Multiple warming does not alter the regulation of DOM by soil layers in a subtropical primary forest

AbstructClimate warming substantially affects the carbon sequestration potential and ecosystem functions of forest ecosystems. As a labile fraction of soil organic matter (SOM), dissolved organic matter (DOM) serves as a pivotal mediator in terrestrial carbon cycling. Nevertheless, the long-term effects of climate warming on DOM dynamics in forests remain poorly understood. In this study, we conducted a five-year in situ infrared warming experiment with multiple temperature gradients (+0℃, +0.8℃, +1.5℃, +3.0℃, and +4.2℃) in the subtropical primary forest of the Ailao Mountains in southwest China. We systematically evaluated the content and compositional characteristics of DOM across different warming levels and soil depths, then identified the predominant factors influencing their variation. The results indicated that warming did not significant affect the overall DOM content, although changes in molecular composition were obvious. Fluorescence analysis showed that humic-like substances generally increased under most warming scenarios and soil layers, especially at +4.2℃. Fourier transform infrared spectroscopy indicated that warming enhanced the degradation of low molecular weight DOM in surface soils but promoted their accumulation in deeper layers. Ultraviolet spectroscopy revealed that DOM aromaticity, humification degree, and molecular weight responded to warming. Structural equation modeling and random forest analysis showed that DOM content was primarily regulated by soil depth and nutrient availability, while the warming effect was not statistically significant. These findings suggest that subtropical primary forests with low-disturbance can buffer the impacts of climate warming on soil DOM dynamics through a synergistic mechanism involving vertical soil stratification and rich nutrient reserves.