root zone and non-root zone were positively correlated with DOC concentration in the root zone soil. The CH4 emission rate was also positively correlated with dissolved CH4 concentration in soil solution.


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  		Table 3 Linear regression to describe the relationship between root zone dissolved organic C (DOC) and CH4 emission rate and dissolved CH4 concentration in a flooded rice soil.{dagger}

 
Root Exudation and Methane Production Response
The root exudation rates, at the age of Week 9, were 0.36 to 0.71 mmol C plant-1 d-1 for the three cultivars (Fig. 5) . Dular and IR72 showed greater exudation rates than IR65598 (P < 0.05). The addition of exudate solution into anaerobic soil immediately stimulated CH4 production (Fig. 6) . Corresponding with the C concentration in exudate solution, the stimulation of CH4 production increased in the order: IR65598 < IR72 < Dular.



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  		Fig. 5 Root exudation rates of three cultivars at panicle initiation stage (9 wk after seeding) grown in solution culture. Bar values followed by the same letter are not significantly different

 


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  		Fig. 6 Methane production rates of a flooded rice soil with amendment of root exudates. Root exudates were collected from Dular, IR72 and IR65598 at panicle initiation (9 wk after seeding). Bars represent standard error. Arrow denotes the addition time of root exudates to the anaerobic incubation

 

   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
Conclusions
 REFERENCES
 
Dynamics of Dissolved Organic Carbon
The results in this study demonstrated substantial increase in DOC concentrations of the root zone soil as compared with the non-root zone soil. The soil used in this experiment contained low labile C as evidenced by its low intrinsic CH4 production potential (Gaunt et al., 1997; Wassmann et al., 1998; Lu et al., 2000). Organic materials were not incorporated into the soil at the start of experiment. Apparently, the increase of DOC in the root zone was most likely the result of organic C released from the plant roots. The maximal concentration of DOC between rice flowering and maturation was consistent with our previous finding that root exudation of rice plants reached a maximum during these stages (Lu et al., 1999). Theoretically, the seasonal increase of DOC in the root zone could also have been the result of accumulation of recalcitrant compounds contained in root-derived materials or soil organic matters with time. However, this alternate hypothesis does not seem likely because DOC concentrations decreased at the end of season, indicating that DOC pool in the root zone was degradable. The decrease of DOC concentrations at the end of season was probably due to the decline of root exudation and the decomposition of DOC pool.

In contrast, DOC in the non-root zone soil showed no substantial fluctuation over the growing season. The root-zone and non-root zone soil was artificially separated by a mesh bag, which allowed DOC in soil solution diffusing freely through the root zone to the non-root soil. The fact that DOC in the non-root zone did not increase during the growing season suggests that the plant-enriched DOC was decomposed in the root zone before moving out, or was rapidly consumed during the diffusion by C-limited microbes in the bulk soil.

The bioavailability of DOC depends on its origin and chemical characteristics (Yano et al., 1998). The labile fraction (e.g., carbohydrate) of DOC in soil solution is easily degradable, whereas recalcitrant C such as humic substances is biologically inert (Yano et al., 1998). The small fluctuation of DOC concentration in the non-root zone indicates that DOC was stable in the non-root zone soil. However, DOC in the root zone is actively produced and rapidly decomposed. Generally, the exudates of fresh roots contain 50 to 80% carbohydrates (Marschner, 1996), which makes the root zone DOC readily available for microbial metabolism. Apparently, not only the quantity but also the quality of DOC pool differ between root zone and non-root zone soil.

Few studies have been reported on DOC dynamics in flooded rice soils. Kimura et al. (1993) reported that water-soluble C and organic acid contents in anaerobic rice soil decreased with time in a system without plant growth. Maie et al. (1997) observed a higher DOC concentration in a planted soil as compared with nonplanted soil. Sigren et al. (1997) reported that soil acetate concentrations in the rice fields increased in the initial stage, but leveled off soon thereafter. In these studies, however, samples of soil solutions were taken from the bulk soil. Therefore, the plant effects on DOC dynamics were not as significant as in this experiment.

Relationship between Methane Emission and Dissolved Organic Carbon
The seasonal peaks of CH4 emissions ranged from 0.6 to 1.3 mmol CH4 plant-1 d-1 among the three cultivars, which were within the range measured in the IRRI farm fields during 1994 to 1996 (0.1–1.6 mmol CH4 plant-1 day-1) under similar environmental conditions (Wassmann et al., 2000). However, only one peak of CH4 emission appeared in this experiment. This differed from previous studies in which two or three CH4 emission peaks were reported (Yagi and Minami, 1990; Sass et al., 1990; Lindau, 1994; Wassmann et al., 1994; Bronson et al., 1997). The early season peaks have been related to CH4 production from soil organic matter and organic amendments, whereas the latter season peak has been attributed to the supply of plant-borne C through root exudates and decaying tissue (Neue et al., 1997). Due to the low soil labile C content and the absence of organic amendments, the early season peak did not appear. Apparently, the increase of CH4 emission rates between rice flowering and maturation indicates the contribution of the root-derived C as the origin of CH4 emission.

