Research Article: Methanogenic Pathway and Fraction of CH4 Oxidized in Paddy Fields: Seasonal Variation and Effect of Water Management in Winter Fallow Season

Date Published: September 12, 2013

Publisher: Public Library of Science

Author(s): Guangbin Zhang, Gang Liu, Yi Zhang, Jing Ma, Hua Xu, Kazuyuki Yagi, Wei-Chun Chin.

http://doi.org/10.1371/journal.pone.0073982

Abstract

A 2-year field and incubation experiment was conducted to investigate δ13C during the processes of CH4 emission from the fields subjected to two water managements (flooding and drainage) in the winter fallow season, and further to estimate relative contribution of acetate to total methanogenesis (Fac) and fraction of CH4 oxidized (Fox) based on the isotopic data. Compared with flooding, drainage generally caused CH4, either anaerobically or aerobically produced, depleted in 13C. There was no obvious difference between the two in transport fractionation factor (εtransport) and δ13C-value of emitted CH4. CH4 emission was negatively related to its δ13C-value in seasonal variation (P<0.01). Acetate-dependent methanogenesis in soil was dominant (60–70%) in the late season, while drainage decreased Fac-value by 5–10%. On roots however, CH4 was mostly produced through H2/CO2 reduction (60–100%) over the season. CH4 oxidation mainly occurred in the first half of the season and roughly 10–90% of the CH4 was oxidized in the rhizosphere. Drainage increased Fox-value by 5–15%, which is possibly attributed to a significant decrease in production while no simultaneous decrease in oxidation. Around 30–70% of the CH4 was oxidized at the soil-water interface when CH4 in pore water was released into floodwater, although the amount of CH4 oxidized therein might be negligible relative to that in the rhizosphere. CH4 oxidation was also more important in the first half of the season in lab conditions and about 5–50% of the CH4 was oxidized in soil while almost 100% on roots. Drainage decreased Fox-value on roots by 15% as their CH4 oxidation potential was highly reduced. The findings suggest that water management in the winter fallow season substantially affects Fac in the soil and Fox in the rhizosphere and roots rather than Fac on roots and Fox at the soil-water interface.

Partial Text

Paddy fields are an important source of the greenhouse gas, methane (CH4), contributing to 5–19% of the total global CH4 emission [1]. Proper water management is considered to be one of the most important options for regulating CH4 emission from paddy fields [2], [3]. Generally, the fields are either intermittently irrigated or continuously flooded during the rice-growing season, and either drained without any irrigation except for rain water or kept flooded in the winter fallow season. Compared with continuous flooding, intermittent irrigation significantly decreases CH4 emission from rice fields during the rice-growing season by 40–70% [4]–[6]. Similarly, drainage, relative to flooding, in the winter fallow season not only prevents CH4 emission from the fields directly in the current season, but also sharply reduces CH4 emission indirectly during the following rice-growing season [7]–[9]. Although effects of water management in the winter fallow season on CH4 flux from the fields are considerably reported, its effect on the processes of CH4 emission, including CH4 production, oxidation and transportation, remains unclear. The stable carbon isotope technique, an important method for identifying processes of CH4 emission from rice fields, has been widely used through measuring carbon isotopic ratios [10]–[12]. In addition, it can be used to quantify contributions of various CH4 sources and provide information about carbon isotopes for global CH4 budget [13], [14]. To our knowledge so far, very little study has been done on the measurement of stable carbon isotopes in the fields during the rice-growing season as affected by water management in the winter fallow season.

Through the field and laboratory experiments, we investigated δ13C in every process of CH4 emission from rice fields as affected by water management in the winter fallow season and further estimated pathways of CH4 production and fraction of CH4 oxidation using the stable carbon isotope technique. Compared with flooding, drainage generally caused the produced CH4 depleted in 13C. Although drainage significantly decreased CH4 emission, it had little effect on δ13C-value of emitted CH4, as well as the transport fractionation factor εtransport. Acetate-dependent methanogenesis dominated in the soil in the late season, but H2/CO2-dependent methanogenesis occurred mostly on the rice roots over the season. Drainage decreased the contribution of acetate to CH4 production by 5–10%. In field conditions, ∼10–90% of the CH4 was oxidized in the rhizosphere, while ∼30–70% at the soil-water interface. In lab conditions, less a half of the CH4 was oxidized in the soil, while almost all on the roots. Moreover, CH4 oxidation was more important in the first half of the season as well as in the rhizosphere. Drainage increased the fraction of CH4 oxidized in the rhizosphere by 5–15%, which is possibly attributed to the fact that CH4 production decreased significantly while CH4 oxidation did not simultaneously. Measuring δ13C-values of the CH4 from different pools in the rice fields is useful for quantifying the methanogenic pathway and the fraction of CH4 oxidized in these fields. More importantly, it is useful for better understanding the processes of CH4 emission, which may provide useful information for setting up an isotope model. Such a model may be of a great help to national or global CH4 budget. Therefore, more attentions should be paid to the paddy fields with more different patterns of agricultural management at a larger scale.

 

Source:

http://doi.org/10.1371/journal.pone.0073982