Efficacy of Indigenously Isolated Rice Field Methanotrophs as Potential Bio-Inoculants for Promoting Rice Plant Growth

Based on the results of pot experiments, a small-scale field trial was designed to test the efficacy of two methanotrophs on rice plant growth. A methanotroph, Methylomonas strain Kb3, and Methylomagnum ishizawai strain KRF4 were found to be promising in promoting rice plant growth in our earlier experiments done on an Indian rice cultivar, Indrayani. The same methanotrophs were used in a small-scale field trial, where the individual methanotrophs and in combination were used. Methylomonas strain Kb3 was helpful in enhancing the plant growth height and yield in the field experiment, compared to the control plants. Methylomonas Kb3 and Methylomagnum KRF4 in combination also showed considerable positive effects. Methylomagnum ishizawai KRF4 alone did not show an increase in grain yield. The nitrogen input in this field, as per the farmers' practice, has been considerably low (~50kg N/ha) compared to the normally used fertiliser input (100 or 150 kg N/ha). The treatment with methanotrophs was done by dipping the plants in the inoculum. Methylomonas strain Kb3, an indigenously isolated methanotroph, was reported in 2014 from a field ~25 km away from the region where trials were taken, and the genome analysis was done in our prior studies, indicating that it has a complete nitrogen fixation pathway. Kb3 showed an increase in plant height of ~ 15% and an increase in grain yield of ~17% compared to the control plants, with no methanotroph treatment. Methylomonas Kb3 also induced early flowering in the rice plants, followed by early grain formation and early maturation. Methylomonas-Methylomagnum combination treatment showed ~15% height increase and ~15% yield increase compared to the control, whereas Methylomagnum ishizawai KRF4 showed comparable growth yields to the control plants.