Long term intensive farming that does not add soil amendment such as manure or fertilizer has been shown to decrease oil organic matter content overtime among the nutrients and nitrogen why is nitrogen essential for life on earth?

It is essential because its availability can affect the rate of key ecosystem processes (primary production and decomposition).

Explanation:

78% of earths atmosphere is nitrogen which is a majority. During nitrogen fixing, atmospheric nitrogen gas is released and converted into soil nitrate. Here, atmospheric nitrogen can be captured by symbiotic organism like the root noodle bacteria-Rhizobium Leguminosarium and non-symbiotic organisms like Azotobacter, Clostridium and Nostoc are converted into soil nitrate. The nitrate is then absorbed by the plants through its roots.