Divergent responses of soil plant-derived and microbial-derived C to phosphorus input in a Chinese fir plantation

Plant-derived and microbial-derived carbon (C) are key components of soil organic C (SOC), and their responses to phosphorus (P) availability and root-derived C, govern SOC dynamics, thereby influencing SOC fate. However, in P-limited regions, how P input affects SOC and its components and underlying mechanisms remain unclear. To address this, we conducted a P addition experiment (50 kg P ha⁻¹ yr⁻¹) in a subtropical Chinese fir (Cunninghamia lanceolata) plantation, incorporating root and mycelium exclusion treatments. We investigated lignin phenols (biomarker for plant-derived C), amino sugars (biomarker for microbial-derived C), soil properties, microbial biomass, and extracellular enzyme activity. We found that 5 −years of P addition significantly decreased lignin phenols. This was due to reduced belowground C allocation and enhanced lignin decomposition (e.g., V- and S-type monomers). In contrast, P addition increased amino sugars by suppressing their enzymatic degradation and promoting conversion of plant-derived C to microbial derived C. In addition, root and hyphal exclusion negated the P-induced lignin decomposition, but did not affect amino sugar accumulation. This suggests that plant-derived C is greater sensitivity to P addition and stronger dependence on root-microbe interactions compared to microbial-derived C. These findings indicate that P addition alters SOC sources by reducing plant-derived C and increasing microbial-derived C, potentially enhancing SOC stability. This study elucidated how P availability regulates SOC and its component’s dynamics in P-limited ecosystems, providing mechanistic insights into the effects of P inputs on SOC storage, offering a theoretical basis for regional soil C management.