Humic acids (HAs) incorporated into urea fertilizers are highly effective at increasing yield and decreasing fertilizer-derived nitrogen (N) loss from soil, but reports of the optimal proportion in fertilizers remain widely inconsistent. In this study, we examined the effects of urea enhanced with 0.2–5.0% HAs (UHAs) on the yield, biomass production, N uptake, and N residue in fluvo-aquic soil in winter wheat cultivated over two growing seasons from 2018 to 2020 in the North China Plain. UHAs application significantly enhanced wheat grain yield, aboveground dry biomass, total and fertilizer-derived N uptake by wheat, and residue in soil, while reducing the loss of fertilizer-derived N. Additionally, UHAs treatments increased fertilizer-N residues in soil, especially in the top 30 cm soil layer, which increased with the proportion of added HAs. These positive effects were attributed to a higher spike number under UHAs treatments compared to conventional urea. Clustering analysis of the different treatments showed that 0.2% HAs were more similar to conventional urea, while 0.5% had similar effects to HAs at higher proportions. UHAs application significantly enhanced wheat grain yield, mainly via increasing spike number, and optimized the fertilizer-N fate. Among UHAs treatments, 0.5% HAs showed the highest increase in economic benefit.

Humic acid (HA) can improve the use efficiency of phosphate fertilizer, although its optimal proportion in fertilizers remains unknown. In this study, we conducted soil column experiments over two growing seasons to compare the effects of different HA proportions in phosphate fertilizer on wheat yield and phosphorus uptake, utilization, and soil available phosphorus. At harvest, grain yields were 0.23%-13.21% higher under HA-enhanced phosphate (HAP) compared to treatments with conventional phosphate fertilizer (P). Among different HAPs, wheat treated with HAP with 1.0% HA (HAP10) or 2.0% HA (HAP20) had the highest 2-year grain yield. Furthermore, total phosphorus uptake and its allocation in grains were both higher under HAP treatments compared to P treatment. The apparent use efficiency, agronomic efficiency and partial productivity of phosphorus in HAP10 and HAP20 treatments were also significantly higher than those treated with P in both growing seasons. In addition, HAP10 and HAP20 treatments resulted in the highest content of available phosphorus in the 0-60 cm soil layer compared with other HAPs or P alone. Incorporating HA into phosphate fertilizer at proportions of 0.5%-5.0% can increase wheat yield, phosphorus uptake, phosphate fertilizer use efficiency, and soil available phosphorus content, 1.0% or 2.0% HA in HAP shows the highest effects on these indexes. Given the economic benefits, 1.0% level is an optimal level for enhancing the effects of phosphate fertilizer in winter wheat crops.