Methane Cycling in Paddy Field: A Global Warming Issue

*Mohammed Mahabubur Rahman and Akinori Yamamoto*

## **Abstract**

Paddy fields are major sources of CH4 emission and a vital source of global warming. Thus, it is important to understand the CH4 cycling in paddy field. The CH4 chemistry, mechanisms of production and emission from paddy fields are also significantly important to understand. This paper discusses about the CH4 cycling, how CH4 emission effect on the global warming, and the mechanisms of CH4 exchange between rice paddy field and atmosphere, factors effecting the CH4 production, oxidation, transportation and calculation. Also try to suggest the CH4 mitigation options of paddy fields. The mitigation of CH4 emission can be achieved by water management, selection of rice cultivar and fertilization. Controlled irrigation can also reduce CH4 production compared to flood irrigation. Cultivation of high-yielding and more heat-tolerant rice cultivars will be promising approach to reduce CH4 emissions and slow down the global warming.

**Keywords:** paddy soil, methane cycling, atmosphere, mitigation, climate

### **1. Introduction**

Methane (CH4) is a key greenhouse gas (GHG), which has global warming potential 25-times higher than that of carbon dioxide (CO2) over a 100-year period [1, 2]. Atmospheric CH4 mixing ratio has increased approximately threefold since the pre-industrial era [3, 4]. The latest analysis of the World Data Centre for Greenhouse Gases (WDCGG) observations, the year-averaged CH4 mixing ratio is 1853 ± 2 ppb in 2016. Atmospheric CH4 mixing ratio has increased by 6.8 ppb y−1 over the last decade [5]. Paddy field is the dominant anthropogenic source of CH4 [6–10], accounting for approximately 10% of global CH4 emissions [11]. Emissions of CH4 from paddy soils are concentrated in irrigated areas; irrigated paddy soils account for 60% of the total rice harvesting area worldwide, but produce 78% of CH4 emissions in rice-producing areas [12]. Globally, around the tropics, sub-tropics and parts of the temperate boreal regions contribute most of the CH4 emissions from paddy rice fields because these regions have a large area of paddy field compared with other regions. This includes areas like Central and Latin America, Africa and Southeast Asia [13]. Southeast Asia emits approximately 10 Mg CH4 km−2 and it contributes 90% to the global rice CH4 emissions chart [14]. Africa and South America contribute 3.5% and 4.7% to the global Paddy rice CH4 budget respectively.

Rice is a vital crop in the world and it is grown on more than 167.25 million hectares of land globally [15]. In Asia, China, India, Indonesia, Bangladesh, and Vietnam are the major dominant rice-producing countries. So, the areas of high rice production are equally the area of high CH4 emissions due to rice cultivation [14, 16]. Considering the large CH4 emission from paddy fields, it is important to understand the CH4 chemistry, mechanisms of CH4 production and emission. Moreover, the reduction of CH4 emission from rice paddy fields has become increasingly important. Therefore, the main objectives of this research are to discuss CH4 cycling in the paddy soil and global warming, the basic understandings of methane chemistry, production, oxidation, transportation, calculation, the mechanisms of CH4 exchange between rice paddy field and atmosphere, final emission and try to give some mitigation options of CH4 emissions from paddy soils to slow down the global warming.
