Objectives In north China, there are many problems such as low nitrogen utilization efficiency and unstable yield in the large area of medium and low yield fields. Therefore, this paper studies the influence of reducing 20% of nitrogen fertilizer and applying different amounts of organic fertilizer on winter wheat yield and fertilizer utilization rate on the basis of farmers' habit of applying nitrogen.

Methods During 2022−2024, a field trial were conducted for two consecutive winter wheat seasons in central of north China plain. Seven treatments were setup, including no nitrogen fertilizer (N0), conventional nitrogen application rate (N1), reducing nitrogen rate by 20% (N2), and combined application of humic acid compound fertilizer (N3), functional bio-organic fertilizer (N4), microalgae (N5), and weathered coal bio-fertilizer (N6) at the same total N input with N2. The photosynthetic parameters of flag leaves of wheat were measured at anthesis stage. The plant samples were taken to investigate the dry matter accumulation, nitrogen uptake, and transport at the key growing stages. And grain yield and protein content were analyzed at harvest.

Results Compared to N2, the net photosynthetic rate increment of wheat flag leaves treated with four different organic fertilizers did not reach a significant level; the transpiration rate increased significantly by 4.3% to 33.5%, with N7's transpiration rate being significantly higher than N6's; N3 reduced the inter-row CO₂ concentration; N3 and N4 treatments increased dry matter accumulation at all stages, while N5 and N6 increased dry matter accumulation during the flag emergence, flowering, and maturity stages, with an increase in above-ground dry matter accumulation of 8.9% to 18.2% during maturity; N3, N4, N5, and N6 treatments increased nitrogen accumulation from green-up to flowering stages, with only N4 increasing nitrogen accumulation during maturity, by 15.8%. Compared to N1, N2 treatments either decreased or remained unchanged in pre-flowering nitrogen transport, whereas N3, N4, N5, and N6 treatments increased pre-flowering nitrogen transport. Compared to N1, N2 reduced grain weight (2022) and effective ear number (2023), while N3, N4, N5, and N6 treatments significantly increased effective ear number, grain weight, and nitrogen fertilizer agronomic efficiency, with N3 and N4 significantly increasing yield; N2, N5, and N6 treatments reduced protein content in grains, while N3 and N4 had no significant effect on protein content. Structural equation analysis results show that fertilization directly affects wheat nitrogen and dry matter accumulation (path coefficients 0.86**, 0.90**), which in turn affects leaf photosynthetic characteristics (0.52*), and dry matter accumulation directly affects grain yield (0. 7*), the comprehensive effect of each fertilizer treatment in two years was ranked as N4>N3>N6>N5>N1>N2>N0.

Conclusions Farmers habitually reducing nitrogen by 20% has no significant impact on wheat photosynthetic efficiency, dry matter accumulation, and nitrogen utilization. Applying functional bio-organic fertilizers and humic acid compound fertilizers on top of a 20% nitrogen reduction can further increase the accumulation of dry matter and nitrogen during all growth stages, promoting the transfer of dry matter and nitrogen to grains. This ultimately significantly boosts wheat yield and protein content, enhancing nitrogen fertilizer efficiency. Applying weathered coal bio-fertilizer on top of a 20% nitrogen reduction can increase nitrogen accumulation before flowering, but the nitrogen accumulation during flag leaf emergence and flowering is notably lower compared to applying functional bio-organic fertilizers after a 20% nitrogen reduction. Considering the large area of medium-and low-yield wheat fields in the North China Plain, it is recommended that applying functional bio-organic fertilizers on top of a 20% nitrogen reduction can achieve cost savings and increased efficiency.