Potential for long-term transfer of DOC from riparian zones to streams in boreal catchments

Transcription

1 Potential for long-term transfer of DOC from riparian zones to streams in boreal catchments José L. J. Ledesma*, Thomas Grabs, Kevin H. Bishop, Sherry Schiff, and Stephan J. Köhler *Department of Aquatic Sciences and Assessment

2 Background/Objectives

3 Where is the soil C? IGBP-DIS (1998) Boreal peatlands and permafrost regions store most of the world s terrestrial carbon (Post et al., 1982; Gorham, 1991; Tarnocai et al., 2009)

4 Inland water CO 2 emissions the importance of streams and rivers % 85 % 45% of all stream and river CO 2 emissions are estimated to occur in the smallest streams, 1 st and 2 nd order 4

5 Fate of DOC in inland waters Headwaters Tranvik et al. (2009). Numbers represent Pg C Over 90% of the total stream network in Sweden belongs to catchments with total area under 15 km 2 (Bishop et al., 2008)

6 DOC source in headwaters? Riparian zones as sources of DOC into the streams (Fiebig, 1990; Dosskey and Bertsch, 1994; Bishop et al., 1995; Hinton et al., 1998; Fölster, 2001; Köhler et al., 2009; Löfgren and Zetterberg, 2011; Strohmeier et al., 2013) Riparian zones act as near-infinite DOC source pools (McGlynn and McDonnell, 2003; Sanderman et al., 2009)

7 Typical riparian zone characteristics Riparian zone Stream Riparian zone Upslope area High biodiversity High organic matter content Near-surface water tables Support anoxic conditions High reaction rates etc. BIOGEOCHEMICAL HOTSPOTS

8 Research question/objectives 1. How long could boreal RZ sustain lateral DOC fluxes as the sole source of exported DOC? Estimate of theoretical turnover times of DOC Specific objectives: 2. Identify the depth range within the RZ that most contributed to the exported DOC, the Dominant Source Layer (DSL). 3. Investigate the quality of riparian DOC fluxes 4. Investigate whether recent or ancient carbon dominate stream water (using DO 14 C)

9 Method

10 Krycklan Catchment Study (KCS)

11 Krycklan Lake Forest Mire Foto: Peder Blomkvist

12 Organic carbon Riparian zones as sources of organic carbon to the streams Especially during hydrological events (Fiebig et al., 1990; Dosskey and Bertsch, 1994; Hinton et al., 1998; Köhler et al., 2009; Sanderman et al., 2009)

13 KCS 13 riparian profiles 18 monitored streams Wet morraine (wetland): 2, 8 Humid morraine: 5, 6, 7, 10 Dry morraine: 1, 4, 9, 12 Sedimentary: 11, 14, km 2

14 Sampling Riparian profiles (DOC) 9 Sampling Campaigns (summer-autumn ) Grabs et al.(2012) Stream sites (DO 14 C) 3 streams 28 samples (1999 and 2007)

15 Hydrology Transmissivity feedback mechanism Groundwater dynamics similar to runoff dynamics Lateral flow to the stream increases when groundwater table rise entering in high conductive layers (Rodhe, 1987) Mobilization of old water during events Perhaps the best PhD thesis in hydrology (J. McDonnell)

16 Riparian Flow-Concentration Integration Model (RIM) Bishop et al., HP (2004); Seibert et al., HESS (2009)

17 Riparian Flow-Concentration Integration Model (RIM) 1. Simulate stream water chemistry (CALIBRATION) 2. Estimate fluxes from soil profiles (NO CALIBRATION)

18 Dominant Source Layer (DSL) Lateral fluxes converge to relatively narrow layer in the riparian soil profile 90% DOC flux Dominant Source Layer (DSL) Stream

19 Turnover times Hydrological approach EXPORT (rate of leaching) VS. POOL (potential source)

20 Riparian soil indices Organic soil depth 4 m from stream Distance Extent of riparian peat from stream Distance Depth Depth

21 Results

22 Lateral DOC fluxes and DSL DOC (mg L 1 ) Dry 3.7 ± 2.4 g m -2 year -1 Depth (cm) Relative proportion of lateral DOC flux Relative proportion of lateral water flux Average DOC concentration Upper and lower DSL limits Humid Wet Till Sediment 12.1 ± 5.3 g m -2 year ± 5.3 g m -2 year ± 2.1 g m -2 year -1

23 DSL size Narrower and less variable in wetter sites 90% DOC flux Dominant Source Layer (DSL) 36 ± 18 cm Stream

24 Riparian Soil Patterns How homogenous are soils along a stream segment?

25 Organic soil depth transect from stream Organic soil depth [cm] Till ,5 0, Distance from stream [m]

26 Organic soil depth transect from stream Organic soil depth [cm] Till ,5 0, Distance from stream [m] Sediment Organic soil depth [cm] ,5 0, Distance from stream [m]

27

28 Hydromorphological controls on Riparian DOC fluxes

29 Turnover times Turnover time in the order of 100s to few 1000s years (median = 618 years) Long-lasting supply of Organic Carbon

30 Recent or ancient carbon mobilization? Net Ecosystem Production (NEP): carbon available for storage or lateral export NEP = 120 g m -2 y -1 (Grelle, 1997; Öquist and Laudon, 2008) NEP = 12 g m -2 y -1 (rough assumption) DOC = 8 g m -2 y -1 (result) DIC = 3.2 g m -2 y -1 (Öquist et al., 2009) DO 14 C Modern (result) 10% (rough assumption)

31 Upslope vs. Riparian DOC Quality SUVA Upslope carbon has Lower SUVA (molecular weight and aromaticity) A 254 /A 365 Greater bioavailability (A 254 /A 365 ) than riparian and stream carbon

32 Stream variability A 254 /A 365 Low temporal variation. (higher bioavailability in forested riparian fens than peatlands) SUVA

33 Conclusions

34 Conclusions

35 Conclusions 1. Long-lasting supply of Organic Carbon

36 Conclusions 2. DSL (36 ± 18 cm) is responsible of most of the terrestrial DOC export into streams

37 Conclusions 3. RZ width controls the DOC flux

38 Conclusions 4. Modern carbon found in stream runoff is supported by recent primary production

39 Thanks

40 Is there any uncertainty? Inherent uncertainties within the RIM approach (Köhler et al., 2009; Seibert et al., 2009; Grabs et al., 2012; Ledesma et al., 2013)