Long term low-level nitrogen addition enhances microbial carbon use efficiency and turnover rate across different seasons

• Low nitrogen addition promoted microbial CUE and accelerated turnover rate. • High nitrogen had either no effect on or inhibited microbial CUE. • Microbial CUE showed obvious seasonal dynamics, which was highest in spring. • Microbial respiration and turnover rate reached their peak in summer. • SOC was significantly positively correlated with microbial CUE and turnover rate. Microbial carbon use efficiency (CUE) is a key parameter in regulating the soil microbial C cycle, influencing C mineralization, turnover, and sequestration. The boreal forest plays a key role in the global C cycle, attributable to its substantial C stocks. However, seasonal variations in microbial CUE in boreal forest ecosystems and their response to enhanced nitrogen (N) deposition are poorly understood. We conducted a N addition experiment (0, 2.5, 5.0, and 7.5 g N m⁻ 2 yr⁻ 1 ) in a boreal forest, employing a substrate-independent 18 O-H 2 O labeling method to evaluate seasonal variations in microbial CUE and turnover rates. N effect on CUE was contingent on sampling season and N addition rate. Throughout the growing season, low N addition increased microbial CUE and accelerated microbial turnover rate. In contrast, high N addition had no significant effect or even decreased microbial CUE. Additionally, microbial respiration and turnover rate were highest in summer across all treatments, while microbial CUE peaked in spring. The findings also revealed a significant positive relationship between microbial CUE and factors such as the fungal-to-bacterial (F/B) ratio, dissolved organic carbon (DOC), and soil organic carbon (SOC), while being negatively correlated with the DOC/dissolved organic N (DON) ratio. Our results indicate that increases in microbial CUE and turnover under LN addition contribute to elevated SOC levels, likely through changes in microbial carbon allocation and stabilization processes. Overall, these findings highlight the crucial role of seasonal microbial CUE dynamics in regulating soil C processes under long-term N addition in boreal forest.