Abstract: Dissolved organic matter (DOM) represents the most labile fractions of organic matter and drives ecosystem carbon turnover in northern peatlands. However, the effects of increased nitrogen (N) availability on DOM dynamics and the underlying mechanisms remain unclear in northern peatlands. We conducted a 14-year N addition experiment (0, 3, 6, and 12 g N·m^-2·a^-1) in a peatland located in the continuous permafrost region of the northern Greater Khingan Mountains. We examined the effects of N addition on DOM chemical composition and biodegradability in surface water and soil pore water (0-15 cm) using the excitation-emission matrix (EEM) fluorescence spectroscopy and laboratory incubation experiments. We found that dissolved organic carbon (DOC) and dissolved total nitrogen (DTN) concentrations in surface water initially increased and then decreased with the increases of N addition levels. Compared with control, N addition at 6 g N·m^-2·a^-1 had 44.8% and 944.5% higher DOC and DTN concentrations, respectively. In contrast, dissolved total phosphorus (DTP) decreased from 0.20 mg·L^-1 to 0.08 mg·L^-1 with increasing N addition levels. These changes resulted in the increases in the DOC:DTP and DTN:DTP ratios, showing the shift in the nutrient limitation status from N to P. Fluorescence analysis revealed that N addition increased the proportion of relatively labile components such as protein-like components, but reduced the proportion of humic-like components. A 42-day incubation experiment showed that DOM biodegradability initially increased and then decreased with increasing N addition levels. In surface water, N addition at 6 g N·m^-2·a^-1 had the highest DOM biodegradability (30.3%), while in soil pore water, N addition at 3 g N·m^-2·a^-1 had the greatest DOM biodegradability (28.8%). Furthermore, the stepwise regression analyses showed that DOM biodegradability correlated significantly with C quality and nutrient availability in surface water, but correlated significantly with the intrinsic aromaticity in soil porewater. Overall, our findings demonstrated that N enrichment could alter DOM chemical composition and biodegradability in northern peatlands, with implications for understanding C cycling in these ecosystems.

Key words: northern peatland, ecological stoichiometry, DOM chemical structure, microbial degradation, nutrient limitation