why does feces of animals catching fire? but it is main source of nitrogen.?

Feces is high in carbon ... it is an organic substance. After drying the carbon compounds can burn. Some feces, especially bird droppings, are very high in nitrogen compounds. Sea birds depositing their droppings on the Ataconda desert in south america built up feet deep and very dry. These have been "mined" for the production of fertilizer and for explosives for many years. The German U-boats were trying to stop that raw material from being delivered by boat to the US to slow the production of explosives being used against them in WW2.