Rice is the staple food for the majority of people in the world, and the production of rice accounts
for 9% of the total arable land area of the earth, or more than 125 million hectares. Nitrogen (N) is
one of the essential macroelements for growth of plant and yield, and many researches on N nutrition of
rice have focused on ammonium (NH+4 ). However, in recent years, researchers have paid more attention
to the nitrate (NO−3 ) nutrition of the rice crop and their results showed that the lowland rice was
exceptionally efficient in absorbing NO−3 formed by nitrification in the rhizosphere. This is important
because rice roots can aerate the rhizosphere by releasing oxygen (O2) and this activity promotes the
process of nitrification, that is, the conversion of NH+4 to NO−3 (Li et al., 2006). The results obtained
by Kirk (2001) showed that substantial quantities of NO−3 can be produced in the rhizosphere of rice
plants through nitrification, and the nitrification by microbes is partially responsible for the maximum
overall rate of microbial O2 consumption. Therefore, even in a flooded paddy soil, rice roots are actually