Organ‐specific ozone and nitrogen legacies strengthen substrate control over litter decomposition

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.