DOC export is exceeded by C fixation in May Creek: A late-successional watershed of the Copper River Basin, Alaska

Abstract

Understanding the entirety of basin-scale C cycling (DOC fluxes and CO2 exchanges) are central to a holistic perspective of boreal forest biogeochemistry today. Shifts in the timing and magnitude of dissolved organic carbon (DOC) delivery in streams and eventually into oceans can be expected, while simultaneously CO2 emission may exceed CO2 fixation, leading to forests becoming stronger CO2 sources than sinks amplifying rising trace gases in the atmosphere. At May Creek, a representative late-successional boreal forest watershed at the headwaters of the Copper River Basin, Alaska, we quantified the seasonality of DOC flux and landscape-scale CO2 exchange (eddy covariance) over two seasonal cycles. We deployed in situ fDOM and conductivity sensors, performed campaign sampling for water quality (DOC and water isotopes), and used fluorescence spectroscopy to ascertain DOC character. Simultaneously, we quantified net CO2 exchange using a 100 ft eddy covariance tower. Results indicate DOC exports were pulse-driven and mediated by precipitation events. Both frequency and magnitude of pulse-driven DOC events diminished as the seasonal thaw depth deepened, with inputs from terrestrial sources becoming major contributors to the DOC pool with decreasing snowmelt contribution to the hydrograph. A three-component parallel factorial analysis (PARAFAC) model indicated DOC liberated in late-season may be bioavailable (tyrosine-like). Combining Net Ecosystem Exchange (NEE) measurements indicate that the May Creek watershed fixes 142–220 g C m-2 yr-1 and only 0.40–0.57 g C m-2 yr-1 is leached out as DOC. Thus, the May Creek watershed and similar mature spruce forest dominated watersheds in the Copper River Basin are currently large ecosystem C sinks and exceeding C conservative. An understanding of DOC fluxes from Gulf of Alaska watersheds is important for characterizing future climate change-induced seasonal shifts.

Stormflows Drive Stream Carbon Concentration, Speciation, and Dissolved Organic Matter Composition in Coastal Temperate Rainforest Watersheds

“…This recent event water likely entered the tributary streams through shallow subsurface flow paths and/or via saturation‐excess overland flow (mainly in wetland sites) given that previous research from the PCTR (D'Amore et al, 2015; Emili & Price, 2006) and other forested regions (Boyer et al, 1997; Hinton et al, 1997; Peralta‐Tapia et al, 2015; van Meerveld et al, 2015) shows that streamflow is generated rapidly when soil water table levels are elevated into hydrologically conductive soil surface horizons. These explicit linkages between water and terrestrial C sources provide one of the few examples in northern forested watersheds where tight catchment‐scale coupling between the water and C cycles has been observed (Tomco et al, 2019).…”

“…Riverine transport of carbon (C), as dissolved inorganic (DIC), dissolved organic C (DOC), and particulate organic C (POC), is increasingly recognized as contributing to the global C cycle (Battin et al, 2009; Cole et al, 2007; Drake et al, 2017; Tranvik et al, 2009). As a result, the quantification of C movement across the land‐water interface, which is estimated at >5.0 Pg yr −1 globally (Butman et al, 2018; Drake et al, 2017), and how the speciation of this flux varies across ecosystem types are essential topics of study within the aquatic and Earth sciences (Csank et al, 2019; Tomco et al, 2019). Previous studies in small forested catchments show that rainfall‐runoff processes are key drivers of the lateral export of terrestrial C to surface waters (Boyer et al, 1997; Hinton et al, 1997; Raymond & Saiers, 2010; Vaughan et al, 2019), with the bulk of annual catchment DOC export typically occurring during high flow events (Raymond et al, 2016; Wiegner et al, 2009; Wilson et al, 2013).…”

“…For example, an increase in DOC concentration during stormflows suggests that extensive soil source pools of DOC exist, but a lack of watershed hydrologic connectivity limits the transport of this DOC across the terrestrial‐aquatic interface. Thus, combining C concentration and speciation data with catchment hydrologic tracers that provide information about streamflow generation, such as stable isotopes of water (δ 18 O and δ 2 H; Benettin et al, 2015; Kirchner, 2016; Stockinger et al, 2016; Tomco et al, 2019), allows for a more nuanced understanding of how different forms of C are transported laterally to surface waters.…”

Subarctic catchment water storage and carbon cycling – Leading the way for future studies using integrated datasets at Pallas, Finland

“…Lateral carbon fluxes, which occur in the form of dissolved organic (DOC), dissolved inorganic carbon (DIC) and particulate C, act as a key link between terrestrial and aquatic ecosystems (Csank et al, 2019;Giesler et al, 2014;Zimmer & McGlynn, 2018). To date, the few studies combining high-frequency atmospheric and aquatic flux measurements at the individual forest or peatland sites indicate that the proportion of aqueous flux in the net C balance may be considerable (Nilsson et al, 2008;Öquist et al, 2014;Worrall et al, 2009), but in other systems, it comprises only a small fraction of catchment C fluxes (Tomco et al, 2019). Furthermore, recent studies highlight the role of headwaters as hotspots of CO 2 losses from streams and show that groundwater contributes disproportionately to CO 2 in headwater streams (Duvert et al, 2018;Rocher-Ros et al, 2019).…”

Fluorescence Characteristics of Coalbed Methane Produced Water and Its Influence on Freshwater Bacteria in the South Qinshui Basin, China

“…A peak of C1 is located in the range of long wave ultraviolet (UVA), belonging to UVA fulvic acid, and is close to the “M” peak [ 45 ], which is similar to the marine humus component [ 44 ]. C2 is a terrestrial humic acid component, and its peaks are close to “A” peak and “C” peak, respectively [ 46 , 47 , 48 ]. It is worth noting that in the three-dimensional fluorescence spectrum, the region where the peak is located may also have fluorescence peaks from artificial additives such as detergent and brightener [ 49 , 50 ], which should be distinguished from terrestrial humus.…”