Effects of combined application of nitrogen and potassium on oil concentration and fatty acid component of oilseed rape (Brassica napus L.)

Nutrient inputs significantly affect the quality of rapeseed, yet comprehensive research on combined nutrient applications is lacking. To investigate the effects of combined application of N and K on rapeseed traits, including N, K, oil, and protein concentrations, fatty acid component, seed yield and oil yield. A 2-year field experiment was conducted in the Yangtze River Basin involving the rapeseed cultivars H9 (low oil) and Z11 (high oil). Sixteen N and K fertilizer combinations encompassed four N rates (0, 90, 180, and 270 kg N ha−1) and four K fertilizer rates (0, 60, 120, and 180 kg K2O ha−1). The results revealed that the combined application of N and K significantly affected rapeseed oil and protein concentrations by modulating the nutrient concentrations. Specifically, applying N reduced the oil concentration while applying K increased the oil concentration. Each 1.0% rise in seed K concentration corresponded to a 14.9% average increase in oil concentration, counterbalancing a 2.8% increase in N concentration. Additionally, the higher oil concentration of Z11was closely linked to its seed K concentration. In contrast, protein concentration increased with the N application and decreased with the K application. Furthermore, applying N reduced palmitic (C16:0) and stearic (C18:0) acid content and increased oleic (C18:1) acid content, which further increased with K. Combining the K application with the N treatment resulted in higher seed yield and oil yield compared to a single N application, which particularly benefited the high oil cultivar Z11. Utilizing an N180K120 combination significantly improved rapeseed oil concentration, fatty acid composition, seed yield, and oil yield. This study details the effects of applying K at various N fertilizer rates on enhancing rapeseed quality and yield, emphasizing the importance of the N and K ration in oilseed rape production.