Sugar maple (Acer saccharum Marsh.)-dominated northern hardwood forests of the Great Lakes Region commonly receive elevated levels of atmospheric nitrate (NO3) deposition, which can alter belowground carbon (C) cycling. Past research has demonstrated that chronic experimental NO3 deposition (3 g N m)2 y)1 above ambient) elicits a threefold increase in the leaching loss of dissolved organic carbon (DOC). Here, we used DOC collected from tension-cup lysimeters to test whether increased DOC export under experimental NO3 deposition originated from forest floor or mineral soil organic matter (SOM). We used DOC radiocarbon dating to quantify C sources and colorimetric assays to measure DOC aromaticity and soluble polyphenolic content. Our results demonstrated that DOC exports are primarily derived from new C (<50- years-old) in the forest floor under both ambient and experimental NO3 deposition. Experimental NO3 deposition increased soluble polyphenolic content from 25.03 + 4.26 to 49.19 + 4.23 lg phenolic Cmg DOC)1, and increased total aromatic content as measured by specific UV absorbance. However, increased aromatic compounds represented a small fraction (<10%) of the total observed increased DOC leaching. In combination, these findings suggest that experimental NO3 deposition has altered the production or retention as well as phenolic content of DOC formed in forest floor, however exact mechanisms are uncertain. Further elucidation of the mechanism(s) controlling enhanced DOC leaching is important for understanding long-term responses of Great Lakes forests to anthropogenic N deposition and the consequences of those responses for aquatic ecosystems.