Identifying controls on the degradation of dissolved black carbon in sunlit surface waters.
EMSL Project ID
49482
Abstract
Black carbon is the residual, charred vegetation remaining after wildfires or biomass burning in terrestrial ecosystems. This large pool of carbon is more recalcitrant to microbial degradation in soils compared to the parent (uncharred) biomass. Thus, the fraction of biomass sequestered in the black carbon pool over long timescales is considered an important sink for atmospheric CO2. However, the capacity of black carbon to act as a C sink by slowing soil organic matter turnover on a global scale is poorly constrained, in large part because controls on black carbon degradation are poorly known. Soil erosion contributes to black carbon degradation by moving this C from dark soils to sunlit surface waters, where sunlight can abiotically break down dissolved black carbon (DBC) to CO2 (i.e., photo-mineralization). Photo-degradation of DBC to CO2 has been proposed as an important loss pathway for the light-absorbing condensed aromatics that comprise the black carbon pool. However, we showed that little CO2 is produced when DBC is exposed to sunlight over time periods relevant for DBC transit in rivers to coastal waters. Instead, we found that DBC leached from charred biomass in arctic soils is primarily partially broken down by sunlight to aliphatic compounds that no longer overlap in chemical composition with black carbon but remain in the dissolved organic matter pool. The photochemical conversion of DBC to aliphatic compounds no longer recognizable as black carbon, instead of mineralization to CO2, suggests that the conversion of DBC to CO2 in sunlit surface waters may be over-estimated. However, photo-degradation of DBC likely depends strongly on the chemical composition of black carbon, making it difficult to predict the conversion of DBC to CO2 vs. aliphatic compounds in sunlit surface waters. To address this knowledge gap, we will quantify DBC photo-degradation to CO2 and to aliphatic compounds as a function of vegetation type and charring temperature, which are well-established controls on the chemical composition of black carbon. For example, as charring temperature increases, the oxygen content (i.e., oxygen to carbon ratio; O/C) of black carbon decreases. Oxygen content has been shown to be an important control on photo-mineralization of dissolved organic matter to CO2, and our preliminary work suggests this may apply to DBC as well. We will test hypotheses on the effect of DBC chemical composition on its photo-degradation using black carbon already produced from two biomass sources and five charring temperatures. We expect a relationship between the chemical composition of DBC, characterized using high-resolution mass spectrometry (FT-ICR MS), and the lability of DBC to photochemical conversion to CO2 or to aliphatic compounds. Quantifying the relative importance of DBC photo-degradation to CO2 vs. aliphatics, and the controls on these degradation pathways, is critical for predicting DBC turnover time in sunlit surface waters. This new information is essential to accurately assess how black carbon quantitatively influences the global carbon cycle, particularly given the expected increase in wildfire activity by the end of this century.
Project Details
Project type
Limited Scope
Start Date
2016-05-03
End Date
2016-07-03
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members