Root chemical legacies outweigh direct nitrogen exposure in controlling fine-root decomposition in poplar plantations

Fine-root decomposition is a key process in soil carbon (C) and nutrient cycling, but it remains unclear whether nitrogen (N) influences decomposition mainly by altering root traits before decomposition begins or by directly affecting roots during decomposition. We addressed this question in a 3-year field experiment in poplar plantations by separating two pathways of N influence: an indirect pathway reflecting pre-decomposition substrate history (roots from long-term N-addition plots were incubated in a common control soil), and a direct pathway reflecting N exposure during decomposition (control roots incubated across an N-addition gradient). Roots with a long-term N-addition history decomposed more slowly and retained more mass, whereas control roots exposed to elevated N during decomposition showed no significant change in decomposition rate, indicating that indirect effects of N mediated through initial substrate traits were more pronounced than direct soil N effects. Across both pathways, decomposition rate was negatively related to root N content and positively related to C:N and lignin:N, suggesting that trait variation in the initial substrate, rather than contemporaneous soil N conditions, played a dominant role over fine-root decay. These results support the N-inhibition hypothesis and suggest that N enrichment can suppress decomposition through its legacy effects on root functional traits. Our study highlights the need to incorporate pre-decomposition trait states into trait-based frameworks for understanding and predicting belowground decomposition responses to N deposition.