Effects of Combined Application of Different Nitrogen Forms on Substrate Nutrient Utilization, Root Microenvironment, and Tomato Yield

In facility tomato production, the excessive application ratio of ammonium nitrogen (NH4+-N) often leads to root acidification and calcium-magnesium antagonism. Although amide nitrogen (urea-N) has better buffering properties, it needs to be hydrolyzed before utilization, resulting in a lag effect. Previous studies have mostly focused on a single nitrogen source or a fixed proportion, and there is still a lack of systematic evidence on the nitrogen supply effects of different nitrogen application combinations at different growth stages of tomatoes. Therefore, in this experiment, tomato cultivar ‘Jingfan 502’ was used. All treatments received the same total nitrogen concentration (15 mM), but the nitrogen was supplied as different combinations of ammonium nitrogen (AN) and amide nitrogen (UN). Six AN–UN ratio treatments were designed: CK (0% AN, 0% UN), T1 (100% AN, 0% UN), T2 (0% AN, 100% UN), T3 (25% AN, 75% UN), T4 (50% AN, 50% UN), and T5 (75% AN, 25% UN). T3 (25% NH4+ + 75% urea) increased single-plant yield by 64.04% and 5.10%, and total N, P, K, and Mg accumulation by 29.0% and 20.7%, relative to T1 and T2. In addition, compared to T1 and T2, the nitrogen fertilizer uptake rate of the T3 treatment increased by 17.00% and 24.90%, respectively, and the electrical conductivity (EC) increased by 27.04% and 44.84%, respectively. Redundancy Analysis (RDA) showed that enzyme activities, total N and electrical conductivity were positively linked to microbial communities in T3 and T4, whereas communities in CK, T1, T2 and T5 correlated with nutrients and pH. Under controlled pot conditions, T3 optimizes the rhizosphere micro-environment, enhances microbial abundance and nutrient uptake, and provides a theoretical basis for precise N management in tomato.