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Dissolved organic matter (DOM) plays a key role in forest carbon biogeochemistry by linking soil organic carbon (SOC) sequestration and water fluxes, which is further shaped by elevated atmospheric nitrogen (N) deposition. Although enhanced SOC sequestration was evidenced in tropical forests due to rising N deposition, it remains unclear how long-term N inputs affect soil DOM composition, which regulates SOC sequestration capability due to its mobility and biological instability. Here, the quantity, optical properties, and molecular-level characteristics of soil DOM based on a simulative N deposition experiment with four N addition levels (0, 5, 10, and 15 g m−2 yr−1) were studied in a primary tropical forest in south China. Results showed that 18-year N additions significantly altered soil DOM composition, with an increasing trend in soil dissolved organic carbon content. Medium- (10 g m−2 yr−1) and high-N addition (15 g m−2 yr−1) markedly elevated DOM average molecular weight by 12% and aromaticity, with specific ultraviolet absorbance at 254 nm increasing by 17%, modified aromatic index by 35%, and condensed aromatics by 67%. Medium- and high-N addition also increased recalcitrant DOM components but decreased other DOM components, with increasing percentages of lignin-like, tannin-like, and carboxylic-rich alicyclic molecule-like compounds, and decreasing percentage of more bioavailable contributions with H/C ratio > 1.5. Importantly, significant correlations of the SOC content of the heavy fraction with optical properties and with recalcitrant DOM components were observed. These findings suggest that long-term N additions may alter soil DOM composition in a way to benefit soil OC storage in the primary tropical forests. It merits focusing on the mechanisms to association of soil DOM dynamics with SOC sequestration.

Dissolved organic matter (DOM) in soils plays a crucial role in biogeochemical cycles during rainy seasons in forest systems. However, spatiotemporal variations in DOM characteristics in forest soil solutions during the rainy season and their responses to increased nitrogen deposition are unclear. Using optical spectroscopy, dissolved lignin phenol analysis, and stable carbon isotope analysis, we characterized DOM in soil solutions collected in May, July, and September at different soil depths (20 and 40 cm) from two tropical plantations with three N addition levels (0, 50, and 100 kg N ha–1 yr–1). Although the dissolved organic carbon (DOC) concentration did not significantly differ between depths, the aromaticity and δ13C values of DOM and dissolved lignin phenol levels were lower at 40 cm than at 20 cm, which could be attributed to the preferential mineral adsorption or microbial consumption of dissolved lignin and light carbon isotopes in the layer between 20 and 40 cm. The DOC concentration decreased and DOM aromaticity (indicated by specific ultraviolet absorbance at 254 nm) increased from May to September, possibly due to increasing consumption of nonaromatic DOM and dilution of DOC in the middle and late rainy seasons. However, 8 years of nitrogen addition did not significantly alter the DOM concentrations or characteristics in the soil solution. These findings enhanced our understanding of the dynamics of DOM quantity and quality in tropical plantations and their drivers under global environmental change.

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