Soybean Effects on Soil Nitrogen Availability in Crop Rotations

Soybean [Glycine mar (L.) Merr.] production contributes significantly to the N supply for a following corn (Zeu 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 LS-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 sativu L.) in corn-soybean-corn-oat-alfalfa (CSCOM) and corn-cornoat-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 lst and 2nd yr after alfalfa, respectively, and 75 kg N ha⁻¹ for the Ist 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 (l987–l991) 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 bt-yr corn is realized at the expense of subsequent reductions in soil N availability 537064299, Research supported by the College of Agricultural and Life Sciences, Univ. of Wisconsin-Madison through Project 3449