Biolability of Fresh and Photodegraded Pyrogenic Dissolved Organic Matter from Laboratory-Prepared Chars

Abstract

Pyrogenic dissolved organic matter (pyDOM) is known to be an important biogeochemical constituent of aquatic ecosystems and the carbon cycle. While recent studies have examined how pyDOM production, composition, and photolability varies with parent pyrogenic solid material type, we lack an understanding of potential microbial mineralization and transformation of pyDOM in the biogeosphere. Thus, leachates of oak, charred at 400 °C and 650 °C, as well as their photodegraded counterparts were incubated with a soil-extracted microbial consortium over 96 days. During the incubation, significantly more carbon was biomineralized from the lower versus higher temperature char leachate (45% vs. 37% lost, respectively). Further, the photodegraded leachates were biomineralized to significantly greater extents than their fresh non-photodegraded counterparts. Kinetic modeling identified the mineralizable pyDOC fractions to have half-lives of 9-13 days. Proton nuclear magnetic resonance spectroscopy indicated that the majority of this loss could be attributed to low molecular weight constituents of pyDOM (i.e., simple alcohols and acids). Further, the quantification of benzenepolycarboxylic acid (BPCA) molecular markers indicated that condensed aromatic compounds in pyDOM were biomineralized to much lesser extents (4.4% and 10.1% decrease in yields of ΣBPCA-C over 66 days from 400 °C and 650 °C oak pyDOM, respectively), but most of this loss could be attributed to the biomineralization of smaller condensed clusters (four aromatic rings or less). These results highlight the contrasting bioavailability of different portions of pyDOM, and the need to examine both to evaluate its role in soil or aquatic heterotrophy and its environmental fate in the hydrogeosphere.Plain Language Summary Given the recent changes in wildfire frequency and intensity in many areas of the globe, it is important to understand the fate of charred biomass in soils. As char is degraded in soils, it dissolves into porewaters, from where it then moves through the soil into rivers and ultimately to the ocean. The current study strives to understand how microbial decomposition destroys or alters fire-derived dissolved organic carbon. To accomplish this, two chars were leached in water and were incubated with soil microbes. This study found that about half of the carbon in these leachates could be readily decomposed, that is, converted back to carbon dioxide (with some variation with char leachate type). However, the other half of the leachate was resistant to microbial utilization. As such, one could expect that this remaining portion might be transported by rivers to the ocean, potentially influencing aquatic ecology and global carbon cycling.

Wildfire-Derived Pyrogenic Carbon Modulates Riverine Organic Matter and Biofilm Enzyme Activities in an In Situ Flume Experiment

“…The increases in DOC concentration in treatment flumes can be explained by leaching of pyrogenic organic matter from wildfire charcoals, as also observed in previous laboratory studies. 18 , 48 , 49 …”

“… 16 In-stream microbiota, especially biofilm communities that are hot spots of microbial functioning, are central to the role of fluvial ecosystems as bioreactors of terrestrial material, 17 but the microbial degradation of pyrogenic DOM remains poorly understood. Bostick and colleagues 18 recently measured considerable degradation of the labile DOC fraction leached from fresh PyC in laboratory-based experiments; however, the metabolization of fresh as well as aged PyC under natural conditions remains elusive. Finally, mechanical processes can physically disintegrate PyC into smaller particles.…”

Supplementary material to "Labilization and diversification of pyrogenic dissolved organic matter by microbes"

“…S4. Data used for the correlation analysis between molecular diversity (as determined by FT-ICR-MS) and 1D NMR (Bostick et al, 2020). Coefficients of determination (R 2 values) are listed for each functional group in the corresponding color.…”

Questions remain about the biolability of dissolved black carbon along the combustion continuum

“…Here, the term biolability refers to carbon fractions with rapid microbial turnover rates (within days to weeks). At high charring temperatures (>450°C), organic matter can undergo aromatization to form polycondensed and graphitic type structures that resist biodegradation 3,4 . Black carbon (BC) refers to the condensed aromatic fraction of thermally altered biomass that accumulates in carbon reservoirs over thousands of years.…”

“…Data in Qi et al (2020) suggest CTO captures more than just graphitic DBC, therefore we have expanded the range for this method and included a question mark to denote that further research is needed to understand the breadth of this DBC fraction. degradation 4 . Therefore, the authors' key finding that biodegradation is the likely mechanism for losses of CTO-derived DBC during transit is unfounded.…”