The influence of soil temperature, methanogens and methanotrophs on methane emissions from cold waterlogged paddy fields

Highlights

• The greatest CH₄ emission was observed from cold-waterlogged fields.

• Rice planting increased CH₄ emissions via provision a “chimney” function.

• mcrA and pmoA copy numbers were positively correlated with CH₄ fluxes.

• Perennial waterlogging added to CH₄ emissions by promoting microbial proliferation.

Abstract

Paddy fields are major sources of atmospheric methane (CH₄). However, CH₄ emissions from cold-waterlogged paddy fields, a major type of paddy soil in China, remain unclear. Here we investigated the CH₄ emissions and associated influential factors in cold-waterlogged paddy fields at two sites (Yangxin County and Daye City) in Hubei Province, South China. Normal paddy fields matched with parental material and cropping system were used as the controls. The CH₄ emissions from cold-waterlogged fields were significantly higher than those from normal fields with (3.0–4.4-fold) or without (3.5–8.6-fold) rice. Rice planting increased CH₄ emissions by 59–78% in cold-waterlogged fields and by 85–247% in normal fields. CH₄ instantaneous fluxes were positively correlated with soil temperature and methanogen mcrA (methyl coenzyme M reductase alpha subunit) and methanotroph pmoA (methane monooxygenase) copy numbers at the annual scale. Under rice planting, mcrA copy number was higher in cold-waterlogged fields than in normal fields at both sites, whereas pmoA copy number had the same trend at the Daye site only. Soil temperature and water content influenced mcrA and pmoA copy numbers in the normal paddy fields, whereas soil organic matter content was more influential in the cold-waterlogged paddy fields. These findings suggest that perennial waterlogging is a prerequisite for substantial CH₄ emissions from cold-waterlogged paddy fields, and it promotes the proliferation of methanogens and methanotrophs under rice planting. Therefore, CH₄ production-oxidation processes are more active in cold-waterlogged paddy fields than in normal paddy fields.

Introduction

Methane (CH₄) is the second most important greenhouse gas after carbon dioxide (CO₂), with 25 times the global warming potential (over a 100-year time frame) of CO₂, and contributing to up to 20% of global warming (Bridgham et al., 2013). Paddy fields are one of the major sources in the global CH₄ budget and release 25–300 Tg CH₄ per year (Bridgham et al., 2013). The CH₄ emissions from paddy fields in China account for 29.2% of the global CH₄ emissions (Yan et al., 2009). CH₄ emissions from paddy fields depend on the balance between CH₄ production and oxidation, which is influenced by abiotic factors (e.g., soil water content, temperature, and substrates), biotic factors (e.g., methanogens and methanotrophs), in addition to their interactions.

Water management is one of the foremost factors influencing CH₄ emissions from paddy fields. Compared to long-term waterlogging, drainage at the middle stages of rice-growing seasons and alternation of wetting and drying can reduce CH₄ emissions markedly (Zou et al., 2005). The results suggested that the alternation of wetting and drying repressed CH₄ production and stimulated CH₄ oxidation, since the practice markedly decreased methanogen abundance and increased methanotroph abundance (Ma and Lu, 2011). Temperature is another key factor influencing CH₄ emissions from paddy fields (Parashar et al., 1993). Under field conditions, the optimal temperature for CH₄ production is generally lower than 30 °C (Svensson, 1984).

Methanogens and methanotrophs are key biotic factors influencing CH₄ emissions in paddy soils. Slurry incubation experiments have demonstrated that methanogen abundance and CH₄ emissions increase after warming in different soil types rich in substrates (Liu et al., 2018). Field experiments have also showed that methanogen abundance and CH₄ emission are positively correlated, both of which substantially increase with an increase in soil temperature (Lee et al., 2014). The influence of temperature on microbial CH₄ oxidation is less than the influence on CH₄ production, which could be due to the strong affinity of some methanotrophs for CH₄. In addition, substrate availability is a major factor influencing CH₄ production in paddy fields (Chen et al., 2019, He et al., 2016). High substrate availability has been reported to lead to increased mcrA copy number and CH₄ emissions (Liu et al., 2018).

Cold-waterlogged paddy fields are strongly gleyed low-yield paddy fields under perennial waterlogging. There are ~3.46 million hectares of cold-waterlogged paddy fields in China, accounting for 15.2% of the total paddy fields in this country (Xie et al., 2015). Such cold-waterlogged paddy fields are characterized mainly by poor drainage, low soil temperature, which is generally 3–5 °C lower than the temperature of normal paddy fields in summer and autumn, and relatively high soil organic matter content compared to the contents in normal paddy fields. In addition, the quantities of cultivable microbes are lower in cold-waterlogged paddy fields than in normal paddy fields, whereas microbes exhibit less activity in the former case. However, to date, CH₄ emissions from cold-waterlogged paddy fields remain poorly understood. In particular, it is unclear whether poor drainage, low soil temperature, and high soil organic matter content lead to higher CH₄ emissions compared to the cases in normal paddy fields, and whether the methanogen and methanotroph abundances vary between cold-waterlogged and normal paddy fields.

In the present study, we measured CH₄ emissions, soil water content, soil temperature, and methanogen and methanotroph gene copy numbers in cold-waterlogged and normal paddy fields at two sites in South China. The objectives of the present study were to investigate the differences in CH₄ emissions between cold-waterlogged and normal paddy fields and to elucidate the influence of abiotic and biotic factors on CH₄ emissions.