Seasonal pattern of CH4 emissions generally followed the pattern of DOC in the root zone, and significant linear correlation existed between two (Table 3). However, at the end of the season CH4 emission decreased (Weeks 12–14 for the three cultivars) before the decline of DOC pool in the root zone (Week 13 for Dular and Week 15 for IR65598 and IR72) (see Fig. 2 and 4). The early decline of CH4 emission probably was due to the decrease of gas transport capacity of the plants during maturation (Aulakh et al., 2000).

The DOC pool in the root zone was enriched by root-derived materials and was easily degradable. Higher dissolved CH4 in the root zone soil compared with non-root zone soil (Fig. 4) suggests that CH4 in the root zone soil was most likely generated locally from the decomposition of the DOC pool. Since DOC in the non-root zone soil was relatively stable, the increase of dissolved CH4 in the non-root zone probably resulted from the diffusion of CH4 from the root-zone to the non-root zone soil. Therefore, the rhizosphere probably serves as an important zone for CH4 production in rice paddies. Reichardt et al. (1997) reported that the population of methanogenic bacteria increased with plant growth and became most abundant on rhizoplane during rice maturation. Großkopf et al. (1998) and Lehmann-Richter et al. (1999) found novel lineages of methanogenic archaea that inhabited the root surface of rice plants. Because methanogens are known to be strictly anaerobic, more studies are necessary to elucidate the mechanisms for the colonization of these bacteria in the rhizosphere where O2 usually leaks.

Cultivar Effects on Dissolved Organic Carbon and Methane Emission
Significant differences in CH4 emission rates existed among cultivars (Fig. 4). Many workers have also reported cultivar differences in CH4 fluxes from rice fields (Parashar et al., 1991; Lin, 1993; Lindau et al., 1995; Watanabe et al., 1995; Butterbach-Bahl et al., 1997; Shao and Li, 1997; Sigren et al., 1997). Our results showed that the cultivar differences in CH4 emissions corresponded with root exudation and DOC production in the root zone. The root exudates collected from our hydroponics experiment included only low-molecular-weight components due to the process of filtration. Furthermore, the factors controlling the exudation processes of root C in hydroponics differ from those in soil. Therefore, caution should be taken when interpreting CH4 emission rate with exudation data. Nevertheless, the amount of C released from rice roots, and consequently the DOC concentrations in the root zone probably provide good indication for the CH4 production potentials of different cultivars. It would be a useful strategy to mitigate the overall CH4 emissions from rice fields by screening for cultivars with low root exudates and hence lower DOC and CH4 emissions. Unfortunately, this study also showed that the rice cultivar with the lowest CH4 emission had lower grain yield than the cultivar with a medium rate of CH4 emission. Therefore, the challenge in the future studies will be to select for rice cultivars that have low CH4 emission but show a high yield potential.

In addition, the CH4 emissions from rice fields also are controlled by the CH4 oxidation and transport capacities of rice plants. Various studies estimated that the proportion of the produced CH4 that was locally oxidized in the soil could range from 58% (Sass et al., 1991) to 80% and higher (Holzapel-Pschorn et al., 1985; Conrad and Rothfuss, 1991; Frenzel et al., 1992). Butterbach-Bahl et al. (1997) indicated that the difference in CH4 emissions of two Italian rice cultivars was controlled mainly by their transport capacities. The high bioavailability of the DOC pool in the root zone soil but low amount of CH4–C in the soil solution (Table 2) indicates that most of produced CH4 probably was emitted and/or oxidized. Cultivar effects on CH4 oxidation and transport are currently not well understood (Wassmann and Aulakh, 2000). Further studies are essential to elucidate different impact mechanisms of rice cultivar on CH4 emissions from rice fields.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 Conclusions
REFERENCES
 
The results of this study suggest that the root zone DOC pool was enriched by root-derived C during the growing season, whereas the non-root zone DOC pool was the least affected. The plant-enriched DOC in the root zone was easily degradable. The dissolved CH4 concentrations in soil solution and CH4 emission rates were positively correlated with the dynamics of DOC pool in the root zone. The majority of the CH4 emitted from soil might have originated from the decomposition of DOC pool in the root zone soil. The differences in CH4 emissions among cultivars corresponded with the differences in organic C release and in the root zone DOC concentrations. The contribution of the decomposition of native SOC to the DOC pool was not identified. Further studies with approaches such as isotopic C labeling are needed to discriminate between the effects of rice plants from that of native soil organic matter.

Our results suggest that the rhizosphere is probably an important zone for methanogenic activity in rice paddies. Since methanogens are known to be strictly anaerobic, it is necessary in future studies to investigate the mechanisms of methanogenesis in the rhizosphere where O2 probably leaks from the roots. Furthermore, because the DOC pool in the root zone is biologically active and shows substantial seasonal fluctuations, it will also be important in future studies to determine the function of DOC in other rhizospheric microbial metabolism and in nutrient transport, uptake, and cycling in rice soils.SAS Institute 1996


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
The research was conducted in Soil and Water Sciences Division, IRRI, Los Baños, Laguna, Philippines.

Received for publication December 1, 1999.


   REFERENCES
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 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 Conclusions
 REFERENCES
 




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