Where does the biggest source of nitrogen come from for plants??

Question

where does the biggest source of nitrogen come from for plants??

Answer 1

The biggest source of N for plants is in Dissolved Organic Nitrogen (basically dead stuff in the soil).  This is part of the internal nitrogen cycle (meaning the nitrogen is already in the system).  This form of nitrogen must be mineralized by microbes before it is available to plants.

The largest N input (new N coming into the system) is from human agricultural systems via atmospheric deposition or biological N-fixation.