Fine root decomposition in poplar plantations: Negative regulation by initial root nitrogen content

Fine root decomposition is essential for the cycling of carbon (C) and nutrients in terrestrial ecosystems. Although nitrogen (N) deposition is known to affect this process, the primary regulatory mechanisms remain uncertain. In this study, we investigated the impact of N addition on fine root decomposition through a three-year experiment, using two distinct N addition timelines: “Before” (fine roots collected from N addition plots and decomposed in control plots) and “During” (fine roots collected from control plots and decomposed in N addition plots). Our findings showed that N addition “Before” significantly inhibited fine root decomposition, while N addition “During” had no noticeable effect. Random forest analyses identified substrate N concentration as the key factor influencing decomposition rates. Specifically, decomposition rates were negatively correlated with N concentration and positively correlated with C:N ratios, regardless of the N addition timeline. These results support the N inhibition hypothesis and emphasize the dominant role of substrate chemistry in regulating fine root decomposition. This study offers valuable insights into ecosystem C cycling under increasing N deposition and underscores the importance of incorporating substrate chemical traits into predictive models.