Hydrological control of dissolved organic carbon dynamics in a rehabilitated <i>Sphagnum</i>-dominated peatland: a water-table based modelling approach

Abstract

Abstract. Hydrological disturbances could increase dissolved organic carbon (DOC) exports through changes in runoff and leaching, which reduces the potential carbon sink function of peatlands. The objective of this study was to assess the impact of hydrological restoration on hydrological processes and DOC dynamics in a rehabilitated Sphagnum-dominated peatland. A conceptual hydrological model calibrated on the water table and coupled with a biogeochemical module was applied to La Guette peatland (France), which experienced a rewetting initiative on February 2014. The model (eight calibrated parameters) reproduced water-table (0.1<NS<0.61) and pore-water DOC concentrations (2<RMSE<11 mg L−1) in a time series (1 April 2014 to 15 December 2017) in two contrasting locations (rewetted and control) in the peatland. Hydrological restoration was found to impact the water balance through a decrease in slow deep drainage and an increase in fast superficial runoff. Observed DOC concentrations were higher in summer in the rewetted location compared to the control area and were linked to a difference in dissolved organic matter composition analyzed by fluorescence. Hydrological conditions, especially the severity of the water-table drawdown in summer, were identified as the major factor controlling DOC-concentration dynamics. The results of the simulation suggest that the hydrological restoration did not affect DOC loads, at least in a short-term period (3 years). However, it impacted the temporal dynamics of DOC exports, which were the most episodic and were mainly transported through fast surface runoff in the area affected by the restoration, while slow deep drainage dominated DOC exports in the control area. In relation to dominant hydrological processes, exported DOC is expected to be derived from more recent organic matter in the top peat layer in the rewetted area, compared to the control area. Since it is calibrated on water-table and DOC concentration, the model presented in this study proved to be a relevant tool in identifying the main hydrological processes and factors controlling DOC dynamics in different areas of the same peatland. It is also a suitable alternative to a discharge-calibrated catchment model when the outlet is not easy to identify or to monitor.

Peatland Contribution to Stream Organic Carbon Exports From a Montane Watershed

“…The coupled monitoring of fDOM, turbidity, and discharge shows that all the stream organic carbon concentration peaks are triggered by a simultaneous discharge elevation. However, stream organic carbon concentration peaks are not proportional to discharge (Figure 3) and other high-frequency parameters such as temperature and water table levels must be included in models to understand their variability (Bernard-Jannin et al, 2018;Binet et al, 2013;Birkel et al, 2017;Tunaley et al, 2018).…”

“…Moreover, the high DOC concentration peaks at the outlet occurred several hours before the low DOC concentration peaks at inlet n°4, confirming that the peatland is the source of DOC in the watershed. Higher DOC concentrations at the outlet result from (1) the transfer of DOC enriched "preevent" porewater previously stored within the peat column (Tetzlaff et al, 2014;Tunaley et al, 2016) and (2) the leaching of subsurface nonsaturated peat by event water (Bernard-Jannin et al, 2018).…”

“…These sensors have revealed unexpected temporal variability for organic carbon concentrations in streams draining peatlands (Austnes, 2010;Grayson & Holden, 2012;Koehler et al, 2009;Pellerin et al, 2011;Ryder et al, 2014;Strohmeier et al, 2013;Tunaley et al, 2016). Associated with high-frequency meteorological and hydrological surveys, stream organic carbon concentration monitoring contributes to a better understanding of the carbon mobilization processes and temporality in the watersheds (Bernard-Jannin et al, 2018;Birkel et al, 2017). High-frequency sensing of streams is also an effective way to compare and question the relevancy of stream carbon export assessments based on sporadic water samples (Walling & Webb, 1985).…”

Drivers of seasonal and event scale DOC dynamics at the outlet of mountainous peatlands revealed by high frequency monitoring

“…Higher temperatures were also shown to enhance evapotranspiration from peatland resulting in a rise in DOC concentration in peat porewater and stream waters during dry summer periods (Fraser et al, 2001). Finally, other studies documented the importance of water table level fluctuations (Bernard-Jannin et al, 2018;Kalbitz et al, 2002;Strack et al, 2008) and acrotelm oxygenation (Freeman et al, 2001) in DOC production and mobilization to streams. DOC concentration monitoring at the outlet of peatlands has generally consisted in a weekly or monthly stream water sampling routine (Clark et al, 2008;Juutinen et al, 2013).…”

“…Water -Jannin et al, 2018;Billett et al, 2006;Clark et al, 2009;Fenner and Freeman, 2011;Ritson et al, https://doi.org/10.5194/bg-2019-372 Preprint. (Austnes, 2010;Ryder et al, 2014) Square-root…”