Experimental design for analyzing the carbon sink potential of dissolved organic matter in water under the background of carbon neutrality.

[Objective]The cultivation of talents with the theoretical knowledge and practical ability of carbon neutrality is urgent for mitigating climate change and achieving a goal of carbon neutrality. Protecting and increasing the carbon sink in ecosystems represents an important pathway to carbon neutrality. The dissolved organic matter (DOM) in aquatic environments contains a large carbon pool; thus, studying the biodegrad ability of bulk DOM and the transformation of different DOM components is crucial. This study designed a comprehensive experiment to analyze the carbon sink potential of aquatic DOM, combining research frontiers and experiment teaching. [Methods]This experiment included the sampling of DOM from typical water bodies and laboratory biodegradation incubation. The river water and wastewater were collected as typical samples, and the DOM was measured before and after the microbial degradation. The dissolved organic carbon (DOC) concentration was measured using a total organic carbon analyzer to quantify the biodegrad ability of bulk DOM. A UV--visible absorption spectrometer and a molecular fluorescence spectrometer were used to further measure the optical properties of DOM. The absorption coefficient and the spectral slope were calculated as indicators of the level of chromophoric DOM (CDOM) and the average molecular weight of DOM, respectively. Humic-like and protein-like peaks were identified from the fluorescence spectra, and the fluorescence intensity of each peak was used to indicate the level of the corresponding fluorescent component. The absorption and fluorescence indices of the initial and degraded samples were compared to decipher the degradation and transformation characteristics of CDOM and different types of fluorescent components. [Results]The bulk DOC decreased by 15.7% and 45.0% after the microbial degradation of river and wastewater DOM, respectively. This indicated the higher biodegrad ability of DOC from wastewater that contained more labile constituents. In contrast, the river DOC contained a higher percent of refractory constituents and thus had a higher carbon sink potential. The CDOM absorption coefficient decreased by 12.8% and 1.5% in the river water and wastewater after biodegradation, respectively. The weaker decrease in wastewater CDOM suggested the potential transformation from labile constituents to CDOM by microbial humification. The absorption spectral slope increased slightly by 4.9% after the biodegradation of the river sample due to the decreased molecular weight by the prevailing degradation process. In contrast, the spectral slope decreased by 19.8% for the wastewater sample, indicating an increase in the molecular weight through microbial humification. The fluorescence of river DOM was dominated by humic-like peaks, whereas that of wastewater showed strong signals for both humic-like and protein-like peaks. The protein-like peaks showed little changes in the river water while the humic-like peaks increased slightly, which indicated net accumulations of refractory components and was consistent with the theory of microbial carbon pump. In contrast, both protein-like and humic-like peaks decreased after the degradation of the wastewater sample, which proved a higher biodegradability and the potential priming effect due to the increased microbial activity with more labile substrates. [Conclusions]This experiment revealed different biodegrad abilities of bulk DOM and different DOM constituents (CDOM and different fluorescent components) from typical aquatic environments (i.e., river and wastewater). The experiment design can be applied to further explore the biodegrad ability of DOM from other water bodies. The results can be used for assessing the carbon sink potential of environmental DOM and for analyzing its underlying mechanisms. This experiment can help students understand the cutting-edge scientific theory of carbon neutrality, master the principle, operation, and data analysis of modern instrumental analysis techniques, and cultivate innovative thinking and practical ability, which will help the cultivation of talents for carbon neutrality. [ABSTRACT FROM AUTHOR]