It is widely recognized that increased nitrogen (N) and phosphorus (P) inputs play critical roles in plant carbon (C) inputs and microbial growth and activity, thereby profoundly affecting the composition and dynamics of soil organic C (SOC). However, whether and how plant- and microbial-derived C and their associated SOC fractions respond to the interaction between N and P additions remain unclear. Here, an 8-year N and P addition experiment was conducted in a subalpine forest on the eastern Tibetan Plateau. We used amino sugars and lignin phenols as biomarkers for quantifying microbial necromass and plant lignin components, respectively, and separated SOC into distinct functional C pools, such as particulate organic C (POC) and mineral-associated organic C (MAOC). The results revealed that N addition decreased the retention of lignin phenols in SOC without P input, whereas no significant changes occurred under N addition with P input. In contrast, N addition increased the microbial necromass contribution to SOC under no P input, whereas N addition effects were absent under P input. Regarding N addition alone, the decrease in plant lignin was likely associated with lower lignin inputs from the root litter, while the increase in microbial necromass was largely attributed to suppressed necromass decomposition via a reduction in N-acquisition enzyme activity. Moreover, the POC and MAOC pools and their ratios to SOC were insensitive to N addition, regardless of P input. Collectively, our findings provide novel insights into the importance of P availability in mediating N addition-induced accumulation of plant lignin and microbial necromass in subalpine forest soil, highlighting the necessity of incorporating the interaction between N and P additions on plant- and microbial-derived components into terrestrial C cycling models to improve the prediction of SOC dynamics and storage under future nutrient enrichment scenarios.