Inorganic N pool that accounts about 1%-2% of the total N in the soil contains nitrate and ammonium mainly.
Some other transitional forms like nitrite, nitrous and nitride N are also contained but at very low level.
The inorganic N has high solubility, smaller molecule size, high mobility, and polarity. These characters make them a high availability for plants in soils, and therefore become the main N source for the plant growth (Zhang and Xu, 2011).