Soils were collected from the frozen permafrost layer (> 90 cm below the surface) at four sites underlying tussock or wet sedge vegetation, and on three glacial surfaces on the North Slope of Alaska during summer 2015.  Sampling and preparation of the permafrost leachates from soils collected in 2015 are described in Ward et al. (2017).

Duplicate permafrost leachates were prepared from permafrost soils collected at each site, except for Toolik moist acidic tundra, which did not have an experimental replicate.  Each permafrost leachate was exposed to 18 hours of simulated sunlight at 20 °C (Atlas Suntest XLS+) in triplicate gas-tight, precombusted 12 mL borosilicate exetainer vials (450 °C; 4 hrs; Labco, Inc.) alongside dark controls.  Light-exposed and dark control waters were then analyzed for dissolved inorganic carbon (DIC) using a DIC analyzer (Apollo SciTech, Inc.) and for chromophoric  dissolved organic matter (CDOM) as described previously (Cory et al., 2014).  φPM,λ was calculated as the light minus dark difference in DIC concentration divided by the light absorbed by CDOM during the light exposure period, assuming the φPM,λ spectrum decreased exponentially with increasing wavelength (Cory et al., 2014).  The amount of light absorbed by CDOM (mol photon m-2 nm-1) was quantified for each vial using absorption coefficients of CDOM (aCDOM,λ) and the photon flux spectrum (Cory et al., 2014).  The photon flux spectrum was quantified from the solar irradiance spectrum at the surface of the Suntest XLS+, which was measured by radiometry.  The photon dose was calculated as the sum of the total photon flux spectrum across all wavelengths.  The amount of light absorbed by CDOM and φPM,λ for each permafrost leachate is reported as the average ± 1 standard error (SE) of the experimental replicate leachates (n = 2, except for Toolik moist acidic tundra, which did not have an experimental replicate).

References: 

Bowen, J. C., C. P. Ward, G. W. Kling, R. M. Cory..  Arctic amplification of global warming strengthened by sunlight oxidation of permafrost carbon to CO2.    In review.

Cory, R. M., C. P. Ward, B. C. Crump, G. W. Kling.  2014.  Sunlight controls water column processing of carbon in arctic fresh waters. Science, 10.1126/science.1253119

Ward, C. P., S. G. Nalven, B. C. Crump, G. W. Kling, G. W., R. M. Cory.  2017.  Photochemical alteration of organic carbon draining permafrost soils shifts microbial metabolic pathways and stimulates respiration. Nat. Commun., 10.1038/s41467-017-00759-2