Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Nitrification at the rice root was investigated. Nitrifiers were found in the rice rhizoplane, rhizosphere and non-rhizosphere soils under flooded conditions. They were greater in the rhizoplane than in the rhizosphere soil and greatest at the heading stage of rice growth. Ammonium-sulfate or NaNO(,2) injected into the flooded soils with growing rice plants did not increase the activities of nitrifiers in the rice rhizoplane. Incubation of the flooded soil with (NH(,4))(,2)SO(,4) or NaNO(,2) did not increase the activities of NH(,4)('+)- or NO(,2)('-)-oxidizers in soils without rice. The (NH(,4))(,2)SO(,4) did not increase denitrifiers, but incubation with NaNO(,2) caused an early increase and subsequent decline. Nitrifiers and denitrifiers in the rhizoplane of rice growing in the (NH(,4))(,2)SO(,4)- and KNO(,2)-treated soils in the closed system increased with additional O(,2) to the highest numbers at 15% additional O(,2) and then decreased with additional O(,2). However, in the closed system where only limited concentrations of O(,2) could be added, the nitrifiers and denitrifiers in the rice rhizoplane in both soils and in the (NH(,4))(,2)SO(,4)-treated soil, respectively, decreased, whereas the denitrifiers in the KNO(,2)-treated soil increased as O(,2) in the system was increased. In (NH(,4))(,2)SO(,4)-amended soils the rates of nitrification and NH(,4)('+)-N loss increased within 28 days of incubation under aerobic conditions. The disappearance of NH(,4)('+)-N in soils with 50 mg NH(,4)('+)-N kg('-1) soil followed first-order kinetics whereas soils with 38 mg NH(,4)('+)-N kg('-1) followed zero-order kinetics. In the anaerobic conditions, reductions of NO(,3)('-)-N in both soils were rapid with 5 days of incubation and followed zero-order kinetics. Losses of NH(,4)('+)-N began after small NH(,4)('+)-N gains had occurred during the first days of anaerobic incubations. Under aerobic conditions, NO(,3)('-)-N concentrations were high in soils with applied NH(,4)('+)-N or without rice plants, and low in soils without applied N or with rice plants. The NO(,3)('-)-N concentrations in soils were highest at 14 days after transplanting and correlated positively with numbers of nitrifiers in the rice rhizoplane. Nitrifiers can survive in the rice rhizoplane under flooded conditions.