ROOT-GROWTH AND DISTRIBUTION ARE AFFECTED BY CORN-SOYBEAN CROPPING SEQUENCE

Corn (Zea mays L.) and soybean [Glycine war (L.) Merr.] yield better w hen grown in rotation than when grown in monoculture, even under high inputs. The explanation for this yield increase due to rotation is not yet known, The objective of this study was to evaluate the effect of cropping sequence on seasonal root development of corn and soybean. We hypothesized that there would be more roots on rotated crops than on crops held in a continuous monoculture. The research was conducted in 1991 and 1992 at the Univ. of Minnesota Southwest Experiment Station o n a Webster clay loam (fine-loamy, mixed, mesic Typic Haplaquoll), Des tructive root sampling via soil cores was performed at the seedling an d flowering stages of both crops. Nondestructive root monitoring was p erformed weekly and recorded on video cassette tapes via an agricultur al research video camera inserted into minirhizotron tubes that had be en placed into the soil beneath crop rows. Our hypothesis that there w ould be more roots on first-year corn than continuous corn was general ly supported. Minirhizotron monitoring suggested that corn grown in ro tation had greater Foot length density than continuous corn, with the exception of the top 12.5 cm, where continuous corn had greater root l ength density early in the season. Soil cores confirmed 22% more seedl ing roots in the top 12.5 cm on continuous corn, Our hypothesis that t here would be more roots on first-year soybean than continuous soybean was generally not supported, Minirhizotron monitoring indicated that when there were differences, continuous soybean had more roots than ro tated soybean, with the single exception of depths of 37.5 to 50 cm, w here rotated soybean had more than twice as many roots at flowering in 1992. Root length density data obtained from soil cores also indicate d that rotated soybean had approximately twice as many roots at depths of 36 and 48 cm at flowering., Growth and distribution of both corn a nd soybean roots were thus affected by cropping sequence, Fewer roots in continuous corn may have been the result of autotoxins from decompo sing roots of the previous corn crop. We do not have an explanation fo r increased soybean roots under monoculture.