1. Nitrate (NO3-)

* This is the preferred form of nitrogen for most plants.

* It is readily absorbed from the soil through the roots.

* Nitrate is the product of nitrification, a process where bacteria convert ammonia to nitrite (NO2-) and then to nitrate.

2. Ammonium (NH4+)

* This form is also used by plants, but it is less preferred than nitrate.

* It is less mobile in the soil and can be toxic at high concentrations.

* Ammonium is produced from the breakdown of organic matter by decomposers, and through nitrogen fixation.

Other Forms:

* Nitrogen gas (N2): This is the most abundant form of nitrogen in the atmosphere, but most organisms cannot directly utilize it. Only certain bacteria, known as nitrogen-fixing bacteria, can convert N2 into a usable form.

* Organic nitrogen: This form is found in various organic molecules, such as amino acids, proteins, and nucleic acids. While organisms can utilize it, it needs to be broken down first.

Note: The exact form of nitrogen that an organism can utilize depends on its specific metabolic pathways and the environmental conditions.