Long-Term Nitrogen Addition Alters the Composition of Soil-Derived Dissolved Organic Matter

Forest soil dissolved organic matter (DOM) is a major source of terrigenous dissolved organic carbon (DOC) that is an important component of biogeochemical cycles. While many studies have shown that DOM dynamics are regulated strongly by nitrogen availability, how continued or increasing deposition of reactive nitrogen to forests alters the molecular composition of soil DOM remains unexplored. We studied the storage and molecular-level composition of soil-derived DOM after 22 years of nitrogen addition in a temperate deciduous forest. Soil DOC quantity changes were small (+19.0 g/m2; ∼10% increase) but the molecular composition changed markedly. Indices based on optical spectroscopy suggested that DOM molecular size and aromaticity were elevated with nitrogen addition. Nuclear magnetic resonance analyses showed that DOM contained more carbohydrates and aromatics but less aliphatic compounds with nitrogen addition. Ultrahigh-resolution mass spectrometric analysis further supports the finding that solid-phase isolated DOM from nitrogen-added soils was larger in molecular size and aromaticity. Condensed aromatic dissolved black carbon, particularly those compounds with high molecular size/carbon number and those stored in deep soils, showed the greatest percent increase with nitrogen addition. These results countered our hypothesis that soluble carbohydrates would be selectively removed from the DOC pool compared to polyphenols and condensed aromatic components under nitrogen enrichment. These changes in DOM molecular composition may also impact microbial communities and downstream metabolism of DOC.