RESEARCH

Why do we track carbon from mountains to the ocean?

Human-induced perturbations to the global carbon cycle constitute one of the most pressing threats to the sustainability of the human-dominated “anthropogenic biosphere” (sensu Ellis, 2015). Public concerns have been focused on rising atmospheric CO2 concentrations and their adverse environmental impacts, initiating a lot of research efforts to identify two major fates of anthropogenic carbon - land and ocean sinks. However, recent studies have suggested that a substantial portion of the anthropogenic carbon stored in terrestrial (land) ecosystems can be released to inland waters. Aquatic carbon, including inorganic carbon, dissolved organic carbon (DOC), and particulate organic carbon (POC), can be transferred back to the atmosphere (increasing atmospheric CO2 and hence accelerating the ongoing climate change), deposited to the bottom sediment (“sequestered” from the active carbon pools), or transported to the ocean (Figure 1; Cole et al., 2007; Battin et al. 2009; Regnier et al., 2013).

A key question remains unanswered - how much of the carbon derived from terrestrial sources is transformed in inland waters including streams, rivers, lakes, and reservoirs? Specifically, we don’t know yet how much carbon is actually released from terrestrial sources; and how much of the organic carbon of terrestrial origin is deposited to the bottom sediment or transformed by microbial respiration or photochemical oxidation during transit to the ocean. Therefore, constraining carbon budgets of inland water systems has global impacts on climate change and hence represents a frontier of carbon cycle research.

Despite decades of research, the fate of terrigenous (land-derived) organic carbon in the ocean remains an unresolved mystery. Terrigenous organic carbon is a complex mixture of highly degraded organic materials and might be resistant to further microbial processing. However, only a small fraction of DOC and POC derived from land (>0.4 Pg C yr-1; 1 Pg = 1 Giga ton) remains either dissolved in seawater or preserved in marine sediments (Hedges et al., 1997). On the other hand, recent syntheses have produced much greater estimates of CO2 and CH4 emitted from inland waters than previously thought, also suggesting that the actual amount of carbon released from terrestrial sources has been significantly underestimated (Figure 2; Bastviken et al., 2011; Raymond et al., 2013). Uncertainties behind the current understanding and estimates of the generation, processing, and export of the aquatic C of terrestrial origin have motivated our research over the past decade in two topic areas as described in the following sections.

Our key research questions:

(1) How do watershed biogeochemical processes respond to rainfall variability and extremes, which are predicted to increase with climate change?

(2) How do human-induced perturbations alter riverine C fluxes in increasingly urbanized watersheds around the globe?

How do watershed biogeochemical processes respond to rainfall variability and extremes?

I am the Poem of Earth, said the voice of the rain,

Eternal I rise impalpable out of the land and the bottomless sea,

Upward to heaven, whence, vaguely form'd, altogether changed, and yet the same,

I descend to lave the drouths, atomies, dust-layers of the globe,

- Walt Whitman, ‘The Voice of the Rain’

Blessings of rain cannot be better appreciated anywhere than in monsoon Asia, particularly when the new monsoon season terminates the preceding drought, opening a summer feast of tree and crop growth. The Asian monsoon climates affect more than half of the humanity and their failures and associated large-scale droughts have been linked to the rise and fall of dynasties and civilizations across the region (Zhang et al. 2008). The Asian summer monsoon is accompanied by heavy rainfalls and typhoons, which can wash away large amounts of soil and associated carbon from “dust-layers the globe” on steep terrain (Hilton et al. 2008). As rainfall variability and extremes are predicted to increase with climate change (Min et al., 2011), the summer monsoon period might become a more important biogeochemical “hot moment” in the future.

To better understand how biogeochemical processes in general and carbon export in particular respond to short-term hydrologic changes during storm events of various duration and intensity, we have been conducting multi-year biogeochemical monitoring in the Haean Basin located in the Lake Soyang Watershed since 2007, using an approach of multiple time-scale monitoring of stream water chemistry combining in-situ continuous instrumental measurements and hourly automatic sampling with the traditional routine monitoring at biweekly to monthly intervals. In line with an emerging view of “pulse” storm events delivering a “large and underappreciated percentage of the terrestrial DOM flux to drainage networks” (Raymond et al., 2015), our research has demonstrated that large pulses of DOC during storm events account for more than half of the annual DOC export from the mountainous forested headwater watersheds. In addition, we have shown that storm pulses of POC could exceed rather moderate increases in DOC export during intense storm events (Jung et al., 2012, Jeong et al., 2012; Jung et al., 2014). These storm pulses of POC have been linked to the co-transport of metals such as Pb (Jo and Park, 2011) and biodegradation and stabilization of particle-associated organic carbon during transport or deposition in floodplain and lake sediments (Jung et al., 2014, 2015). An ultimate goal of our research is to redress the view of mountainous streams and rivers as a “passive conduit” of carbon by examining DOC and POC biodegradation under more realistic conditions.

Major publications

• Jung B-J, Jeanneau L, Alewell C, Kim B, Park J-H. 2015. Downstream alteration of the composition and biodegradability of particulate organic carbon in a mountainous, mixed land-use watershed. Biogeochemistry 122:79-99.

