What are the different types of Rice farming?:

  • Irrigated - fully controlled floodwater is kept shallow
  • Rain-fed - fluctuate according to season and growing period from dry to up to 50cm
  • Deepwater -  floodwater can rise to more than 50cm (even up to several metres)
  • Upland (only 13% of farming is upland)

Upland Dry vs. Wet Rice Farming in Orissa

Why Flood a Rice Plant?:

  • Increases soil fertility
  • Stabalizes soil moisture and temperature
  • Prevents soil erosion
  • Depresses soil-borne disease and weed growth

How does flooding produce Methane?

  • Flooding cuts off oxygen supply to the soil which causes anaerobic fermentation of which methane and carbon dioxide are major end products

    • What is anaerobic fermentation?
    • What is methane?
  • Methane is released through diffusion and ebullition through the stems and roots of rice plants

    • What is diffusion and ebullition (gas bubbles)
    • Rice has developed aerenchyma tissue which allows methane to escape the soil through the plant structure.  This is a hollow series of tube-like structures within the plant that transport air bubbles through the rice plant.
  • Methane that does not escape during flooding is stored in the soil and released when the water is drained away
  • Methane production is increased with higher soil temperatures (tropical sites), higher carbon content, and with higher grow rates such as during the cropping season

The Methane Record:

  • One of the most common greenhouse gasses, Methane (CH4) is 20 times more powerful that CO2 at warming the atmosphere by weight, and thus one of the most prolific contributors to global warming and climate change. (Xiong et al 2008)
  • Methane production is increased by certain cultural farming practices such as:

    • Wet tillage
    • Puddling
    • Harrowing
    • Weeding
    • Harvest  
  • It is therefore important to look at social practices before relying on the methane level to reconstruct rice agriculture in prehistory.  The analysis of modern analogue systems allows us to apply these techniques to the archaeological record in a comparative manner.