Optimizing nitrogen and potassium fertilization enhances yield-protein balance in soft wheat

Nitrogen and potassium fertilization are key factors influencing the yield and quality of soft wheat. However, most existing studies have focused on single responses such as yield or grain protein content, and a systematic understanding of nitrogen-potassium interactions in regulating nitrogen metabolism and transport-partitioning processes, as well as their roles in the coordinated "high yield-low protein" mechanism, remains insufficient. This study investigated two soft wheat cultivars, Wanximai 0638 and Yangmai 20, under four nitrogen application levels (0, 150, 180, and 210 kg·ha⁻¹), three N basal-to-topdressing ratios (8:2, 7:3, and 6:4), and two K fertilization treatments (single basal application versus split application at a 5:5 basal-to-topdressing ratio). The topdressing fertilizers were applied at the jointing stage. By analyzing indicators, including grain yield (GY), grain protein content (GPC), flour solvent retention capacity (SRC), nitrogen accumulation and translocation, nitrogen use efficiency (NUE), physiological nitrogen use efficiency (NPE), chlorophyll content (SPAD), and key nitrogen metabolism enzyme activities, the comprehensive effects of nitrogen application and potassium split application on soft wheat yield and quality were elucidated. The results revealed that the nitrogen rate, topdressing nitrogen ratio, and potassium split application method significantly influenced GY, GPC, SRC, nitrogen accumulation and translocation, NUE, NPE, SPAD, and nitrogen metabolism enzyme activities. With increasing nitrogen rate and topdressing proportion, activities of nitrogen metabolism enzymes were enhanced. Compared with 180 kg·ha⁻¹ nitrogen and 7:3 or 8:2 base-to-topdressing ratios, applying 210 kg·ha⁻¹ nitrogen with a 6:4 ratio increased pre-anthesis nitrogen translocation contribution to grains and post-anthesis nitrogen accumulation, leading to GY increase of 18.2%-33.5%. However, NUE declined by 5.3%-8.2%, and GPC exceeded the standard for soft wheat. Optimizing potassium fertilization revealed that under a nitrogen rate of 180 kg·ha⁻¹, combining a 7:3 nitrogen base-to-topdressing ratio with potassium split application significantly improved NUE compared to one-time basal potassium application. This was achieved by increasing the number of effective spikes and grains per spike, leading to a 13.3% increase in GY. Additionally, potassium application negatively regulated wheat nitrogen assimilation and utilization, thereby reducing GPC (total effect = - 0.042), exhibiting a usual "protein dilution effect" that contributes to the quality formation of soft wheat. In conclusion, under the production conditions of rice-wheat rotation in the Jianghuai region of China, a recommended nitrogen application rate of approximately 180 kg·ha⁻¹, combined with a 7:3 base-to-topdressing ratio and potassium split application (5:5), can effectively maintain high GY while controlling GPC in soft wheat.