Appearance of Recalcitrant Dissolved Black Carbon and Dissolved Organic Sulfur in River Waters Following Wildfire Events

Increasing wildfire frequency, a consequence of global climate change, releases incomplete combustion byproducts such as aquatic pyrogenic dissolved organic matter (DOM) and black carbon (DBC) into waters, posing a threat to water security. In August 2022, a series of severe wildfires occurred in Chongqing, China. Samples from seven locations along the Yangtze and Jialing Rivers revealed DBC, quantified by the benzene poly(carboxylic acid) (BPCA) method, comprising 9.5–19.2% of dissolved organic carbon (DOC). High concentrations of BPCA-DBC with significant polycondensation were detected near wildfire areas, likely due to atmospheric deposition driven by wind. Furthermore, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) revealed that wildfires were associated with an increase in condensed aromatics, proteins, and unsaturated hydrocarbons, along with a decrease in lignins. The condensed aromatics primarily consisted of dissolved black nitrogen (DBN), contributing to abundant high-nitrogen-containing compounds in locations highly affected by wildfires. Meanwhile, wildfires potentially induced the input of recalcitrant sulfur-containing protein-like compounds, characterized by high oxidation, aliphatic nature, saturation, and low aromaticity. Overall, this study revealed the appearance of recalcitrant DBC and dissolved organic sulfur in river waters following wildfire events, offering novel insights into the potential impacts of wildfires on water quality and environmental biogeochemistry.


Text S1. Analysis of FT-ICR-MS data
Double bond equivalents (DBE) must be a whole number.The calculation of DBE is as follows: 1, 2 DBE = 1 + C − 0.5H + 0.5N (1) The normal oxidation state of carbon (NOSC) was used to reflect the redox potential of a given formula.The positive values mean the compounds at the oxidized state, the negative values mean that at the reduced state, and zero means that it is in this neutral state. 3The nominal oxidation state of carbon (NOSC) was calculated based on the equation: 4 Based on the assigned formulae of each DOM molecule, the intensity-weighted average (wa) parameters such as double-bond equivalence (DBEwa), molecular weight (MWwa), H/Cwa, O/Cwa were calculated based on the following equations: [2][3][4] DBEwa=∑(DBEn × Mn) (3) O/Cwa=∑(O/Cn × Mn) H/Cwa=∑(H/Cn × Mn) ( (AImod)wa=∑((AImod)n × Mn) NOSCwa=∑(NOSCn × Mn) (8) DBC-Cwa (%) = ∑(DBC-Cn × Mn)/ ∑(DOM-Cn × Mn) (9) Where wa means an intensity-averaged calculation; n means list number of each assigned molecular formula; M is the relative intensity of each formula.

Figure
Figure S1.Chongqing's map and location on the map of China.

Figure S2 .
Figure S2.Pictures of several sampling locations.

Figure S3 .
Figure S3.The amount of precipitation (mm) in Chongqing before and after wildfire events.Data were obtained from the weather records in China (https://www.tianqi24.com/historycity/).

Figure S4 .
Figure S4.Daily average atmospheric OC and BC concentrations in Chongqing before and after wildfire events.Data obtained from the Tracking Air Pollution in China dataset (TAP, http://tapdata.org.cn/).

Figure S5 .
Figure S5.The correlation analysis of tested water quality parameters and DOC

Figure S9 .
Figure S9.The correction between DOC concentration of DOM and intensity-averaged abundance of tannins, carbohydrates and lipids.

Table S1 .
Information about sampling locations.

Table S3 .
Calibration curves of BPCAs target compounds.

Table S4 .
Molecular parameters of DOM with different molecular compositions.

Table S5 .
The correlations between DOC and molecular parameters of DOM with different molecular compositions (red color indicates a positive correlation, and green color indicates a negative correlation)

Table S6 .
Molecular parameters of DOM with different elemental compositions.