Factors influencing the concentrations and biodegradation of dissolved organic carbon in the middle reaches of the Yellow River

Riverine aquatic systems play a key role in both regional and global carbon cycles. However, the extent to which nutrient regulation and changes in dissolved organic matter (DOM) chemistry control the proportion dissolved organic carbon (DOC) biodegraded by microorganisms (%BDOC) in rivers of the Loess Plateau is still not well quantified. Based on a 56-day indoor incubation experiment, %BDOC, temperature sensitivity (Q10), optical properties of DOM, and nutrients and their stoichiometric ratios were investigated in the middle reaches of the Yellow River. The results showed that BDOC (mean [SD]) at 35 °C (54% [21%]) was significantly higher than that at 25 °C (38% [22%]) with an average (mean [SD]) Q10 value of 1.7 (0.6). Consistent decreases in absorbance at 254 nm (a254) and 350 nm (a350), supported by a concomitant reduction in humic-like fluorescence, indicated decomposition of aromatic compounds and lignin fractions. By contrast, the increase in the absorbance ratios (E2:E3, A250/A365) and the decrease in the E4:46 ratios (A465/A665) showed that macromolecular DOM had not been completely degraded and enriched after 56 days of incubation. In addition, %BDOC was negatively correlated with DOC, TN:TP, and DOC:TP in the study, further suggesting that phosphorus availability was a major limiting factor for aquatic microbial respiration in rivers of the Loess Plateau. These findings advance the mechanistic understanding of DOC biodegradation, informing predictions of carbon dynamics and their implications for the global carbon cycle under future climate scenarios.