Greenhouses, commonly used for vegetable production, are experiencing large nitrogen (N) inputs in North China, which leads to soil acidification, increases soil N availability, and affects microbial community structure and composition. However, it remains unclear how N enrichment influences soil microbial functional activities in this region. In this study, we conducted a two-year pot experiment in a greenhouse to evaluate the effects of four different rates of N addition (0, 334, 668, and 1002 kg N ha⁻¹ year⁻¹) on cucumber soil properties, extracellular enzyme activities, and community level physiological profiles (CLPP). We found that high-N addition (1002 kg N ha⁻¹) caused a massive accumulation of inorganic nitrogen and soil acidification, which was not beneficial to soil microbial activities. The color development (AWCD) values for the metabolism of microbial carbon sources and the activities of soil extracellular enzymes also showed a significant decrease in high N(N3) treatment. Additionally, the activity of leucine aminopeptidase (LAP) and polyphenol oxidase (PPO) of N3 decreased by 36% and 50% compared to the N0 and could be a good predictor for microbial functional diversity and microbial biomass carbon (MBC). Structural equation modeling (SEM) confirmed that the reduction of microbial functional diversity is mainly coregulated by the decline of soil pH and the change of cucumber BGB (belowground biomass) resulting from soil C and N imbalance. Overall, excessive N-fertilizer amendment can be more dangerous to microbial community functional diversity, especially for carbohydrate utilization which adversely affects cucumber yield in current intensive management.

Facility-based agriculture has rapidly advanced due to its capacity for high-intensity and year-round crop cultivation. This study evaluated the effects of different nitrogen fertilizer application rates on the growth of greenhouse tomatoes, while utilizing ¹⁵N tracing technology to explore nitrogen utilization efficiency during the growth process of facility-grown tomatoes. The results indicate that nitrogen application rates within the range of N60–N80 (93–128 kg N ha⁻¹) can optimally balance yield, nitrogen-use efficiency, and crop growth. Application rates exceeding this range do not enhance yield and lead to reduced nitrogen-use efficiency. Tomato plants exhibited a low N requirement during the seedling stage, relying primarily on native soil N stocks during the flowering stage. Fertilizer-derived N use increased during the fruiting stage. These findings demonstrate that excessive N inputs lead to diminishing returns and potential nutrient imbalances, while fully utilizing soil N stocks during the seedling and flowering stages is essential. This study emphasizes the importance of adjusting nitrogen input according to the developmental stages of the crop to optimize yield and resource utilization.