Storm pulses and varying sources of hydrologic carbon export from a mountainous watershed
Highlights
► Storm pulses of C export represent soil C losses from mountainous watersheds. ► Storm pulses of POC can be a dominant pathway of C export overwhelming DOC export. ► POC sources can rapidly change with varying rainfall intensity during storms. ► Coupled soil and sediment stable isotope measurements are useful in tracing C export.
Introduction
Soil erosion is one of the oldest environmental problems in human history and has expansive on- and off-site impacts worldwide (Pimentel et al., 1995). While most soil erosion occurs on agricultural land, steep mountainous terrain often also displays very high erosion rates (Montgomery, 2007, Cerdà et al., 2009). Erosion in steep terrain can occur as a natural process in high mountain ranges such as the Himalayas (Galy et al., 2007). However, surface erosion and landslides in steep mountain areas in East Asia have often been linked to human influences including unsustainable forest management practices and rapid land use change, with off-site environmental impacts such as river siltation (Sidle et al., 2006, Zhang et al., 2009, Park et al., 2011). The loss of soil from mountain forests represents an unaccounted C loss in the usual C budgeting. Recent reports from East Asia have shown that soil erosion during extreme events can result in very rapid, but substantial soil C losses (Goldsmith et al., 2008, Hilton et al., 2008, Kim et al., 2010). Despite the importance of storm-induced soil C losses, little is known about short-term dynamics and the relative significance of DOC and POC exported from mountainous watersheds during intense rainfall events.
POC in eroded soil particles, along with DOC in soil leachate, plays a crucial role in the global C cycle, including the delivery of terrestrial organic C to the oceans (Battin et al., 2008), sediment trapping in reservoirs and lakes (Syvitski et al., 2005), and C stabilization in riverine floodplain soils (Zehetner et al., 2009). According to recent syntheses of regional or global C budgets, soil erosion, along with leaching, transfers a substantial portion of the annual soil C increment to aquatic systems, leading to a loss of terrestrial C and either enhanced CO2 outgassing or burial in sedimentary deposits in aquatic systems (Battin et al., 2009, Aufdenkampe et al., 2011). Many previous studies have considered POC as a minor component of aquatic organic C based on measurements in large rivers, which showed that DOC/TOC ratios were between 0.6 and 0.8 in lowland rivers and lower than 0.5 for highland river systems (Meybeck, 1982). However, unusually high concentrations and export of POC exceeding those of DOC have been observed in streams draining upland forested watersheds and small mountainous rivers during storm events (Fisher and Likens, 1973, Wallace et al., 1991, Dawson et al., 2002, Coynel et al., 2005, Kim et al., 2010).
Because sediment sources constrain the biological and physicochemical characteristics of SS transported by stream and river systems, detailed information on sediment sources and transport pathways is crucial in understanding the fate of C, other nutrients, and contaminants in eroded soil particles (Walling, 2005). Many approaches of tracing or ‘fingerprinting’ sediment sources have been developed since the 1970s to differentiate geochemical properties between sediments and potential source materials, including sediment mineralogy (Wall and Wilding, 1976), fallout radionuclides (Zapata, 2003), stable isotopes (Papanicolaou et al., 2003), and a combination of several tracers (Mukundan et al., 2010). Among geochemical tracers, fallout radionuclides (e.g., 137Cs, 210Pb, and 7Be) have been used to trace sources of sediments in a wide range of environments including mountainous areas (Rogowski and Tamura, 1965, Walling and He, 1999, Alewell et al., 2009). The use of fallout radionuclides to investigate watershed erosion processes has some limitations resulting from either the non-uniform distribution of artificial radionuclides (e.g., 137Cs) and/or their short half-life (e.g., 7Be and 210Pb; Mabit et al., 2008). It is also difficult to use radionuclides to comparatively identify erosional processes in watersheds with diverse land surface features, such as hillslope vs. floodplain erosion (Fox and Papanicolaou, 2007).
