Carbon, nitrogen, and phosphorus are examples of nutrients that organisms need to stay alive. Nitrogen can be the Choose... in an ecosystem when its available forms are in short supply.

Nitrogen is present in the soil in the form of ammonium and nitrates.

Explanation:

Nitrogen present in large amount in the atmosphere about 78 % by volumes but in an ecosystem such as land and water its available forms are in short supply because very less amount of nitrogen is produced due to the process of decomposition and some quantity of nitrogen comes from the atmosphere with the rainfall and mixes with soil and present in the soil in the form of ammonium and nitrates.