• Jung B-J, Lee J-K, Kim H, Park J-H. 2014. Export, biodegradation, and disinfection byproduct formation of dissolved and particulate organic carbon in a forested headwater stream during extreme rainfall events. Biogeosciences 11: 6119-6129.

• Bartsch S, Pfeiffer S, Shope CL, Arnhold S, Jeong J-J, Park J-H, Eum J, Kim B, Fleckenstein JH. 2013. Monsoonal-type climate or land-use management: Understanding their role in the mobilization of nitrate and DOC in a mountainous catchment. Journal of Hydrology 507: 149-162.

• Jeong J-J, Bartsch S, Fleckenstein J, Matzner E, Tenhunen J, Lee SD, Park SK, Park J-H. Differential storm responses of dissolved and particulate organic carbon in a mountainous headwater stream, investigated by high-frequency in-situ optical measurements. Journal of Geophysical Research-Biogeosciences 117: G03013.

• Jung B-J, Lee H-J, Jeong J-J, Owen JS, Kim B, Meusburger K, Alewell C, Gebauer G, Shope C, Park J-H. 2012. Storm pulses and varying sources of hydrologic carbon export from a mountainous watershed. Journal of Hydrology 440-441: 90-101.

• Park JH, Inam E, Abdullah MH, Agustiyani D, Duan L, Hoang TT, Kim K-W, Kim SD, Nguyen MH, Pekthong T, Sao V, Sarjiya A, Savathvong S, Sthiannopkao S, Syers JK, Wirojanagud W. 2011. Implications of rainfall variability for seasonality and climate-induced risks concerning surface water quality in East Asia. Journal of Hydrology 400: 323-332.

• Jo K-W, Park J-H. 2010. Rapid release and changing sources of Pb in a mountainous watershed during extreme rainfall events. Environmental Science & Technology 44: 9324-9329.

• Park J-H, Duan L, Kim B., Mitchell MJ, Shibata H. 2010. Potential effects of climate change and variability on watershed biogeochemical processes and water quality in Northeast Asia. Environment International 36: 212-225.

How do human-induced perturbations alter riverine C fluxes in urbanized watersheds?

The importance of inland waters in the global C cycle and anthropogenic perturbations of the C dynamics in various inland water components have been examined extensively over the recent decades. However, it remains very challenging to quantify anthropogenic modifications of the pools and fluxes across the aquatic continuum from terrestrial sources through inland waters to the oceans (Regnier et al., 2013; Figure 3). In a recent synthesis of anthropogenic perturbations to C fluxes across the aquatic continuum, Regnier et al. (2013) estimated the anthropogenic perturbation flux at 1.0 Pg C yr-1 out of the total terrestrial export to aquatic systems of 2.5 Pg C yr-1, including erosion of soil C (0.8 Pg C yr-1), enhanced weathering (0.1 Pg C yr-1), and sewage (0.1 Pg C yr-1).

The quantity and cascading effects of C contained in urban sewage and runoff have been constrained more poorly compared to the anthropogenic perturbations associated with erosion and weathering (Butman et al., 2015). The composition and lability of organic C can significantly change along streams and rivers draining urbanized watersheds (Kaushal et al., 2014). However, chemical characteristics and lability of urban runoff and wastewater treatment plant (WWTF) effluents have rarely been examined in the context of organic C mineralization and CO2 evasion in large rivers and estuaries (Griffith and Raymond, 2011). Information gap is particularly daunting when we address organic C transformations and CO2 outgassing in streams and rivers draining rapidly urbanizing watersheds in developing countries across Asia. Rivers in many parts of Asia are highly polluted with domestic waste, agricultural runoff, and industrial waste, with the total volume of wastewater generated annually, excluding agricultural drainage, estimated at about 142 km3 (Evans et al., 2012). Rapid biological transformation of organic C in urban inland waters and resulting enhancement of CO2 outgassing represents not only an emerging research topic for environmental scientists but also a pressing challenge for government policies on water management and climate change mitigation.

We are now investigating riverine C fluxes in urbanized watersheds in the Han River, Korea, Mekong-Tonle Sap, Cambodia, and Ganges-Brahmaputra, Bangladesh. Our primary goal is to obtain a better understanding of how human-induced perturbations, water pollution and damming of waterways in particular, modify the quantity and composition of DOC and POC, altering the emission of major greenhouse gases - CO2, CH4, and N2O - from affected aquatic systems.

References

• Butman DE, Wilson HF, Barnes RT, Xenopoulos MA, Raymond PA. 2015. Increased mobilization of aged carbon to rivers by human disturbance. Nature Geoscience 8: 112-116.

• Evans AEV, Hanjra MA, Jiang Y, Qadir M, Drechsel P. 2012. Water quality: Assessment of the current situation in Asia. International Journal of Water Resources Development 28: 195-216.

• Griffith DR, Raymond PA. 2011. Multiple-source heterotrophy fueled by aged organic carbon in an urbanized estuary. Marine Chemistry 124: 14-22.

• Kaushal SS, McDowell WH, Wollheim WM. 2014. Tracking evolution of urban biogeochemical cycles: past, present, and future. Biogeochemistry 121: 1-21.

• Regnier P, Friedlingstein P, Ciais P, et al. 2013. Anthropogenic perturbation of the carbon fluxes from land to ocean. Nature Geoscience 6: 597-607