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.

Incorporating humic acid (HA) into phosphate fertilizers to produce HA-enhanced phosphate fertilizers (HAPs) can improve the migration and availability of fertilizer-derived phosphorus (P) in soil. However, the optimal proportion of HA remains inconsistent. Here, we investigated the effects of HAPs with different HA proportions (0.1–10% w/w) on water-soluble P fixation rate, P migration, P transformation, and soil microorganisms, and analyzed the main P forms in HAP using Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy. The results showed that incorporating 0.1% HA had no impact on P migration and transformation, whereas incorporating 0.5–10% HA increased the migration distance and cumulative migration of fertilizer-derived P by 0–5 mm and 17.1–30.3%, respectively, compared with conventional phosphate fertilizer (CP). Meanwhile, HAPs with 0.5–10% HA significantly reduced the water-soluble P fixation rate by 18.3–25.6%, and significantly increased the soil average available P (AP) content in 0–40 mm soil layer around the P application site by 6.2–12.9% relative to CP, partly due to the phosphate monoesters in HAPs. Clustering analysis revealed that 0.5% HA had similar effects relative to higher HA proportions (1% and 5%), and the inhibition of HAP with 0.5% HA on bacteria and fungi was also greater than that of CP due to the high concentration of soil P. Overall, 0.5% was determined to be the optimal amount of HA for HAP production, which provided a theoretical basis for the development of high-efficiency phosphate fertilizer.

Weathered coal is known to have potential inhibitory effects on urease activity, thus reducing the loss of nitrogen from fertilizer such as ammonia. This means that it can be used as a urea enhancer to promote urea utilization efficiency. However, the variation in its composition and structure has impeded the optimal utilization of this resource. In this study, we collected Chinese weathered coal from six representative geographical locations and analyzed its elemental and substance composition, as well as determined its chemical structure via Fourier transform infrared spectroscopy and investigated its effects on urease (soybean meal) activity. The results showed evident variation in the composition and structure among the different weathered coal samples, especially in the pH values, humic acid and ash content, and aromaticity. All six weathered coal samples significantly inhibited urease activity, and the inhibitory effect was enhanced with the elevated proportion of weathered coal introduced to urea. When the additive proportion of weathered coal increased, the weathered coal, characterized as having a higher humic acid content and a more aliphatic structure, showed a more rapid increase in the urease activity inhibition rate, while there was only a slight effect when the weathered coal had a low humic acid content and high atomicity. Therefore, the former type of weathered coal was more sensitive to the additive proportion. Furthermore, there was no consistent rule when the same proportion of weathered coal from different geographic locations was blended into urea, which might be attributable to other unexplored factors.