Under elevated CO2 concentration, the increase in CH4 and N2O emissions from paddy fields is decreased with the appropriate agricultural management practices

This study was led by Dr. Guangbin Zhang, Jing Ma, and Hua Xu (Institute of Soil Science, Chinese Academy of Sciences). Data extraction from the previous studies was conducted by Haiyang Yu, Tianyu Wang, Qiong Huang, and Kaifu Song (Institute of Soil Science, Chinese Academy of Sciences). They analyzed 175 independent observations (CH₄, 112 groups, and N₂O, 63 groups, respectively), which involved study sites and agronomy management (see Figure 1 below).

The team found that, overall, elevated CO₂ (ECO₂) significantly increased CH₄ emissions from rice fields by 23% (P < 0.05), but it reduced N₂O emissions by 22% (P < 0.05). However, with a longer duration of ECO₂ (≧10 years), ECO₂ significantly reduced CH₄ and N₂O emissions by 27% and 53%, respectively (P < 0.05). Along with the increasing levels of ECO₂, the stimulating effect of ECO₂ on CH₄ emissions showed a trend of “weakening firstly and then strengthening”, while its effect on N₂O emissions changed from stimulation to inhibition (see Figure 2a below). "The results suggest that the responses of CH₄ and N₂O emissions to ECO₂ might change with its duration and concentration gradients," Hua Xu says.

The researchers also elaborated that agricultural management practices (e.g., nitrogen application rates, straw incorporations, water regimes, and rice cultivars) affected the responses of CH₄ and N₂O emissions to ECO₂. With no or half amount of straw incorporation, ECO₂ increased CH₄ emissions by 27% or 49% from paddy fields, respectively (P < 0.05), while non-significant effects were observed on CH₄ emissions under full straw incorporation. With the increasing amount of straw incorporation, the reductions in N₂O emissions from paddy fields were enhanced by ECO₂. Compared with continuous flooding, intermittent irrigation weakened the promoted effect on CH₄ emissions but stimulated the inhibited effect on N₂O emissions from paddy fields under ECO₂ (see below, Figure 2b). “Therefore, under ECO₂ conditions, the increase in CH₄ and N₂O emissions from paddy fields is decreased with the appropriate agricultural management practices,” Guangbin Zhang says.

The decrease in CH₄ and N₂O emissions during the long-term ECO₂ (≧10 years) is a new insight into the current scientific community because it has been generally accepted that the ECO₂ increases CH₄ and N₂O emissions from paddy fields. “These opposite results need further validation. In the future, it is necessary to conduct comprehensive studies at multi-scale, with multi-factor, and by multi-method to effectively reduce the uncertainty in the quantifying the response of CH₄ and N₂O emissions from paddy fields to the ECO₂,” Haiyang Yu says.

See the article:

Haiyang YU, Tianyu WANG, Qiong HUANG, Kaifu SONG, Guangbin ZHANG, Jing MA, Hua XU. 2022. Effects of elevated CO₂ concentration on CH₄ and N₂O emissions from paddy fields: A meta-analysis. SCIENCE CHINA Earth Sciences, 65(1): 96-106, https://doi.org/10.1007/s11430-021-9848-2

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