Nitrogen in biosphere exists in three forms primarily: gaseous N in the air, organic and inorganic N
pools in soils. Although N2 accounts for 78% in the air, they can be absorbed by a small population of plants (leguminous plants and some other plants with root nodules) only through a process called bio N fixation (Zhang, 2007). The organic N pool in soils consists of the residues and compost of animal bodies, waste, plants, and microorganisms. Besides, some N-rich amino acids, amide, and urea, most of the organic N is hydrophobic and cannot be easily dissolved into soil solution to supply the plant growth (Wang, 1999).