Reply to: “Questions remain about the biolability of dissolved black carbon along the combustion continuum”

Abstract

B lack carbon (BC) is broadly defined as the product of incomplete biomass and fossil fuel combustion, including biochar and soot1. While this definition is intuitively clear, the exact chemical composition and structure of this ubiquitous material are difficult to differentiate. With many proposed pathways of BC formation, it is now well accepted that BC is carbon-rich, structurally condensed, and contains abundant aromatic compounds such as polycyclic aromatic hydrocarbons (PAHs). The terminology of BC has also been confused with pyrogenic carbon (PyC) and elemental carbon (EC). We feel that PyC is probably the best description of this material that eliminates the confusion based on its chemical composition that has never been precisely determined. Dissolved black carbon (DBC) is the fraction of BC dissolved in natural aquatic systems and has been studied extensively in recent years due to its potential importance in global carbon cycling. However, isolation of DBC in natural waters and determination of its chemical composition is more challenging2. The literature to date has been exclusively based on the methods used for the isolation and quantification of DBC which has led to inevitable uncertainty and controversy2,3. Using the solid-phase extraction (SPE) and chemothermal oxidation (CTO) method3, we investigated DBC in four large and two small mountainous rivers in China; the Yangtze River and Yellow River estuaries; the East China Sea (ECS); and the North Pacific Ocean basin (NP). We measured carbon isotopic (13C and 14C) values of DBC and found that the carbon isotopic signatures of DBC are relatively homogeneous. Further, DBC 14C in rivers is predominantly young derived mainly from biochar, aging during continuum transport, and cycling in the ocean4. Combined with results from biochar leaching and degradation experiments, we found that DBC is dissolved from biochar and degraded by bacteria in river water. This suggests that a fraction of riverine DBC could be labile and respired during transport and mixing into the ocean as well as that residual DBC is likely cycled and aged on the same time scales as bulk dissolved organic carbon (DOC) in the ocean4. After the publication of our study, a Matters arising (MA) article5 raised concerns regarding the bioliability of DBC as we proposed. We respect these concerns and the following is our response. We feel that the main controversy lies in the uncertainty of the methodology and the one-sidedness of the definition of DBC. First and foremost, we believe that the experiments we conducted were robust and support the conclusion that the removal of the dissolved compounds leached from biochar was due to microbial biodegradation as supported by the bacterial abundance measurements. During the long-term incubation (1700 days), we compared the biochar leachate in bacteria-inhibited river water to the biodegradation of biochar leachate in bacteria-active river water4. The degradation is real. Obviously, we could not create duplicates for each treatment but the data from the experiments was not a single-point measurement. It was a long-term experiment with continuous measurements and consistent results. This kind of experiment has been used to examine biochar solubility6. Because the well-burned locust wood biochar, by definition, was BC, we believe that compounds leached from the biochar were a fraction of BC but we did not determine whether they were aromatic compounds or not. Previous studies have well demonstrated that both low-molecular-weight (LMW) compounds and aromatic compound (PAHs) were leached from biochar in natural environment7–9 and the biodegradation of PAHs was identified a long time ago10. The question is should we define the uncharacterized biochar leachate in our experiments as DBC or something else. A second controversy between our studies is related to the different methods we used to measure BC (or PyC). The BC https://doi.org/10.1038/s41467-021-24478-x OPEN