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Quantifying variability in peatland porewater chemistry in the context of a large-scale climate change experiment Natalie A. Griffiths1 and Stephen D. Sebestyen2 1Oak Ridge National Laboratory 2USDA Forest Service, Northern Research Station Northern peatland ecosystems - Saturated, nutrient-poor, acidic ecosystems. - Disproportionately large carbon pools. - Primarily located in northern latitudes. - Important to understand the effects of climate change on carbon dynamics and associated processes in peatlands. (Yu et al. 2010) Large-scale climate change experiment: SPRUCE Elevated CO2 (+500 ppm) Ambient CO2 +0°C +4.5°C +6.75°C +6.75°C +2.25°C +4.5°C +0°C +9°C +9°C +2.25°C mnspruce.ornl.gov phenocam.sr.unh.edu/webcam Water chemistry measurements in SPRUCE Porewater chemistry (0-3m) Lateral outflow (stream) chemistry How does climate change affect porewater chemistry? H1: Elevated temperature will increase production and decomposition rates and result in higher DOC and nutrient concentrations in porewater. H2: Increased CO2 concentrations will increase production and result in greater leaching of DOC into porewater. What is the temporal and spatial variability in porewater chemistry? Provides key information on bog ecosystem function. Baseline data necessary to determine sampling regime. Estimate the change needed to detect a significant effect. How does porewater chemistry in the study bog compare to that in nearby peatlands? Important to assess the representativeness of an experimental site for upscaling and when incorporating processes into Earth System Models. Porewater chemistry at multiple scales Temporal variability Spatial variability Cross-peatland variability Accounting for variability when interpreting treatment effects Focus on total organic carbon (TOC). Temporal variability in porewater chemistry 0 1 2 3 0 20 40 60 (C) 2013 0 6/1/12 6/4/12 7/13/12 8/2/12 9/7/12 11/16/12 1 Depth (m) 8/16/11 9/2/11 9/19/11 9/27/11 10/15/11 10/22/11 10/29/11 11/11/11 11/19/11 11/26/11 12/3/11 12/10/11 2 3 80 6/28/13 7/26/13 8/27/13 10/3/13 11/1/13 1 2 3 0 Total organic carbon (mg/L) Total organic carbon (mg/L) Depth (m) (B) 2012 Typical WL fluctuations Depth (m) (A) 2011 0 20 40 60 Total organic carbon (mg/L) 80 0 20 40 60 80 Total organic carbon (mg/L) - TOC concentrations decrease with depth possibly due to decomposition. 80 70 60 - Greater variation in TOC in deeper peat. Drivers of variation not clear. Vertical flow paths slow, but present. 50 40 2011 2012 2013 30 20 50 100 150 200 250 Day of Year 300 350 400 - Surface water TOC seasonally variable likely reflecting recently produced photosynthate. Spatial variability in porewater chemistry - Chemistry can vary at small spatial scales (10’s m). TOC concentrations variable across the bog. - Lower TOC concentrations in the southern part of the bog may reflect processing of labile OC along lateral flow paths. Depth (m) 0 1 2 South North 3 20 30 40 50 60 70 80 Total organic carbon (mg/L) South 90 Cross-peatland variability in porewater chemistry Bog Lake Fen S1 bog S2 bog Depth (m) Bog Lake Fen S2 bog 0 Depth (m) 0 S1 bog 1 2 Bog Lake Fen S1 bog S2 bog 1 2 0 1 2 3 4 Ammonium (mg N/L) 5 6 20 30 40 50 60 70 80 90 100 Total organic carbon (mg/L) - Peatlands fall along a gradient from ombrotrophic bogs to minerotrophic fens. - S1 bog porewater chemistry resembles a weakly ombrotrophic bog. Porewater at 0 m Total organic carbon (mg/L) Total organic carbon (mg/L) At what scale is porewater chemistry most variable? 80 70 60 50 40 30 temporal spatial cross-site - In near-surface porewater, variability in TOC is largest over time and across sites. - In deeper porewater, variability in TOC is similar across scales. Porewater at 2 m 70 60 50 40 30 20 temporal spatial cross-site Pools (kg) Min Mean Max TOC (temporal) 1981 2716 3412 TOC (spatial) 2412 3105 3758 - Estimated TOC pools can vary by almost 2-fold; highlighting the importance of sampling over space and time when upscaling. Total organic carbon (mg/L) Initial porewater responses to warming 160 140 120 100 80 +0°C (plot 6) +0°C (plot 19) +2.25°C (plot 11) +2.25°C (plot 20) +4.5°C (plot 4) +4.5°C (plot 13) +6.75°C (plot 8) +6.75°C (plot 16) +9°C (plot 10) +9°C (plot 17) pre-treatment warming 60 40 20 0 01/13 07/13 01/14 07/14 01/15 07/15 01/16 - TOC concentrations increased in 0 m porewater with warming, possibly due to increased leaching or mineralization. - Small or no changes observed at deeper depths. - Some changes occurred prior to warming; possibly due to lack of lateral outflow from plots resulting in concentration of TOC. Conclusions: variation - Porewater chemistry is spatially and temporally variable. Variation within a peatland can be as great as variation among peatlands. - S1-bog porewater chemistry resembles a weakly ombrotrophic bog; important for upscaling. - Interpreting effects of ecosystem experiments requires spatially and temporally resolved measurements in the pre-treatment period. Conclusions: warming responses - Up to a doubling of near-surface porewater TOC concentrations with whole-ecosystem warming, possibly due to increased leaching or mineralization. - Continued sampling will examine longer-term responses, including changes in solute fluxes and hydrology. Acknowledgements Keith Oleheiser, Robert Nettles, John Larson, Ben Munson, Reid Peterson, Paul Hanson, Randy Kolka, the SPRUCE team. Funding: U.S. Department of Energy’s Office of Science, Biological and Environmental Research and the USDA Forest Service.