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Though there have been numerous studies on the effect of nitrogen (N) fertilization on soybean [Glycine max (L.) Merr.], relatively few have investigated early season N application in the unique environment of the northern Great Plains. The objective of this research was to investigate the impact of starter N fertilization on soybean yield and quality in this cool environment. To achieve this objective a field experiment was established within a 2-yr corn (Zea mays L.)-soybean rotation, using a split-plot design with four replications. Whole plots were tillage [no-tillage (NT) and conventional tillage (CT)] with starter fertilizer (N source by rate) as the split plot treatments. Nitrogen was band applied at planting as either ammonium nitrate (AN) or urea (UR), at rates to supply 0, 8, 16, and 24 kg N ha 21. Yields were greater for the 2004 growing season than 2002 and 2003, possibly due to more favorable environmental conditions. In 2 of the 3 yr there was an increase in grain yield and early (V3-V4 and R1) plant biomass and plant N due to starter N. The initial increase in plant vigor resulted in a grain yield increase compared to the no N treatment. Analysis pooled over the 3 yr of the experiment showed an average yield increase of 6% for the 16 kg N ha 21 rate, compared to the no N treatment, with no difference in grain N or oil concentration. This research demonstrates that applying N as starter has the potential to increase soybean yield and early plant growth, but this may or may not translate into improved grain quality in the unique environments of the northern Great Plains.

N made available to crops that follow legumes in rotation. An estimate of soil mineralizable N is needed to determine crop While fertilization guides use total organic matter and needs for N fertilizer. The objective of this research was to estimate previous crop as indicators of N mineralization for the soil net N mineralization in soils maintained in continuous corn (Zea mays L.) (CC), corn-soybean [Glycine max (L.) Merr.] (CS), and coming season, a variety of direct and indirect lab methcorn-soybean-wheat (Triticum aestivum L.)/alfalfa (Medicago sativa ods may be used for more precise predictions (Fox and L.)-alfalfa (CSWA) rotations that have been managed since 1990 Piekielek, 1978; Hong et al., 1990). Laboratory tests with zero N (0N), low N (LN), and high N (HN) fertilization. Soil allow compositing and homogenizing soil samples to samples were taken from 0-to 20-cm depth in plots planted to corn decrease the standard deviation and required replicain 1998. In order to produce more realistic time-series data of net N tion. Aerobic incubation for 120 to 252 d is commonly mineralization, soils were incubated in filtration units in a variableused to estimate the size and decay rates of mineraliztemperature incubator (VTI) that mimicked field soil temperatures able N pools (Stanford and Smith, 1972; Cabrera and under a growing corn canopy. Rotation and N fertilization significantly Kissel, 1988). Temperature and matric potential of incuaffected net N mineralization in soil samples. Cumulative net N minerbated soils affect the rate and cumulative N mineralized. alized in a 189-d field temperature incubation averaged 133 Ϯ 6 kg ha Ϫ1 in CC, 142 Ϯ 5 kg ha Ϫ1 in CS, and 189 Ϯ 5 kg ha Ϫ1 in CSWA. Within ordinary field soil matric potentials from Ϫ1.85 Across rotations, average net N mineralized was 166 Ϯ 9 kg ha Ϫ1 in to Ϫ0.01 MPa, temperature has a greater influence on 0N plots, 147 Ϯ 10 kg ha Ϫ1 in LN plots, and 152 Ϯ 10 kg ha Ϫ1 in N mineralization than does matric potential (Zak et al., HN plots. Inclusion of a legume, particularly alfalfa, in the rotation 1999). Most N mineralization laboratory experiments increased net N mineralized. Generally, more net N was mineralized are incubated at 35ЊC, considered the ideal temperature from plots receiving no fertilizer N than from soil with a history of for maximum N mineralization. Nitrogen mineralized N fertilization. Variable-temperature incubation produced realistic in laboratory incubations at 35ЊC represents potential time-series data with low sample variability.

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