Abstract
AbstractOnce thawed, up to 15% of the ∼1,000 Pg of organic carbon (C) in arctic permafrost soils may be oxidized to carbon dioxide (CO2) by 2,100, amplifying climate change. However, predictions of this amplification strength ignore the oxidation of permafrost C to CO2 in surface waters (photomineralization). We characterized the wavelength dependence of permafrost dissolved organic carbon (DOC) photomineralization and demonstrate that iron catalyzes photomineralization of old DOC (4,000–6,300 a BP) derived from soil lignin and tannin. Rates of CO2 production from photomineralization of permafrost DOC are twofold higher than for modern DOC. Given that model predictions of future net loss of ecosystem C from thawing permafrost do not include the loss of CO2 to the atmosphere from DOC photomineralization, current predictions of an average of 208 Pg C loss by 2,299 may be too low by ~14%.
Highlights
To test for controls on the φPM,λ and the chemical composition of dissolved organic carbon (DOC) photomineralized to CO2, each light exposure experiment was conducted using DOC leached from five permafrost soils varying in DOC composition and iron concentration (Table S1, Figure S1; Ping et al, 1998; Trusiak et al, 2019)
For objective (ii), the 14C and 13C compositions of dissolved inorganic carbon (DIC) in the permafrost leachates were measured upon exposure to light-emitting diodes (LED) at 309 and 406 nm alongside dark controls, and compared to the 14C and 13C compositions of DOC leached from the permafrost soil (Figure S1; see Methods)
The Sagwon moist acidic tundra permafrost leachate was exposed to the LEDs for 30 hours with photon doses ranging from 5 to 28 mol photon m-2, which resulted in an average of 4.04 ± 0.22 mol photon m-2 of light absorbed by CDOM at all wavelengths (± 1 SE; n = 5)
Summary
Light exposure experiments were conducted to (i) characterize the spectral (wavelength) dependence of the photomineralization yield (φPM,λ) for permafrost dissolved organic carbon (DOC), (ii) quantify the radiocarbon (14C) and stable carbon (13C) isotopic compositions of carbon dioxide (CO2) produced from photomineralization of permafrost DOC, and (iii) quantify changes in the chemical composition of permafrost DOC from light exposure. The φPM,λ (objective (i)) was measured upon exposure of permafrost DOC to LEDs with peak emission at 278, 309, 348, 369, and 406 nm (Figure S1; see Methods). For objective (ii), the 14C and 13C compositions of dissolved inorganic carbon (DIC) in the permafrost leachates were measured upon exposure to LEDs at 309 and 406 nm alongside dark controls, and compared to the 14C and 13C compositions of DOC leached from the permafrost soil (Figure S1; see Methods). For objective (iii), we used permafrost soils collected in 2013 and 2015 from the same sites as in 2018 to quantify shifts in major functional groups of permafrost DOC by 13C nuclear magnetic resonance (NMR) upon exposure to broadband light relative to dark controls (Figure S1). Measured φPM,λ spectra for permafrost DOC from objective (i) were used to calculate surface water rates of photomineralization as a function of increasing permafrost DOC in the DOC pool in surface waters
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