Forest soil-derived organic matter (SDOM) represents one of the largest mobile and reactive terrestrial carbon pools. It is also a major component in carbon cycling that transports carbon from land to the ocean. Current and future environmental changes (e.g., wildfires and nitrogen deposition) to forest soils may shift both the quantity and quality of SDOM and consequentially alter the environmental impacts of SDOM. Based on optical spectroscopy and ultrahigh-resolution mass spectrometry, we studied the molecular-level composition of SDOM in various forests that were subject to wildfires or nitrogen addition. Wildfires or nitrogen addition did not alter the SDOM quantity but significantly altered the SDOM quantity. Specifically, low-intensity burning shifted the SDOM to less soluble carbohydrates and more polyphenolics, while high-intensity burning yielded more low-mass molecules and condensed aromatics in the SDOM. In addition, short-term (<10 yrs) nitrogen addition did not significantly alter SDOM chemistry, while long-term nitrogen (>20 years) addition increased the molecular size and aromaticity of SDOM, particularly for the condensed aromatics. Such alterations in the SDOM with environmental changes are expected to impact the microbial communities and the cycling of carbon and pollutants. Our results unequivocally highlight that advanced molecular-level methods are valuable in identifying the underlying biogeochemical mechanisms in the impacts of environmental changes on carbon cycling and environmental quality.