Soybean Seed Yield Productivity and Biological Nitrogen Fixation Soybean Seed Yield Productivity and Biological Nitrogen Fixation in Kansas in Kansas

Summary Soybean [ Glycine max ( L. ) Merr.] productivity (seed yield) and biological nitrogen fixation (BNF) were evaluated in response to different fertilization strategies. The study comprised four different locations in Kansas during the 2021 growing season, two irrigated (Topeka and Scandia) and two dryland (Kiro and Ashland Bottoms) sites. Greater seed yields were recorded in Topeka and Kiro (80 bu/a) relative to Scandia (55 bu/a) and Ashland Bottoms (51 bu/a), without observing fertilizer effects on yields. Overall, the relative abundance of ureides (% RAU), an indicator of the level of BNF, increased as the crop matured and showed a negative association with the soil N level. The main objective of this study was to identify how different levels of nitrogen (N) and sulfur (S) fertilization affect the seed yield and the biological nitrogen fixation (BNF) in soybean.


Soybean Seed Yield Productivity and Biological Nitrogen Fixation in Kansas Soybean Seed Yield Productivity and Biological Nitrogen Fixation in Kansas
Greater seed yields were recorded in Topeka and Kiro (80 bu/a) relative to Scandia (55 bu/a) and Ashland Bottoms (51 bu/a), without observing fertilizer effects on yields.
Overall, the relative abundance of ureides (% RAU), an indicator of the level of BNF, increased as the crop matured and showed a negative association with the soil N level.
The main objective of this study was to identify how different levels of nitrogen (N) and sulfur (S) fertilization affect the seed yield and the biological nitrogen fixation (BNF) in soybean.

Weather
Topeka and Kiro accumulated approximately 23 inches of rain each during the growing season, while Ashland Bottoms had only 14.5, and Scandia had 12 inches. Both Topeka and Kiro locations had 6.2 inches after the first month, Scandia had 3.8 inches, and Ashland Bottoms only had 2.5 inches. Maximum temperatures higher than 95°F were reported during 19 days at Ashland Bottoms and 7 at Scandia. Between June and September, both Kiro and Topeka recorded 6 days with temperatures above 95°F ( Figure 2).

Soil Fertility
Topeka and Kiro soils had considerably more clay and greater NO 3 and SO 4 -2 than the other two sites in both sampled depths. Soil organic matter (SOM) was also found in greater levels at Topeka and Kiro, respectively, 2 and 3% (Table 2).

Kansas Field Research 2022
Kansas State University Agricultural Experiment Station and Cooperative Extension Service Treatments A total of five treatments were tested (Table 3): (1) a Check (0 N, 0 S); (2) an omission plot for S (N); (3) an omission plot for N (S), a low N rate combined with S (NS); and (4) a high rate of N combined with S (Full). Fertilizer sources, nutrient rates, and timing are described in Table 3.

Soil and Plant Sampling
A compound (6 cores) sampling at 0-6 inches depth was performed to describe initial fertility, and also 0-8 and 8-24 inches sampling was done to describe NO 3 -N, NH 4 -N, and SO 4 -2 -S prior planting. During the cropping season, additional soil samplings were collected at phenological stages of R2 (full-bloom), R4 (full-pod), and R6 (full-seed) (Fehr et al., 1971) at depths of 0-8 inches and 8-24 inches for all soil N and S determinations (Table 3).
Soybean tissue samples were collected as whole plants at the phenological stages of R2, R4, and R6 to measure biomass. Main stem samples were taken for the determination of relative abundance of ureide-N (RAU) (Moro et al., 2021). In early to mid-October, all the trials had the mid-rows cut by a combine, which provided seed yield adjusted to 13% in bushels per acre.

Data Analysis
Seed yield and relative abundance of ureides (RAU) were tested with an analysis of variance (ANOVA) under mixed effect models. For seed yield, treatment and site were considered as a fixed factor, and block as random. In the case of RAU, treatment, site, and stage are considered fixed, and blocks as random. When significant effects were found (P ≤ 0.05), comparisons were performed using Tukey's test. Analysis was accomplished with R software (R Core Team, 2020) and packages lme4 and emmeans.

Seed Yield
Soybean seed yield did not show an interaction nor fertilization effects (P > 0.05) but differed between sites. Topeka and Kiro yields reached 80 bu/a while Scandia and Ashland Bottoms yielded on average 55 and 51 bu/a, respectively (Figure 1). The abundance of water in the initial development of the soybean and greater soil fertility must have been the main drivers for the higher yields in Topeka and Kiro.

Relative Abundance of Ureides
The RAU did not show an interaction nor treatment effect (P > 0.05), but site and stage differed statistically. The treatment with the highest N input (300 lb/a), combined N fertilization with AMS and urea, was applied at planting and R3 stages, and even though not significant, it decreased RAU. The decay in this BNF activity occurred in R4 and R6 phenological stages, and compared to the other treatments, the decrease in ureide content in the xylem sap reflected in an RAU until 40% lower (Figure 3), emphasizing the effect of the mineral N in the N 2 fixation dynamics. From biomass, samplings were calculated as dry biomass per acre, and it has a positive correlation with crop growth during the season (data not shown).