Stable C and N isotopes have been suggested to overcome some of the limitations of fallout radionuclides (Papanicolaou et al., 2003, Fox and Papnicolaou, 2007). 13C natural abundance measurements, usually in combination with other stable (e.g., 15N) or radioactive (14C) tracers, have been used to trace sources and transport of organic matter in soils and aquatic systems (Ehleringer et al., 2000, Raymond and Bauer, 2001, Sanderman et al., 2009). Recently, the potential of C and N stable isotopes in tracking soil erosion at the watershed level has been recognized (Papanicolaou et al., 2003, Fox and Papnicolaou, 2007, Alewell et al., 2008, Alewell et al., 2009, Mukundan et al., 2010). δ13C and δ15N have been used as biogeochemical tracers of watershed-level soil erosion processes (Fox and Papanicolaou, 2007) or to identify sources of organic matter in river systems (Jennerjahn et al., 2008), assuming that differences in sediment δ13C and δ15N can be linked to differences in vegetative cover, soil characteristics, and other land use patterns. Previous studies have usually focused on site-specific differences in sediment isotope composition (Jennerjahn et al., 2008) or diagenesis along the hydrologic transport pathways in large river systems such as the Amazon (Aufdenkampe et al., 2007). Changes in the isotopic composition of DOC during storm events have recently been linked with storm-induced shift in DOC sources (Sanderman et al., 2009, Lambert et al., 2011). However, few studies have utilized δ13C and δ15N to study short-term changes in sources and characteristics of POC in eroded SS during storm events.
The objective of this study was to investigate short-term dynamics of hydrologic C export associated with soil erosion and leaching during storm events, using intensive storm streamwater sampling combined with C and N stable isotope measurements. Six intensive storm samplings, complemented with routine biweekly stream sampling, were conducted during two summer monsoon periods in a mountainous watershed in northern South Korea. Short-term changes in DOC and POC concentrations and SS δ13C and δ15N during storm events in a headwater forest stream and a downstream watershed outlet receiving agricultural runoff were compared to provide information on sources and hydrologic transport of soil-derived organic C.
Section snippets
Site description
The study site (Haean Basin) is a bowl-shaped mountainous watershed in northern South Korea (38°15′–38°20′N; 128°05′–128°10′E; 400–1304 m asl), 1–2 km south of the demilitarized zone (DMZ) between South and North Korea (Fig. 1) (Jo and Park, 2010). The bedrock in Haean Basin consists of highly weathered biotite granite at the basin bottom, surrounded by metamorphic rocks forming mountain ridges (Kwon et al., 1990). Mixed deciduous forests along the mountain ridges and steep slopes comprise 58% of
Seasonal variations and storm pulses of SS, POC, and DOC
Concentrations of TSS, POC, and DOC in the forest stream showed relatively small variations throughout the year with the exception of generally higher values during the summer monsoon period from late June through August (Fig. 2). Higher concentrations of TSS and POC in the forest stream during the monsoon period might be ascribed to frequent rainfalls, as indicated by the significant correlations between the solute concentrations and hydroclimatic variables including 3-d antecedent
Significance of hydrologic soil C loss
The rapid and drastic changes in streamwater TSS and POC concentrations during storm events (Fig. 3a and b) compared to small temporal variations in solute concentrations observed by the routine sampling (Fig. 2) suggest that the routine low-frequency stream monitoring at weekly to monthly time scales cannot fully capture storm pulses of SS and associated POC that typically last for a few hours during a storm event. If routine stream sampling at weekly to monthly time scales cannot adequately
Conclusions
The results of this study highlight that coupled SS and soil δ13C and δ15N measurements, in concert with intensive streamwater sampling, can be very useful in investigating short-term dynamics of soil erosion and C export in mixed-land use watersheds. In particular, measuring δ13C and δ15N for size-fractionated SS provided information on size fraction-specific temporal variations in potential sources and characteristics of eroded soil C over the course of storm events. On the basis of these
Acknowledgements
This research was supported by the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (Basic Science Research Program 2010-0015205; ERC 2009-0093460). The international collaboration between Kangwon National University and University of Bayreuth was supported through the TERRECO project funded by the National Research Foundation of Korea and DFG (German Research Foundation; IRTG 1565).
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