Litter decomposition regulates the quantity and quality of plant-derived carbon (C) and nitrogen (N) inputs to soil and is closely associated with microbial community structure. However, how elevated ozone (O₃) and nitrogen (N) addition interactively affect residual litter inputs and their associations with soil microbial communities remains poorly understood, especially in agroforestry systems. Here, we conducted a 12-month in situ litter decomposition experiment using two poplar clones (107 and 546) under ambient or elevated O₃ with or without N addition (60 kg N ha⁻¹ yr⁻¹) at an O₃-FACE platform in northern China. Litter mass and chemical traits were measured during decomposition, and endpoint soil microbial community structure was characterized using phospholipid fatty acid (PLFA) profiling. Treatment effects and litter–microbe associations were evaluated using linear mixed-effects models, correlation analysis, and redundancy analysis (RDA). Endpoint litter mass remaining was significantly affected by O₃, clone identity, and their interactions with N addition, while endpoint litter chemical traits showed trait-specific responses. PLFA-derived microbial community indices also showed treatment- and clone-dependent responses, particularly in bacterial groups, AM fungi, and the fungal-to-bacterial ratio. Endpoint litter mass remaining showed the strongest statistical association with PLFA-derived microbial community structure, whereas individual nutrient concentrations showed weaker independent effects. These findings suggest that O₃- and N-induced changes in residual litter quantity and quality are associated with shifts in PLFA-derived microbial community structure. Because PLFA characterizes microbial community structure rather than process rates, these findings should be interpreted as evidence of structural microbial reorganization associated with altered residual litter inputs, rather than direct evidence of changes in C or N cycling rates.

Elevated tropospheric ozone (O ₃ ) and nitrogen (N) deposition are concurrent atmospheric changes that strongly influence terrestrial carbon cycling, yet their combined effects on below‐ground decomposition remain poorly understood. Using a free‐air O ₃ concentration enrichment system (O ₃ ‐FACE), we investigated how O ₃ –N interactions influence the decomposition of poplar leaf and fine root litter by separating growth stage changes in litter chemistry from environmental effects in the decomposition stage. Decomposition responses were strongly organ‐specific. Elevated O ₃ and N addition had only minor effects on leaf litter decay, whereas fine root decomposition was markedly suppressed, indicating a synergistic inhibition of below‐ground carbon turnover. Lower quality substrates decomposed more slowly, released fewer nutrients (with phosphorus release reduced by up to 52.9%) and retained more mass (up to 10.6%), highlighting impaired nutrient recycling under concurrent atmospheric changes. Random forest modelling and path analyses identified starch concentration (leaves) and lignin/N ratios (fine roots) as the dominant controls on decomposition. Microbial community shifts influenced fine‐root decay but played a smaller role than substrate chemical traits. Substrate legacies formed during plant growth exerted stronger control over the dynamics of decomposition than the decomposition environment. Collectively, our results demonstrate that atmospheric O ₃ and N inputs reprogram decomposition trajectories primarily through changes in substrate chemical quality, resulting in a decoupling between substrate chemistry and microbial regulation that constrains nutrient cycling and soil carbon stabilization. These findings provide mechanistic insight into how concurrent global atmospheric drivers regulate below‐ground processes and shape soil carbon feedbacks under global change. Read the free Plain Language Summary for this article on the Journal blog.