Long-Term Nitrogen Addition Regulates Plant-Soil 15N–13C Coupling Through Species Traits and Temporal-Spatial Dynamics in a Temperate Forest

Nitrogen deposition is a critical driver of plant-soil interactions in forest ecosystems. However, the species-specific coordination of nitrogen uptake and carbon assimilation—traced using 15N- and 13C-labeled compounds—under varying nitrogen forms, depths, and time points remains poorly understood. We conducted a dual-isotope (15NH4Cl, K15NO3, and Na213CO3) labeling experiment in a temperate secondary forest to investigate nutrient uptake and carbon assimilation in three understory species—Carex siderosticta, Maianthemum bifolium, and Oxalis acetosella—across three nitrogen treatments (control, low N, and high N), two soil depths (0–5 cm and 5–15 cm), and two post-labeling time points (24 h and 72 h). We quantified 15N uptake and 13C assimilation in above- and belowground plant tissues, as well as 15N and 13C retention in soils. C. siderosticta exhibited the highest total 15N uptake (2.2–6.9 μg N m−2 aboveground; 1.4–4.1 μg N m−2 belowground) and 13C assimilation (58.4–111.2 mg C m−2 aboveground; 17.6–39.2 mg C m−2 belowground) under high ammonium at 72 h. High nitrogen input significantly enhanced the coupling between plant biomass and nutrient assimilation (R2 > 0.9), and increased 15N-TN and 13C-SOC retention in the surface soil layer (13,200–17,400 μg N kg−1; 30,000–44,000 μg C kg−1). Multifactorial analysis revealed significant interactions among nitrogen treatment, form, depth, and time. These findings demonstrate that ammonium-based enrichment promotes nutrient acquisition and carbon assimilation in responsive species and enhances surface soil C—N retention, highlighting the integrative effects of nitrogen form, species traits, and spatial–temporal dynamics on forest biogeochemistry.