Summertime variation of methane oxidation in the rhizosphere of a Carex dominated freshwater marsh
Introduction
Estimating CH4 source and sink strength is necessary for understanding the temporary rise in atmospheric CH4 concentrations and its comprehensive effect on global climate change. Natural wetlands are generally regarded as the greatest natural source of CH4 emitted into the atmosphere and they account for about 20% of the total annual emissions (IPCC, 2001). In wetlands, the net CH4 emissions are determined by the balance of CH4 production, CH4 oxidation, and CH4 transport. The presence of vascular plants not only facilitates CH4 emissions by providing the CH4 emission pathway through the aerenchymatic system and labile organic compounds available as substrates for methanogens, but also attenuates CH4 emissions by transporting O2 into the rhizome and rhizosphere to accentuate the potential for CH4 oxidation (Chanton and Dacey, 1991).
Previous studies using the laboratory incubation revealed that 30–90% of CH4 produced in the anaerobic environment is oxidized before reaching the atmosphere, hence CH4 oxidation likely played a key role in CH4 emissions (Holzapfel-Pschorn et al., 1985; Bosse and Frenzel, 1998). However, recent experiments using methyl fluoride as the CH4 oxidation inhibitor showed that CH4 oxidation attenuates seasonal emissions by less than 20% in a Carex dominated fen (Popp et al., 2000), by 16.1% in the reed marsh and by 34.7% in the bulrush marsh (Van der Nat and Middelburg, 1998). The CH4 production in the in vitro incubation minus CH4 emission technique probably overestimates CH4 oxidation rates (Popp (1998), Popp et al (2000)). Meanwhile, the in vitro incubation by using temperature as a unique variable showed that CH4 oxidation rates increased with an increase in temperature (Saarnio et al., 1997; Segers, 1998). This pattern has been used in some models for predicting CH4 oxidation rates and the flux of CH4 emissions from wetlands (Khalil et al., 1998; Segers, 1998). However, the variation of CH4 emissions over the growing season could not be explained smoothly by temperature (Khalil et al., 1998). Recent studies pointed out that the CH4 oxidation rate was greatest early in the growing season (Van der Nat and Middelburg, 1998; Popp et al., 2000). To accurately estimate CH4 emissions from wetlands by using the model related to the representative parameters, it is essential to understand the seasonal variation pattern of CH4 oxidation and its underlying influencing factors, and to evaluate the effect of temperature on CH4 oxidation in the rhizosphere over the growing season.
Based on the annual measurement of CH4 emissions from the freshwater marsh vegetated with Carex plants (Ding et al., 2004), we examined CH4 fluxes as well as CH4, dissolved organic carbon (DOC) and CO2 concentrations in porewater in the Carex marsh in the Sanjiang plain, China before and after the plants were covered with a black cloth or acetylene was applied during the growing season. Temperatures and redox potentials in the vertical profile as well as the aboveground and underground plant biomass were simultaneously measured. The objective was to elucidate the seasonal variation of CH4 oxidation in the rhizome and rhizosphere in a Carex dominated marsh and to find out the key factors regulating CH4 oxidation.
Section snippets
Study site
Our study site was located at the Sanjiang Mire Wetland Experimental Station, Chinese Academy of Sciences, in Tongjiang, Heilongjiang Province, China (47°35′N, 133°31′E). The average elevation is 56 m, the mean annual precipitation is 600 mm and the mean annual temperature is 1.9°C. Water and soil in the marsh are completely frozen from October to April and begin to melt in late April. The highest and lowest temperatures occur in July and January. As the depth of standing water increases (Ding et
Methane emission
The summertime variation of CH4 emissions from the freshwater marsh vegetated with C. lasiocarpa and C. meyeriana showed that the flux increased significantly from June to July and August (Table 1). The relatively low fluxes in August compared to those in July were due to the carry-out of the measurement after 5-day’s 64-mm heavy rainfall, which greatly diluted CH4 concentration in porewater resulting in low CH4 emissions (Table 2). After a black cloth was used to cover the plants or acetylene
Summertime variation of methane oxidation
O2 transported into the rhizome and rhizosphere comes either from the atmosphere via plant stomata or from the plant photosynthesis as a by-product at the presence of light (Larcher, 1980). Oremland and Taylor (1977) found that O2 concentrations in plants could be higher than in the atmosphere at daytime. CH4 was mainly produced from the plant litters in the freshwater marsh (Ding et al., 2002), no more than 1% of which was resulted from the 14C-labeled CO2 fixed by the plant photosynthesis in
Acknowledgements
This work received financial support from the Institute of Soil Science (ISSASIP 0206), the Chinese Academy of Sciences (KZCX2-302), and the Global Environment Research Fund, Ministry of the Environment, Japan. The authors would like to thank Changchun Song, Baixing Yan, Youzhe Li, and Mingyue Ji for their help with gas sampling and providing the data of plant biomass, and two anonymous referees for their helpful suggestions and comments that greatly improved the manuscript.
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