Quantifying the Contribution of Paddy Rice Field to Climate Change Mitigation through Determining of the Net Ecosystem CO2 Exchanges and Its Environmental Responses

Thuong Nguyen Thi Hoai¹, Hanh Nguyen Thi Hong¹, Trang Bui Thi Thu¹, and Miura Takeshi²

1. Hanoi University of Natural Resources and Environment, Vietnam

2. Okayama University, Japan

Abstract: Evaluation net ecosystem exchange of carbon (NEE) and its responses to key environmental factors is essential to understanding ecosystem behaviors and its contribution to climate change mitigation. Agricultural development and expansion of cropland have had a greater impact on the global climate. Unlike forest ecosystems or grassland, the CO2 exchange of agricultural crops is very complex, continuously change over time and mainly driven by farming practice.

The study measures NEE at paddy rice cultivated field using eddy covariance technique. In the same time, the Michaelis-Menten equation and exponential curve fitting method were used to analyze the relationship between PAR and GPP and the effect air temperature on Re at night.

The result shows nighttime NEE varied exponentially with air temperature. The temperature sensitivities (Q10) of the rice crop was 2.55. Diurnal variation of NEE was significantly in growing season. The trend of diurnal variation of NEE was negative in daytime and positive in nighttime. There was considerable change in seasonal variation of NEE, GPP, and Re. The highest daily average CO2 absorption was 10.28 gCm-2d-1. During the main growing seasons, NEE was controlled by GPP which was mainly influenced by PAR and LAI. Re dominated NEE in pre-vegetative and after harvest stage, the temperature became a key factor controlling NEE. The ratio of Re/GPP was 0.47, the field of paddy rice cropping in the southern part of Okayama was a carbon sink with the average seasonal NEE of 414 gCm-2 in cultivation period.

Key words: paddy rice field, eddy covariance, carbon exchange, gross primary production, ecosystem respiration