Relationship between CH4 and N2O flux from soil and their ambient mixing ratio in a riparian rice-based agroecosystem of tropical region

Abstract

Temporal variations of the ambient mixing ratio of greenhouse gas (CH₄ and N₂O) in a riparian rice-based agro-ecosystem of tropical region were studied during 2005–2006 in coastal Odisha. The endeavour was made with the hypothesis that the ambient mixing ratio of CH₄ and N₂O depends on the changes in the flux of CH₄ and N₂O from the rice fields in the riparian rice ecosystems. A higher ambient mixing ratio of CH₄ was recorded during the tillering to grain filling stages of the rice crop, during both dry and wet seasons. The higher ambient mixing ratio of CH₄ during the wet season may attribute to the higher CH₄ emission from the rice field. The average mixing ratio of CH₄ was recorded as 1.84 ± 0.05 ppmv and 1.85 ± 0.06 ppmv during 2005 and 2006, respectively. The ambient CH₄ mixing ratio was recorded negatively correlated with the average ambient temperature. The N₂O mixing ratio ranged from 261.57 to 399.44 ppbv with an average of 330.57 ppbv during 2005. However, the average mixing ratio of N₂O was recorded as 318.83 ± 20.00 ppbv during 2006. The N₂O mixing ratio was recorded to be negatively correlated with rainfall and average ambient temperature. Significant negative correlation (r = −0.209) of N₂O with sunshine hours may attribute to the photochemical break down of N₂O. The temporal variation of N₂O flux from the rice field does not affect the ambient mixing ratio of N₂O in the same way as in the case of the ambient mixing ratio of CH₄. However, the higher mixing ratio of N₂O during the fallow period of the post monsoon period may attribute to the N₂O flux from soil. Results indicate that intensively cultivated coastal ecosystems can be a major source of ambient greenhouse gas.