There are numerous reports of the beneficial effects of rotating corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. However, few studies have been specifically designed to document the important corn‐soybean rotation effect. The objective of this study was to determine the impact of various corn and soybean cropping patterns on the yield of both crops. The 9‐year field study conducted at two locations was managed for maximum production. Cropping sequences consisted of: continuous monoculture with the same cultivar; continuous monoculture with cultivars alternated; annual rotation of the two crops; and 1, 2, 3, 4, and 5 yr of monoculture following 5 yr of the other crop. Annually rotated corn yielded 10% better, and first‐year corn yielded 15% better than corn under monoculture. Annually rotated soybean yielded 8% better, and first year soybean yielded 17% better than soybean under monoculture. With monoculture of either crop, alternating two different cultivars annually resulted in the same yield as continuous cropping of just one cultivar. There were differences in the response of the two crops to increasing years of monoculture: the lowest corn yield was from second year corn; the lowest soybean yield was from extended monoculture. Total corn dry weight was affected by cropping sequence but soybean dry weight was not. Our data suggest that, from a yield standpoint, a superior cropping sequence for Minnesota would include at least three, and possibly more crops.

Earlier studies showed that mycorrhizal fungi selectively proliferate in soils cropped in monoculture to corn (Zea mays L.) or soybean [Glycine max (L.) Merr.]. This study evaluated whether the dominant mycorrhizal fungi, based on spore numbers present in soil, affected growth and nutrient uptake of the following crop. Plots at two locations in Minnesota with a continuous corn or continuous soybean history were planted to both corn and soybean. The relationship between spore numbers of proliferating species of mycorrhizal fungi and crop yield and nutrient concentrations were assessed using simple correlation analysis. Spore populations of mycorrhizal fungi which proliferated in corn were generally negatively correlated with the yield and tissue mineral concentrations of corn, but were positively correlated with the yield and tissue mineral concentrations of soybean. Spore populations of soybean proliferators exhibited the reciprocal relationship, although less clearly. We suggest that, compared to other fungi, proliferating VAM fungal species may be less beneficial (or perhaps detrimental) to the crop in which they proliferate. We propose a mechanism to explain how vesicular‐arbuscular mycorrhizal (VAM) fungi could cause yield depressions associated with monoculture, and outline research needed to test this hypothesis.

SUMMARYCommunities of vesicular-arbuscular (VA) mycorrhizal fungi were studied in a long-term crop rotation experiment at two locations (Waseca and Lamberton, Minnesota, USA). Spores of mycorrhizal fungi were counted and identified in experimental plots with a cropping history of either corn (Zea mays L.) or soybean \Glycine max (L.) Merrill], Mycorrhizal fungal communities were affected by both location and cropping history. At Waseca, Glomus aggregatum Schenck & Smith, G. leptotichum Schenck & Smith and G. occulttmi Walker spores were more abundant in soil with a corn history than a soybean history, while spores of G. microcarpum Tul. & Tul. exhibited the reciprocal pattern. Approximately 90 % of the spores recovered at Lamberton were G. aggregatum and did not vary with crop history. However, the spores of three other species: G. albidum Walker & Rhodes, G. mosseae Gerdemann & Trappe, and G. occultum, were more abundant in plots with a corn history than a soybean history. Densities of G, aggretatiim spores were negatively correlated with soil pH at Waseca, but were unrelated to pH at Lamberton were the mean soil pH was lower. Our results indicate that mycorrhizal fungal species are individualistic in their responses to cropping history and edaphic factors.

The positive yield effect of crop rotation may be linked to enhanced water uptake and associated efficient use. We hypothesized that a crop grown in rotation might deplete soil water more than the same crop grown under monoculture because some negative factor associated with monoculture was alleviated. Water use efficiency (WUE) might also be improved by rotation. The objective of our study was to de· termine whether altered water uptake or altered WUE was associated with the yield increase observed when corn (Zea moys L.) and soybean [Glycine max (L.) Merr.) are rotated. A long-term corn-soybean ro· tation experiment was monitored in 1987 and 1988 to determine sea· sonal soil water status to 1.50m. Corn and soybean sequences monitored were: monoculture, first-year crop following S yrs of the other crop, second-year crop following S yrs of the other crop, and an annual alteration of the two crops. Compared with monoculture, yield was increased up to 30% when corn followed soybean and up to 11% when soybean followed corn. Average soil water depletion during the season by first-year corn was 16 mm greater than by continuous corn. The WUE in corn was related to cropping sequence at a moderate level of significance (p = 0.14). Seasonal soil water depletion by soybean was not changed by cropping sequence but overall WUE by first-year soybean was higher than by continuous soybean (p = 0.05). When there was a period with only sparse rainfall in 1988, corn had a greater water depletion than soybean and also a deeper zone of depletion. Both crops exhibited a greater water depletion when rotated. Frequent rainfall in 1987 sufficient to wet the soil below 0.30 m obliterated patterns of depleted soil water. Increased water use observed in firstyear corn and increased WUE observed in first-year soybean (compared to monoculture) in both years suggest that rotation allows these crops to increase root surface or root activity and hence to improve grain yield.

Although crop rotation may change soil mineral status, particularly N, there may also be a rotation effect beyond that which can be explained by soil mineral status alone. Research has shown that leaf mineral‐composition can vary between crop sequences at high fertilizer levels. We hypothesized that the rotation effect observed in longterm sequences of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] might be due to an increased nutrient concentration, not just an increased accumulation. A corn‐soybean rotation study in Minnesota managed at high nutrient fertility was used to test our hypothesis. The high management level was appraised by soil test levels. Corn and soybean sequences evaluated were monoculture, first year, second year, and annually‐alternated. These crop sequences were evaluated for their effects on plant nutrient concentration, accumulation, or both. The growth stage at which differences in plant nutrient concentration or accumulation might affect final yield was also evaluated. A positive effect of rotation on yield was observed in both crops. Shoot concentrations and total accumulations of N, P, and K were higher in first year corn compared to monoculture, suggesting that the increased corn yield associated with rotation may have been due to a general improvement in plant nutrition. Cropping sequence had less of an effect on soybean nutrient concentration than corn. Nutrient accumulation in soybean was not generally affected by crop sequence.