Sign up for email alerts of new Fluid Journal issues!
Fluid Journal : Fall 2017
12 The Fluid Journal Fall 2017 nutrition, with N nutrition as the main nutrient limiting soybean yields and seed quality (Ciampitti et al., 2016). Interaction between soybean genotypes and fertilizer N response is not yet well understood. Rowntree et al. (2013) documented an annual genetic U.S. soybean yield gain of approximately 25 kg ha-1 for maturity group (MG) III released from 1920’s to 2,000’s when planted around May. Previously reported soybean yield gain was achieved in detriment of the protein concentration (Rowntree et al. 2013). Thus, it is valid to hypothesize that high yielding soybeans will need higher nutrient demand to sustain protein levels. Soybean plants have the capacity of fixing N from the atmosphere. Biological N fixation (BNF) is the result of the conversion of atmospheric N2 into ammonia (NH3), and later on into N-containing organic components that will become available to the plant (Wright & Lenssen, 2013). However, it had been documented that BNF process is not able to supply the total N requirement of the plant. Overall, only 50 to 60% of soybean N demand is usually met by the BNF process (Salvagiotti et al., 2008). An unanswered scientific knowledge is still related to the ability of the BNF process to satisfy soybean N demand at varying yield levels. In summary, for the genotype × N interaction, the main question is “do high yielding soybeans need to be fertilized with N?” The understanding of genetic gain × N under high yield potential is a critical factor for advancing soybean yield improvement. Objectives The objectives of this study were to: • Evaluate the yield performance and seed N content of historical and modern soybean genotypes released from the 1980s to 2010s • Study the contribution of N in soybeans under different N nutrition scenarios: 1) soybeans planted under normal production conditions - only inoculated-, 2) all N requirements met by N fertilizer, and 3) inoculated with an additional N rate applied in reproductive stages. Methodology Sites. A total of 4 sites were evaluated during the 2016 growing season in Ottawa (East Central KS), Ashland Bottoms and Rossville (Central KS) in the United States (US), and Oliveros (Santa Fe province) in Argentina (ARG) (Figure 1). Experimental design. The study was conducted on experimental plots with 10 ft. wide by 50 ft. long at Ottawa (Figure 2) and Ashland (US), and 10 ft. by 30 ft. long at Rossville (US). Target seeding rate was 140,000 seeds per acre at Ottawa, 180,000 seeds per acre at Ashland, and 103,000 seeds per acre at Rossville. At Oliveros (ARG), experimental plots were 8.5 ft wide and 23 ft long and seeding rate was 146,000 seeds per acre. Site characteristics. Soil samples were collected before planting at 6 and 24-inch depth for US locations (Ottawa, Ashland, and Rossville). Parameters analyzed from these samples collected at 6-inch depth were pH; Mehlich-P; cation exchange capacity (CEC); organic matter (OM); Ca, Mg, and K availability; and for the soil samples at 24-inch soil depth, only N-nitrate (N-NO3) concentration. In Oliveros (ARG), all soil samples were collected at 8-inch soil depth, parameters analyzed were pH, Bray P1, OM, and N-NO3 (Table 1). Fertilizer applications. The fertilizer applications were performed using fluid urea ammonium nitrate (UAN at 32-0-0) at all sites as needed per each treatment combination. The three N strategies were the same at all locations. Strategy 1 (S1) was the common practice (control) with no N applied, only inoculated; strategy 2 (S2) all N was provided by fertilizer at a rate of 600 lbs. N ac-1 , equally split in 3 timings: planting, R1 and R3; and finally Figure 1. Map of the state of Kansas and Argentina highlighting all four sites conducted during 2016 growing season: Ottawa (Franklin Co., KS), Ashland (Riley Co., KS) and Rossville (Shawnee Co., KS) in US (left panel) and Oliveros (Santa Fe province) in ARG (right panel). Figure 2. Experimental plots in Ottawa, Kansas 2016. No soybean history was recorded at this location for the past 20 years, 4 rows of corn non-N-fertilized were planted on the side as the check for the biological N fixation (BNF) determination.