Seasonal and Depth-Dependent Dynamics of Soil Nitrogen Fractions and Microbial Responses to Mineral and Biological Fertilization in Sugar Beet Under Hot and Dry Growing Conditions

Seasonal nitrogen dynamics in irrigated sugar beet reflect interacting thermal, soil-water, depth, and fertilization conditions, yet responses of individual N fractions remain insufficiently resolved. A two-year field experiment (2025–2026) examined NH4+-N, NO3−-N, hydrolysable N, and total N in the 0–20 and 20–40 cm layers under mineral NPK and biological amendments, together with microbial indicators, FDA hydrolytic activity, crop growth, and root yield. During April–August 2025, air temperature exceeded the 2000–2024 mean by 2.35 °C, and precipitation was 31.6% lower. However, gravimetric soil moisture increased between spring and summer sampling in both years despite lower pre-sampling rainfall. Over the same sampling periods, 0–20 cm NO3−-N declined by 41.2% in 2025 and 24.8% in 2026, while NH4+-N increased in 2025, resulting in a 7.3% decrease in total mineral N rather than a proportional decline in both mineral fractions. Soil depth significantly affected all N variables, biological amendment affected total N (p = 0.015), and the first two PCA axes explained 64.4% of standardized variance. NPK increased mean root yield by 26.6% (p = 0.001), whereas biological-amendment and interaction effects were non-significant. These findings highlight seasonally and vertically differentiated soil N dynamics under irrigation without establishing rainfall-induced soil drought or a causal temperature effect.