Article pubs.acs.org/est
Carbon Monoxide Photoproduction: Implications for Photoreactivity of Arctic Permafrost-Derived Soil Dissolved Organic Matter Jun Hong,†,‡ Huixiang Xie,*,‡ Laodong Guo,§ and Guisheng Song‡,∥ †
School of Environmental Studies, China University of Geosciences, Wuhan 430074, P. R. China Institut des Sciences de la Mer de Rimouski, Université du Québec à Rimouski, Rimouski, Quebec G5L 3A1, Canada § Shool of Freshwater Sciences, University of Wisconsin-Milwaukee, 600 E. Greenfield Avenue, Milwaukee, Wisconsin 53204, United States ‡
S Supporting Information *
ABSTRACT: Apparent quantum yields of carbon monoxide (CO) photoproduction (AQYCO) for permafrost-derived soil dissolved organic matter (SDOM) from the Yukon River Basin and Alaska coast were determined to examine the dependences of AQYCO on temperature, ionic strength, pH, and SDOM concentration. SDOM from different locations and soil depths all exhibited similar AQYCO spectra irrespective of soil age. AQYCO increased by 68% for a 20 °C warming, decreased by 25% from ionic strength 0 to 0.7 mol L−1, and dropped by 25−38% from pH 4 to 8. These effects combined together could reduce AQYCO by up to 72% when SDOM transits from terrestrial environemnts to open-ocean conditions during summer in the Arctic. A Michaelis−Menten kinetics characterized the influence of SDOM dilution on AQYCO with a very low substrate half-saturation concentration. Generalized global-scale relationships between AQYCO and salinity and absorbance demostrate that the CO-based photoreactivity of ancient permaforst SDOM is comparable to that of modern riverine DOM and that the effects of the physicochemical variables revealed here alone could account for the seaward decline of AQYCO observed in diverse estuarine and coastal water bodies.
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INTRODUCTION It is estimated that northern terrestrial ecosystems have accumulated up to 48% of the global soil organic carbon (SOC) with nearly 90% of it stored in permafrost.1,2 The transport and fate of this vast permafrost SOC, predominantly of old age, is one of the central concerns about the consequences of climate warming over the Arctic.3,4 Guo et al.5 demonstrated that the particulate organic carbon in the Yukon, Sagavanirkok, and Mackenzie Rivers is presently dominated by old SOC of permafrost origin. Although it lacks consistent evidence for increases in aged dissolved organic carbon (DOC) in present Arctic rivers, long-term warminginduced permafrost thaw will likely lead to larger amounts of old SOC being released into Arctic rivers and ultimately into the Arctic Ocean.4,6 The mobilization of this enormous SOC pool could potentially influence the carbon cycle regionally and globally.4−6 Dissolved organic matter (DOM) in natural waters is subjected to microbial and photochemical processing. Distributions of DOC and/or lignin across land−ocean transects imply removal of terrestrial DOC during its transport from large Arctic rivers to the Arctic Ocean.7,8 A few studies have revealed that DOC in Arctic rivers and streams can undergo substantial biodegradation (20−40%) during incubations lasting from a few weeks to several months.9−12 Furthermore, Holmes et al.10,11 have showed younger, surface biomass© 2014 American Chemical Society
derived DOC discharged during the spring freshet to be much more labile than pre- and postfreshet DOC which is older and supposedly contains a larger fraction of permafrost SOC. A more recent study,12 however, has elucidated that ancient permafrost DOC in Siberian thaw streams is also highly biodegradable. Photooxidation has long been recognized to profoundly impact the fate of terrestrial chromophoric DOM (CDOM) by labilizing it to metabolic attack and by directly oxidizing it to CO2 and carbon monoxide (CO).13 CDOM photooxidation in the Mackenzie shelf surface water has been estimated to mineralize up to 6% of the DOC discharged from the Mackenzie River.14−16 Similar extents of DOC photomineralization have been reported for the Kolyma River water in the Siberian Arctic.11 The photomineralization capacity increases if it takes into account the concordant photoproduction of biolabile organic compounds17 and the more efficient photooxidation occurring in offshore waters where the competition for solar radiation by particles declines and the photoactive zone deepens. Notably, DOM in Arctic river waters is presently dominated by contemporary sources.5,18 It remains Received: Revised: Accepted: Published: 9113
January 12, 2014 July 15, 2014 July 16, 2014 July 16, 2014 dx.doi.org/10.1021/es502057n | Environ. Sci. Technol. 2014, 48, 9113−9121
Environmental Science & Technology
Article
330 nm ranging from 0 to 6 m−1. The AC2 leachate was also utilized to examine the behavior of AQYCO when the leachate was mixed with varying fractions of 0.22-μm filtered seawater collected from the Canada Basin (sampling depth 950 m; coordinates 72.01° N, 131.33° W). The dilution and mixing experiments were conducted at 4 °C and employed fullspectrum radiation only. Parallel dark incubations were conducted for all sample irradiations to assess potential thermal CO production. Thermal production was mostly minor and subtracted as a blank (SI Section S2). Irradiation time varied from 5 to 100 min, depending on the sample’s initial absorbance, to ensure significant CO production but minimum absorbance loss. Analysis. Soil samples were pretreated with 1 N HCl to remove carbonate before being analyzed for total organic carbon (TOC) contents on a PerkinElmer 2400 elemental analyzer standardized with acetanilide.18 DOC samples were acidified to pH ∼2 with 2 N HCl to remove dissolved inorganic carbon and analyzed in triplicate using a Shimadzu TOC-Vcpn Total Carbon Analyzer calibrated with potassium biphthalate. The system was checked, at intervals of seven consecutive sample analyses, against standards and references including Hansell’s low-carbon (DOC 1−2 μmol L−1) and deep Sargasso Sea (DOC ∼44 μmol L−1) waters. The coefficient of variation of triplicate measurements was
