Responses of Dissolved Organic Carbon to Nitrogen Application in Soils with Different Carbon Contents in the Black Soil Region of Northeast China

It was showed that DOC concentration increased significantly with SOC content, accompanied by a greater proportion of macromolecular humic-acid-like components. In the low-carbon soil (Changtu), N application significantly increased DOC concentration (by 13.3% at 320 kg N·hm−2 compared to the control), the specific ultraviolet absorbance (SUVA) by 162%, and the fluorescence index (FI) by 23%; protein-like components increased while short-wavelength humic-like components decreased, indicating that N addition promoted the formation of low-molecular-weight aromatic components mainly through enhanced microbial transformation.

In the high-carbon soil (Hailun), N application did not affect DOC concentration, but decreased the humification index (HIX) by 31.1%, with an increase in protein-like components and a decrease in humic-acid-like components, while FI remained unchanged, suggesting that abiotic dissolution processes dominated the changes in DOC composition. No significant changes were observed for any parameters in the medium-carbon soil (Heihe).

Two-way ANOVA further revealed that the interaction between SOC level and N addition was not significant for DOC concentration, but significant interaction effects (P < 0.05) were found for SUVA, HIX, FI, and fluorescence components protein-like components C1 and humic-like components C3, indicating that the response pattern of DOC molecular composition to N addition differed markedly among carbon levels. In summary, the response mechanisms of DOC to N addition exhibit clear carbon-gradient dependence.

In low-carbon soils, short-term N application may accelerate the turnover of the labile carbon pool, but its long-term ecological consequences require further validation. This study provides a basis for understanding soil carbon-nitrogen coupling mechanisms and active carbon pool transformations in the black soil region.