High-Resolution Molecular Analyses Reveal Non-additive Impacts of Chronic Warming and Nitrogen Addition on...

Dissolved organic matter (DOM) plays a central role in soil carbon (C) cycling as the most mobile and reactive C fraction in forests, regulating the microbial metabolism, nutrient availability, and C export. However, molecular-level DOM responses to environmental stressors such as warming and nitrogen (N) deposition remain poorly constrained, particularly under their combined influences. Thus, we investigated how 14 years of soil warming, N-addition, and combined heat + N influence soil-derived DOM quantity and chemistry. Using solution-state NMR spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry, we resolved DOM composition across molecular size, biochemical class, mobility, and oxidation state. While the DOM quantity remained unchanged, warming enhanced microbial processing and oxidative transformation, yielding DOM enriched in oxidized, structurally complex compounds, whereas N-addition suppressed decomposition, limiting the release of plant-derived biopolymers and shifting DOM toward more microbial-derived constituents. Heat + N produced the most compositionally diverse DOM, with molecular shifts more closely resembling warming-induced responses, indicating that temperature-driven decomposition dominates under interacting stressors. These results demonstrate that chronic warming and N addition influence C cycling through distinct, yet nonadditive molecular pathways not captured by single-factor studies. This underscores the necessity of multifactor experiments to accurately capture the current and future ecosystem responses to interacting environmental stressors.