Population dynamics of Bradyrhizobium Japonica (Kirchener) Jordan in rice-soybean rotation

Date

4-1995

Abstract

Field experiments were conducted to study the populations of indigenous and introduced strains of bradyrhizobia in soil and rhizosphere in a rice-soybean cropping sequence. The experiments were done in Lipa soil series (Fine clayey, mixed, shallow isohyperthermic Typic Eutropepts). The first soybean crop was planted in February, the rice crop in June and the second soybean crop in November, 1992. Antibiotic resistant mutants of B. japonicum strain USDA 110 were used to inoculate the two soybean crops.

Indigenous bradyrhizobia population in the soil increased 100-fold from planting to harvest of the first soybean crop while the introduced inoculant, USDA 110 SpcRif recovered from the soil was 100 items lower. Counts of bradyrhizobia in the rhizospheres were 200 times higher than in the soil for both indigenous and introduced strains. Bradyrhizobia number in the flooded soil declined during rice growth but remained higher in the rice rhizosphere. During the growth of the second soybean crop, the first crop inoculant fluctuated from100-10,000 cells/g dry soil in the presence or absence of the second crop inoculant (USDA 110 StrRif) whose number equilibrated around 1,000 cells/g dry soil. In the rhizosphere both inoculants maintained a population of 106 cells/g dry root in the presence or absence of competition.

The first crop inoculant formed 54% of the nodules of the initial soybean crop and 28% of the nodules of the second soybean crop. With reinoculation, the first crop inoculant occupied only 17% of the nodules while the second crop inoculant dominated nodule occupancy by 43%. Results show that bradyrhizobia introduced for the first time abundantly colonized soybean roots and significantly increased nodulation, nitrogen uptake and yield of soybean. The inoculum strain survived flooding of soil and persisted up to the next soybean crop after rice but failed to dominate nodule occupancy. Inoculation of every soybean crop after rice is therefore necessary to ensure successful nodule occupancy.

The effect of flooding and the rice plant on the survival of introduced and indigenous rhizobia was also studied in a pot experiment. The survival of rhizobia in the soil and rice rhizosphere was not affected by flooding nor the presence of the rice plant. Rhizobia survived better on the surface oxidized layer of flooded soils and were more abundant in the presence of the rice plant.

Document Type

Dissertation

Degree

Doctor of Philosophy in Soil Science

College

Graduate School (GS)

Adviser/Committee Chair

Erlinda S. Paterno

Committee Member

Santiago N. Tilo, Eduardo P. Paningbatan, Teodoro C. Mendoza

Language

English

LC Subject

Crop rotation, Rice, Soybean

Location

UPLB Main Library Special Collections Section (USCS)

Call Number

LG 996 1995 S72 S58

Notes

Doctor of Philosophy (Soil Science)

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