YIELD AND NITROGEN RECOVERY FROM FIELD-GROWN SUPERNODULATING SOYBEAN

Crop rotations that include soybean [Glycine max (L.) Merr.] increase yields and reduce N fertilizer requirements of subsequent nonlegume cr ops such as corn (Zen mays L.) or wheat (Triticum aestivum L.), Nitrog en from biological fixation is often given credit for this benefit. Ho wever, recent studies suggest that soybean may remove more N from the soil than it contributes. The availability of soybean lines capable of supernodulation may provide the opportunity to supply more N for subs equent crop use. The objectives of this research were to (i) determine the yield and growth characteristics of a soybean with supernodulatio n and (ii) to determine the influence of supermodulation on the yield and grow-th characteristics of winter wheat planted after soybean. Fie ld studies were conducted for 3 yr (1993-1995) under rainfed condition s at Arlington, WI, on a piano silt loam (fine-silty, mixed, mesic Typ ic Argiudoll) and under irrigated conditions at Rancock, WI on a Plain field sandy loam (loamy-sand, mixed, mesic Typic Udipsamment). Four so ybean genotypes were evaluated at R3, R7 and R8 growth stages included supernodulating, the normal nodulating parent of the supernodulating line, a nonnodulating line, and the normal nodulating parent of the no nnodulating line. The nonnodulating line provided a baseline of soybea n performance without N-2 fixation capabilities. 'Merrimac' winter whe at planted after all soybean plots were harvested was evaluated at Z23 7, Z50, and Z90 growth stages. Depending on location, the supernodulat ing genotype increased soybean nodule density 43 to 135%, soil nitrate N Pt R3 growth stage 34 to 48% and biomass N 10 to 19% compared with the normal nodulating genotypes. However, at the R7 and RS growth stag es, soil nitrate N, plant biomass N and forage and grain yields were n ot different between the normal and supernodulating genotypes. Winter wheat grown after supernodulating soybean had no difference in soil ni trate N, biomass N, or forage or grain yield compared with normal nodu lating genotypes. Based on these results, there does not appear to be any agronomic benefit to using these supernodulating soybean compared with normal nodulating lines.