Factors influencing fine root decomposition rate in response to nitrogen addition vary across root characteristics

Nitrogen (N) deposition strongly affects litter decomposition and nutrient release , thereby reshaping carbon cycling in terrestrial ecosystems. Although most studies have focused on above-ground litter, the effects of N addition on fine root decomposition and their underlying drivers, particularly across different root characteristics, remain poorly understood. We synthesized a meta-analysis of 144 observations from 30 studies to quantify the response of fine root decomposition rates to N addition and to identify the key factors that regulate these responses across root characteristics. On average, N addition reduced fine root decomposition rates and mass loss by 8.52% and 6.24%, respectively. Negative responses to N addition were most pronounced in roots ≤2 mm in diameter and in first- and second-order roots. Soil microbial biomass N and clay content were the dominant drivers of these responses in roots ≤2 mm. Within different root characteristics, soil microbial biomass N and soil N content were the most important factors regulating the response of decomposition rates to N addition for roots ≤1 mm, while N addition amount was the key determinant for third- to fifth-order roots. These results suggest that the controls on fine root decomposition under N addition vary with root characteristics. This highlights the need to incorporate root variability into ecosystem models to improve predictions of below-ground carbon cycling under future climate change.