Contribution of soil-borne bacteria to the rotation effect in corn

Few efforts have been directed at understanding how the rhizosphere microbiology of continuous corn may effect crop yields. This relationship may explain, in part, the decreases in yield associated with continuous corn as compared to the corn in rotation with a second crop. This study was conducted to determine the importance of soil-borne microorganisms to yield declines in long term continuous corn. Continuous corn (Zea mays L.) or rotated corn-soybean (Glycine max L.) field plots, established in 1975, under either fall plowing or no-till tillage treatments were used. Treatments consisted of methyl bromide applied at 48.8 gm⁻² 3 days prior to planting in all four combinations. Total plant samples from both the fumigated and non-fumigated areas were collected 14 days after planting. Rhizosphere bacteria were recovered and tested for their ability to impact plant growth. Bacterial assessments were made in a test tube bioassay where germinated corn was transported in to agar containing a bacterial isolate. In the first year of the study a highly significant interaction of fumigation and rotation was indicated. With fumigation continuous corn yields were similar to that of rotated corn-bean. Rotated corn yields were less affected by fumigation. In the second year, the effects were similar but less significant. Over 130 bacterial isolates were tested for their effect on plant growth. Approximately 22% were able to inhibit plant growth. Of these, 72% were from the continuous corn system. Clearly, the interaction of rotation and yield is at a microbiological level. The suggestion that microorganisms similar to those isolated are responsible for controlling early plant growth in the continuous corn system is indicated.