Soybean Effects on Soil Nitrogen Availability in Crop Rotations

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Abstract
Soybean [Glycine max (L.) Merr.] production contributes significantly to the N supply for a following corn (Zea mays L.) crop, even though soybean N budget studies indicate that N removed in grain may substantially exceed biological fixation. Information on the N status of cereal crops during the 2nd yr following soybean may help resolve this issue. This study reports on N effects of soybean on yield response of succeeding cereal crops and soil N availability based on data from a 15‐yr crop rotation experiment (1977–1991) on a Rozetta silt loam soil (Typic Hapludalfs) at Lancaster, WI. We evaluated the yields of corn and oat (Avena sativa L.) succeeding soybean and alfalfa (Medicago sativa L.) in corn‐soybean‐corn‐oat‐alfalfa (CSCOM) and corn‐corn‐oat‐alfalfa‐alfalfa (CCOMM) crop rotations. Fertilizer N (0, 56, ll2, and 224 kg ha−1) was applied only to corn, but NO3‐N carryover usually affected oat yields as well. The legume fertilizer N replacement values based on check plot yields and the response function of 3rd‐yr corn in a corn‐corn‐corn‐alfalfa‐alfalfa sequence, were equivalent to 153 and 36 kg N ha−1 for the 1st and 2nd yr after alfalfa, respectively, and 75 kg N ha−1 for the 1st yr after soybean. In the 2nd yr after soybean (CSCOM), oat yields were significantly lower than following corn in the CCOMM rotation. Preplant soil NO3 and oat N uptake (1987–1991) indicated that oat yield differences were due to lower soil N availability in the CSCOM rotation. The average soybean effect on soil N availability in the 2nd yr was equal to a soil N debit of 36 kg N ha−1. This indicates that part of the N contribution of soybean to 1st‐yr corn is realized at the expense of subsequent reductions in soil N availability.

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