Toposequential variation in methane emissions from double-cropping paddy rice in Northwest Vietnam

Abstract

Understanding the spatial and temporal variations in toposequential methane (CH4) emission is essential for assessing and mitigating CH4 emission from rice cascades in mountainous watersheds. To assess the toposequential variation in CH4 emission among different field positions, two cascades of double-cropping paddy rice fields were investigated in Yen Chau district, Northwest Vietnam. The cascades were divided into fertilized and non-fertilized parts and CH4 measurements at 10days intervals were conducted at top, middle and bottom fields of each part. The results showed that the rate and cumulative amount of CH4 emissions in non-fertilized part were higher than that of fertilized one in both spring and summer rice seasons due to the stimulation of CH4 oxidation by urea and sulfate containing fertilizers. The spatial variation in CH4 emissions among the field positions was high in both cropping seasons with the highest emissions in the bottom fields and the lowest emissions were found in the top fields (i.e. bottom field CH4 emissions 1.8–3.0 times higher than the top field). The differences among field positions were influenced by clay content, total nitrogen and total carbon content which showed toposequential differences. The average CH4 fluxes ranged from 1.0 to 5.1mgCH4m−2h−1 being largest at later growth stages for spring rice and during early growth stages for summer rice. Cumulative CH4 emissions for spring rice ranged from 3.1 to 13.7gCH4m−2 and that for summer rice from 4.3 to 23.5gCH4m−2. 61.7% was emitted during summer rice season and 38.1% from spring rice season. The higher values for summer crops were due to higher availability of fresh organic substrates under higher soil temperature during the early growing period. The average total CH4 emissions from double-cropping paddy rice fields were 14.8gCH4m−2 for cascade 1 and 27.3gCH4m−2 for cascade 2. The higher emission for cascade 2 might be due to the lower soil Eh and higher clay content especially in the lower lying fields. The results highlight that large toposequence differences in CH4 emissions require different site specific management practices for each toposequence position in order to mitigate CH4 emission in paddies in mountainous watersheds.