Corn–Soybean Rotation Effects on Soil Water Depletion

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Abstract
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 determine whether altered water uptake or altered WUE was associated with the yield increase observed when corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] are rotated. A long‐term corn‐soybean rotation experiment was monitored in 1987 and 1988 to determine seasonal soil water status to 1.50m. Corn and soybean sequences monitored were: monoculture, first‐year crop following 5 yrs of the other crop, second‐year crop following 5 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 first‐year 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.

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