Effects of Long‐Term Nitrogen Additions on Root Traits, Microbial Activities, and Organic Carbon Pools Along Soil Depth Profiles Under <i>Miscanthus</i>  ×  <i>giganteus</i>

ABSTRACT Nitrogen (N) fertilizer is a common management practice to enhance the growth of bioenergy crops. Increasing aboveground yield and belowground carbon (C) (e.g., root biomass, microbial biomass, and organic carbon [OC]) is a win‐win strategy to address energy and climate crises. However, it remains poorly understood how N additions affect belowground C dynamics, especially in deep soil layers where the majority of OC is stored. In this study, we investigated the effects of 12 years of N additions (0, 30, 60, 120, and 240 kg N ha −1 year −1 ) on Miscanthus × giganteus root traits, microbial biomass, enzyme activities, and OC pools across 0–120 cm soil depth profiles. High N additions at 120 and 240 kg N ha −1 year −1 increased root N contents and leaching, but decreased root biomass along the soil depth profiles. Biomass‐specific activities of cellulose‐ and lignin‐degrading extracellular enzymes (both for C acquisition) increased, but microbial basal respiration and biomass C decreased at 120 and 240 kg N ha −1 year −1 at 0–20 cm. Despite the changes in soil N availability and microbial activities, the magnitudes of salt‐extractable OC, particulate OC, and mineral‐associated OC did not significantly vary with the N additions across the soil profiles. We only observed reductions in these soil OC pools as soil depth increased. Our findings demonstrate that increases in soil N availability and root N contents do not always translate into notable changes in belowground C storage and stability under Miscanthus × giganteus after 12 years of N additions.