Abstract: It is important to maintain crop yield, control nitrogen input, and improve nutrient use efficiency for agricultural green development in China. In this study, we carried out a field experiment with two nitrogen (N) reduction levels (20% and 40%) and N reduction (20% and 40%) + humic acid, with the conventional N application (urea, 250 kg·hm^-2) as control. We measured soil N supply, plant N uptake, photosynthetic characteristics, N assimilationrelated enzyme activity and yield components of maize to evaluate the potential and mechanism of nitrogen reduction and efficiency enhancement by humic acid addition in maize cultivation. The results showed that compared with conventional N application, maize yield was not reduced and 20% N use efficiency was improved by 35.8%-48.3% in the treatment with 20% N reduction and humic acid addition, while maize yield was reduced by 20.6%-24.4% and N use efficiency was improved by 20.4%-31.5% under the treatment of 40% N reduction. The 20% N reduction treatment improved root N uptake and photosynthetic N use efficiency and increased the harvest index. Compared with conventional N fertilization, 20% N reduction with humic acid application increased soil nitrate and ammonium content and root N uptake, enhanced leaf photosynthesis, increased total N uptake by 1.2 times and glutamine synthase activity by 19.9%, and reduced barren tip length by 42.9%. Regardless of humic acid application, the 40% N reduction reduced soil nitrate content, total N uptake, net photosynthetic rate, and photosynthetic N use efficiency of maize. The 20% N reduction and combination of 20% N reduction with humic acid application can achieve no reduction in maize yield and improve N fertilizer use efficiency. Compared with urea application alone, urea application with humic acid addition has advantages in improving N uptake and use efficiency.

Key words: maize, nitrogen fertilizer, humic acid, N use efficiency, nitrogen assimilation enzyme, yield.