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.

Aim: Forest soils contain large amounts of terrestrial organic carbon (C), but the formation pathway of soil organic C (SOC) remains unclear. Recent evidence suggests that microbial necromass is a significant source of SOC, yet a global quantitative assessment across the whole-soil profile is lacking. We aimed to assess the vertical distribution and control of microbial-derived SOC in forest soils. Location: Global forests. Time period: 1996-2019. Major taxa studied: Soil microbial necromass carbon. Methods: We evaluated the proportions of fungal and bacterial necromass C in total SOC in the litter layer, O horizon soil, and various depths of mineral soil in forests using microbial biomarker (glucosamine and muramic acid) data. Results: The total microbial necromass C increased significantly with soil depth, ranging from 30% of SOC in O horizon soil to 62% of SOC in mineral soils below 50 cm. However, only bacterial necromass C followed this increasing trend with soil depth; fungal necromass C showed little variation across the whole-soil profile. Higher fungal and bacterial necromass C was observed in soils with lower C/N ratios and smaller aggregate sizes. Soil C/N ratio and microbial biomass C dominantly determined microbial necromass C in surface soil (above 20 cm), but soil clay content was the primary factor in subsoil (below 20 cm). Main conclusions: Microbial necromass C accounted for high percentages of the total SOC in forest soils (particularly at depths >20 cm), but its long-term stabilization may be governed by different mechanisms at different soil horizons. Substrate quality regulates microbial activity and then controls biomass turnover in surface soil, while aggregate occlusion could facilitate mineral protection of microbial necromass C in subsoil. These differential controls of microbial-derived organic C could be applied in Earth system studies for predicting soil organic C dynamics in forests.

Additional file 1. Supplementary Dataset S1. Current measurements of microbial necromass C under N addition. Supplementary Dataset S2. Effect sizes for microbial necromass C in response to N addition.

Additional file 1. Supplementary Dataset S1. Current measurements of microbial necromass C under N addition. Supplementary Dataset S2. Effect sizes for microbial necromass C in response to N addition.