Responses of fine root dynamics to nitrogen application in global temperate forests and grasslands: inconsistencies between biomass and turnover

Abstract Fine root dynamics are crucial for terrestrial ecosystem productivity and nutrient cycling. However, the effects of nitrogen (N) deposition on fine root dynamics in temperate ecosystems remain poorly understood. In this study, we used a meta-analysis to explore the general patterns and key drivers of fine root biomass and turnover in temperate forests and grasslands in response to N application. We found that N application significantly reduced fine root biomass compared to the control group (no N application), with notable differences across N forms. However, the impact of N application on fine root biomass remained consistent across ecosystem types, soil depths and root diameters. In terms of fine root turnover rate, N application had no significant overall effect, and the response did not vary across N forms, ecosystem types, soil depths or root diameters. However, significant differences were observed across methods for estimating fine root turnover rate. Multiple regression analysis showed that mean annual temperature (MAT) and experimental factors (including duration and N application rates) were the primary determinants of fine root biomass response to N application. In contrast, fine root turnover was not significantly influenced by any of the factors analyzed. Overall, our findings highlight the negative impact of N application on fine root biomass and the neutral effect on fine root turnover, and also suggest that find root dynamics are closely associated with experimental factors, including experiment duration and N application rate. This study provides an important advancement in understanding the feedback between root dynamics and global change, offering insights for developing management strategies to address belowground ecological processes under global change scenarios.