Excessive nitrogen fertilizer application in the black soil region of Northeast China leads to nitrate leaching and gaseous nitrogen loss, posing environmental risks. This study aimed to evaluate the effectiveness of controlled-release urea (CRU) mixed with conventional urea in synchronizing nitrogen fertilizer supply with maize nitrogen requirements, improving nitrogen fertilizer use efficiency (NUE), and increasing economic benefits. A two-year field trial (2023–2024) tested six nitrogen fertilizer application strategies, all with a total nitrogen application rate of 168 kg N ha⁻¹, including no nitrogen fertilizer application (CK), conventional fractionated urea application (C0), and four controlled-release urea–urea mixed application schemes, where CRU supplied 100%, 70%, 50%, and 30% of the total nitrogen (C100, C70, C50, and C30). The results showed that the C70 treatment had the highest maize grain yield and protein yield, at 12,502.92 kg ha⁻¹ and 1567.65 kg ha⁻¹, respectively, and NUE increased by 10.07% in 2024 compared to the C0 treatment. The C70 strategy also reduced nitrate concentrations in deeper soil layers, decreasing nitrogen loss by 29.04–31.21% compared to the C0 treatment. Furthermore, the C70 strategy yielded the highest net benefit, reaching $2817 ha⁻¹. These results indicate that in black soil systems, a single basal application of C70 mixed fertilizer is an effective strategy for increasing maize yield, improving nitrogen fertilizer use efficiency, and reducing environmental risks.

[Objective] Soil microorganisms are the most active parts in soil and are sensitive to soil additives. This study aimed to clarify the impacts of the addition of biomass materials (corn straw and biochar) and nitrogen application on the compositions of the soil microbial community in moderately saline soils (salt content was 0.4%). [Method] Indoor constant-temperature cultivation experiments were conducted to study the effects of biomass materials and nitrogen application on the microbial diversity and community structure in moderately saline soils. This experiment had a two-way factorial design, with the biomass materials and nitrogen application rates as the treatments. The biomass materials included no addition of biomass materials as a control (C0), corn straw (C1, 0.64 g/pot), and biochar (C2, 0.85 g/pot), and the nitrogen application rates included 0 g N (N0), 0.015 g N (N1), and 0.03 g N (N2). There were nine treatments, as follows: C0N0, C0N1, C0N2, C1N0, C1N1, C1N2, C2N0, C2N1, and C2N2. [Results] (1) The different biomass materials and nitrogen application levels significantly influenced the α-diversity and composition of the bacterial community. At the initial stage of cultivation, the soil bacterial diversity was relatively high, and it significantly decreased after 35 days of cultivation. Moreover, the improvement of the bacterial community structure by the biochar treatment was better than that of corn straw. After 35 days of cultivation, the relative abundance of Actinobacteria, Bacteroidetes, and Firmicutes in the soil significantly increased, while the relative abundance of Gemmatimonadete, Chloroflexi, Acidobacteria, Planctomycetes, and Patescibacteria significantly decreased. Ammonium nitrogen, nitrate nitrogen, and nitrate reductase were the main environmental factors affecting the bacterial community. (2) The different biomass materials and nitrogen treatments significantly affected the richness of the fungal communities. The fungal richness index significantly increased after adding the corn straw and biochar treatments, and the addition of corn straw promoted an increase in the beneficial bacterial abundance in the moderately saline soil. Soil nitrate reductase and ammonium nitrogen were the main environmental factors affecting the fungal community. [Conclusions] In summary, biomass materials and nitrogen application can effectively increase the diversity of soil microbial communities and optimize the structure of microbial communities, thereby ameliorating the ecosystem health of moderately saline soil.