Question

How submerging green plants release methane gas that causes greenhouse effect? Please explain with respect to disadvantages of constructing dams

Answer 1

Methane comes from a variety of sources, including gas leaks, forest fires, and, of course, cow burps. Microbes in wetland soil can produce methane anaerobically (without using oxygen), but the idea that it can be produced aerobically (using oxygen) by plants, and on a large scale, is still extremely controversial.
But Conventional wisdom is that plants only make methane (which makes the atomsphere warm by not letting the heat to escape) when they're submerged, without oxygen, in places like swamps.

 The buildup of water dams is also dangerous for plant life that grows on the natural shoreline of the water. The plant life is submerged and dies. In addition, the beneficial sediment that normally is washed down the river is blocked, which decreases the fertility of the soil downriver from the dam.