Effects of N and S Fertilizers On Soybean Seed Composition Under Irrigated and Non-Irrigated Environments in the Early Soybean Production System.

Effects of nitrogen (N) and sulfur (S) fertilization on soybean [(Glycine max (L.) Merr.)] seed composition in the Early Soybean Production System (ESPS) under irrigated (I) and non-irrigated (NI) environments in the midsouthern US have not been well investigated. Therefore, a 3-yr field experiment was conducted from 2005 to 2007 to evaluate effects of different S rates and S with N (S+N) rates on seed N, S, protein, oil, and fatty acids (palmitic, stearic, oleic, linoleic, and linolenic). Treatments were 0 kg S ha^-1 + 0 kg N ha^-1 (control); 22 kg S ha^-1 + 0 N; 45 kg S ha^-1 + 0 N; 67 kg S ha^-1 + 0 N; 0 kg S + 112 kg N ha^-1; 22 kg S ha^-1 + 112 kg N ha^-1. No yield differences were observed with S or S+N treatments under I or NI environments. Under I conditions, S at a rate of 45 kg S ha^-1 alone  or with N at 112 kg N ha^-1  resulted in a consistent increase in seed protein and oleic acid concentrations (g constituent/kg seed dwt), and a decrease in oil content and linolenic acid concentrations compared with the control (C). Sulfur + nitrogen  application resulted in an increase in seed protein by 8.6% in 2005, 11.4% in 2006, and 8.4% in 2007 compared to the control. Oleic acid increases were 32.7% in 2005, 48.5% in 2006, and 26.4% in 2007. Oil decreased by 4.4% in 2005, 3.0% in 2006, and 8.8% in 2007. The increase of protein and oleic acid was accompanied by a higher percentage of leaf (R5-R6) and seed N and S. Under NI conditions, seed protein and oleic acid concentrations were significantly higher in C than in any S or S+N treatments, but the oil and linolenic acid were significantly lower. Total (kg constituent/ha) protein and oleic were greater in S and S+N than C under I. However, the total protein and oleic acid were greater in C than other treatments under NI. The results indicate that S or S combined with N under irrigated condition can alter seed composition, especially protein, oil, oleic and linolenic acids. The results could be useful for the soybean processors to estimate total seed protein and oil in soybean produced in ESPS, and for soybean breeders to select traits for specific seed composition constituent under environmental stress such as drought.