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 crops such as corn (Zea mays L.) or wheat (Triticum aestivum L.). Nitrogen from biological fixation is often given credit for this benefit. However, 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 subsequent crop use. The objectives of this research were to (i) determine the yield and growth characteristics of a soybean with supernodulation and (ii) to determine the influence of supernodulation on the yield and growth characteristics of winter wheat planted after soybean. Field studies were conducted for 3 yr (1993–1995) under rainfed conditions at Arlington, WI, on a Piano silt loam (fine-silty, mixed, mesic Typic Argiudoll) and under irrigated conditions at Hancock, WI on a Plainfield sandy loam (loamy-sand, mixed, mesic Typic Udipsamment). Four soybean genotypes were evaluated at R3, R7 and RS growth stages included supernodulating, the normal nodulating parent of the supernodulating line, a nonnodulating line, and the normal nodulating parent of the nonnodulating line. The nonnodulating line provided a baseline of soybean performance without N₂ fixation capabilities. ‘Merrimac’ winter wheat planted after all soybean plots were harvested was evaluated at Z37, Z50, and Z90 growth stages. Depending on location, the supernodulating genotype increased soybean nodule density 43 to 135%, soil nitrate N at 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 stages, soil nitrate N, plant biomass N and forage and grain yields were not different between the normal and supernodulating genotypes. Winter wheat grown after supernodulating soybean had no difference in soil nitrate N, biomass N, or forage or grain yield compared with normal nodulating genotypes. Based on these results, there does not appear to be any agronomic benefit to using these supernodulating soybean compared with normal nodulating lines. Research QuestionRecent research has demonstrated that current high-yielding soybean cultivars can remove more N from the soil than they add through fixation of atmospheric N. Given the potential to remove soil N, alternative soybean genotypes have been tested that increase N₂ fixation and reduce use of soil N. In the 1980s, a supernodulating soybean mutant referred to as “NTS Supemod” was developed. Most research conducted on supernodulating mutants has been performed under glasshouse conditions, so it is not known how well NTS Supernod will perform under field conditions. The purpose of this research was to (i) determine the yield and growth characteristics of a soybean with supernodulation and to (ii) determine the influence of supernodulating soybean on the yield and growth characteristics of winter wheat planted after soybean. Litrature SummaryPrevious supernodulating soybean research has concentrated on the characteristics relating to nodule infection and development and early vegetative growth compared with normal nodulating soybean lines. Research with supernodulating soybean has been focused on developing a soybean with increased N₂ fixation capacity. The practical benefit of supernodulating soybean beyond its use as a tool for research of the nodulation process has not been demonstrated. With the exception of unpublished reports from the University of Georgia, Mississippi State University, and the University of Arkansas, no reports were found that examined the yield and growth performance of NTS Supemod under field conditions. Study DescriptionFour soybean genotypes (supernodulating, the normal nodulating parent of the supernodulating line, a nonnodulating line, and the normal nodulating parent of the nonnodulating line) were evaluated for yield and growth characteristics for 3 yr (1993–1995) under rainfed conditions at Arlington, WI, on a silt loam soil and under irrigated conditions at Hancock, WI, on a sandy loam soil. Growth and yield of winter wheat planted after soybean was used to determine the influence of these soybean genotypes in crop rotations. In addition to soybean nodule density, measurements on biomass yield, biomass N, soil nitrate concentration, chlorophyll value, grain yield, grain N, and seed weight were determined for both crops. Applied QuestionsWhat are the yield and growth characteristics of a soybean with enhanced nodulation? Soybean with enhanced nodulation had 2.1 times more nodules/plant than the normal parent and 1.6 times more nodules/plant than the normal nodulating parent of nonnodulating soybean (Table 1). Nodule appearance was similar between nodulating genotypes, except the supernodulating line had more nodules on the lateral roots than the other nodulating genotypes. The supernodulating line had higher early season soil nitrate N and above ground biomass N levels than both normal nodulating and nonnodulating genotypes. Above ground biomass and grain yield of supernodulating soybean was not different from either normal nodulating genotype, but was greater than the nonnodulating line and its normal nodulating parent (Table 1). What is the influence of soybean with enhanced nodulation on the yield and growth characteristics of winter wheat planted after soybean? Overall, winter wheat growth and performance was similar following all nodulating soybean genotypes, but was reduced following nonnodulating soybean. Winter wheat biomass and grain yields were not different following supernodulating or either normal nodulating soybean genotype (Table 1). The early season advantage of supernodulating soybean was not maintained throughout the growing season. Based on our results and the parameters we measured, there does not appear to be any agronomic or economic benefit to growing soybean with enhanced nodulating capabilities over a normal nodulating soybean. Table 1Mean values for nodule density, biomass, and grain yield of four soybean genotypes and biomass and grain yield of winter wheat following four soybean genotypes. Arlington and Hancock, WI, 1993–1995. Soybean yieldWinter wheat yieldBiomassGrainBiomassGrainGrowth stageGrowth stageSoybean genotype†Nodule densityR3R7R8Z37Z50Z90no./planttons/acrebu/acretons/acrebu/acreSPN1862.53.659.70.831.8340.8NP912.43.663.10.851.8839.4NN01.11.522.90.581.3032.2NOR1212.13.152.60.951.9040.2 LSD(0.10)‡160.30.44.70.160.211.9†SPN = Supernodulating, NP = Normal parent of supernodulating, NN = Nonnodulating, NOR = Normal parent of nonnodulating. ‡Least significant difference among soybean genotypes at the 0.01 level.