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Boreal peatlands, a crucial stock of dissolved organic matter (DOM), are susceptible to increased reactive nitrogen (N) inputs. However, the response of DOM characteristics to elevated N availability in these ecosystems is still obscure. Here, we investigated the seasonal variability of DOM quantity and characteristics in surface water and soil pore water after 5-year multi-level N addition (0, 3, 6, and 12 g N m−2 year−1) in a permafrost peatland of Northeast China. In this study, we used the excitation-emission matrix fluorescent spectroscopy combined with fluorescence regional integration, parallel factor analysis, and fluorescence parameters to decipher the effects of N addition on DOM composition, origin, and fate. Irrespective of N addition level, increased N availability reduced dissolved organic carbon concentrations in surface water and soil pore water. However, DOM characteristics are more responsive to N addition in soil pore water than in surface water. Nitrogen addition did not affect DOM composition in surface water, but altered the proportions of tyrosine-, tryptophan-, fulvic acid-, and humic-like DOM in soil pore water. Moreover, the changing trends of DOM composition were diverse during the growing season. In addition, compositional transformations of DOM did not change its biogenic signals, freshness, and humification in both surface water and soil pore water across the growing season. Our results suggest that increased N availability reduces DOM quantity and alters DOM chemical composition, which will improve our understanding of soil organic matter dynamics in boreal peatlands.

[Display omitted] • Nitrogen addition decreased DOC concentration across growing season. • Nitrogen addition altered DOM chemical composition in soil pore water. • Compositional transformations of DOM did not change its biogenic signals. • Seasonality of biotic/abiotic drivers constrained inner links of DOM characteristics. • Permafrost-induced retention reinforced edaphic DOC as a potential substrate. Boreal peatlands, a crucial stock of dissolved organic matter (DOM), are susceptible to increased reactive nitrogen (N) inputs. However, the response of DOM characteristics to elevated N availability in these ecosystems is still obscure. Here, we investigated the seasonal variability of DOM quantity and characteristics in surface water and soil pore water after 5-year multi-level N addition (0, 3, 6, and 12 g N m−2 year−1) in a permafrost peatland of Northeast China. In this study, we used the excitation-emission matrix fluorescent spectroscopy combined with fluorescence regional integration, parallel factor analysis, and fluorescence parameters to decipher the effects of N addition on DOM composition, origin, and fate. Irrespective of N addition level, increased N availability reduced dissolved organic carbon concentrations in surface water and soil pore water. However, DOM characteristics are more responsive to N addition in soil pore water than in surface water. Nitrogen addition did not affect DOM composition in surface water, but altered the proportions of tyrosine-, tryptophan-, fulvic acid-, and humic-like DOM in soil pore water. Moreover, the changing trends of DOM composition were diverse during the growing season. In addition, compositional transformations of DOM did not change its biogenic signals, freshness, and humification in both surface water and soil pore water across the growing season. Our results suggest that increased N availability reduces DOM quantity and alters DOM chemical composition, which will improve our understanding of soil organic matter dynamics in boreal peatlands. [ABSTRACT FROM AUTHOR]