Objectives Controlled-release urea (CRU) is designed to release nitrogen (N) into soils at rates of closely matching crop demands for N. Fulvic acid (FA) can regulate the transformation of soil and fertilizer nutrients. Both CRU and FA have been proved the effects of increasing fertilizer use efficiency. The study was to investigate the combination effects of CRU with FA on the wheat (Triticum aestivum L.) yields and soil physical and chemical properties, providing scientific basis for appropriate application of CRU.

Methods A pot experiment was conducted using wheat as tested crop on brown soil. The full-dose urea (U) and reduced-dose urea (U2/3) were used as controls, in full dose control the fertilizer rates were N 225 kg/hm², P₂O₅ 150 kg/hm² and K₂O 75 kg/hm²; the FAs treatments included full-dose urea plus FA 45 kg/hm² (U + FA) and reduced-dose urea with FA (U2/3 + FA), CRUs treatments including full-dose CRU (CRU, the same N rate as in U) and reduced-dose CRU (CRU2/3), combined treatments including full-dose CRU plus FA (CRU + FA) and reduced-dose CRU with FA (CRU2/3 + FA). In controls and FAs treatments, 50% fertilizers were applied before planting and the left at elongation stage; in the treatments of CRUs and combinations, all fertilizers were once applied before planting. The SPAD values and plant height of wheat at different growing stages were measured, and the NO₃^– and NH₄⁺ contents in 0–20 cm soil layer were determined at the same time; the yield and yield components at harvest were investigated, and the nitrogen use efficiency was calculated.

Results 1) At the same N application rates, wheat yields and net income of CRUs were significantly increased by 7.3% and 24.9%, compared with regular urea treatments. There was no significant difference in yield and nutrient use efficiency between CRU treatments with reduced rate and U treatments with full dose. Wheat yields of CRU integrated with FA resulted in 6.4% more yield and 12.6% more net income than CRU treatment at the same nitrogen rate, while the nitrogen use efficiency was similar. The treatment of U2/3 + FA produced 10.6% more yield than U2/3, but lower yield than U treatment. 2) At the same rate of N, CRU treatment or CRU2/3 treatment had the similar plant height and SPAD value to respective treatment of U or U2/3, there was no significant difference in plant height and SPAD value between treatments of CRU or CRU 2/3 and those of CRU + FA or CRU 2/3 + FA at jointing. 3) Soil NO₃^–-N and NH₄⁺ -N were significantly improved at the late stages of wheat growth in CRUs treatments. Compared with U, the NO₃^–-N contents of CRU treatments were increased by 54.7%, but those of CRU2/3 treatments were not. The soil available phosphorus content of U + FA and CRUs + FA treatments decreased first and then increased and finally decreased in the whole growth stage. At jointing, soil available phosphorus content in CRU treatment was not significantly different from U treatment, but CRU2/3 + FA and U2/3 + FA increased soil available phosphorus content by 18.6% and 20.6%, respectively, compared with CRU treatment. At jointing and flowering, soil acidification was alleviated. Treatment U + FA significantly reduced soil pH by 0.11 units at the jointing but did not affect soil pH in other stages, compared with treatment U.

Conclusions Basal application of CRU in combination with FA could meet the wheat requirement for N during the whole growth period, and significantly improves the supply intensity of soil available nitrogen after jointing stage, therefore, improves wheat yield, fertilizer use efficiency and net income by their synergistic interactions.