High soil solution carbon and nitrogen concentrations in a drained Atlantic bog are reduced to natural levels by 10 years of rewetting

Abstract

Abstract. Anthropogenic drainage of peatlands releases additional greenhouse gases to the atmosphere, and dissolved carbon (C) and nutrients to downstream ecosystems. Rewetting drained peatlands offers a possibility to reduce nitrogen (N) and C losses. In this study, we investigate the impact of drainage and rewetting on the cycling of dissolved C and N as well as on dissolved gases, over a period of 1 year and a period of 4 months. We chose four sites within one Atlantic bog complex: a near-natural site, two drained grasslands with different mean groundwater levels and a former peat cutting area rewetted 10 years ago.Our results clearly indicate that long-term drainage has increased the concentrations of dissolved organic carbon (DOC), ammonium, nitrate and dissolved organic nitrogen (DON) compared to the near-natural site. DON and ammonium contributed the most to the total dissolved nitrogen. Nitrate concentrations below the mean groundwater table were negligible. The concentrations of DOC and N species increased with drainage depth. In the deeply-drained grassland, with a mean annual water table of 45 cm below surface, DOC concentrations were twice as high as in the partially rewetted grassland with a mean annual water table of 28 cm below surface. The deeply drained grassland had some of the highestever observed DOC concentrations of 195.8 ± 77.3 mg L −1 with maximum values of > 400 mg L −1 . In general, dissolved organic matter (DOM) at the drained sites was enriched in aromatic moieties and showed a higher degradation status (lower DOC to DON ratio) compared to the nearnatural site. At the drained sites, the C to N ratios of the uppermost peat layer were the same as of DOM in the peat profile. This suggests that the uppermost degraded peat layer is the main source of DOM. Nearly constant DOM quality through the profile furthermore indicated that DOM moving downwards through the drained sites remained largely biogeochemically unchanged. Unlike DOM concentration, DOM quality and dissolved N species distribution were similar in the two grasslands and thus unaffected by the drainage depth.Methane production during the winter months at the drained sites was limited to the subsoil, which was quasipermanently water saturated. The recovery of the water table in the winter months led to the production of nitrous oxide around mean water table depth at the drained sites.The rewetted and the near-natural site had comparable DOM quantity and quality (DOC to DON ratio and aromaticity). 10 years after rewetting quasi-pristine biogeochemical conditions have been re-established under continuously water logged conditions in the former peat cut area. Only the elevated dissolved methane and ammonium concentrations reflected the former disturbance by drainage and peat extraction. Rewetting via polder technique seems to be an appropriate way to revitalize peatlands on longer timescales and to improve the water quality of downstream water bodies.

Drained coastal peatlands: A potential nitrogen source to marine ecosystems under prolonged drought and heavy storm events—A microcosm experiment

“…Therefore, a large portion of NH 4 + -N remains bound to cation exchange sites on the soil complex, which suggests storms can only flush out a small amount of NH 4 + -N to the downstream. In line with other studies (Bragazza and Limpens, 2004;Frank et al, 2014;Qualls and Richardson, 2003), DON was the dominant form of nitrogen in the discharge water (Fig. 3).…”

“…The further reduction of soil moisture possibly limited nitrogen mineralization, resulting in much lower NMR afterward this period. Similar to what other studies found (Dominik and Jörg, 2007;Frank et al, 2014;Laine et al, 2013), ammonium was the dominant inorganic nitrogen and gradually accumulated in peat during drought (Figs. 1c,d).…”

Nitrogen and dissolved organic carbon (DOC) losses from an artificially drained grassland on organic soils

“…While DOC release and consumption are mainly controlled by biogeochemical processes, hydrology -particularly the water balance -becomes an important factor when comparing losses from different peatlands (Fraser et al, 2001;Gibson et al, 2009). Drainage has frequently been found to increase DOC concentrations in peatlands due to increased microbial activity (Frank et al, 2014). In contrast to these results, some boreal peatlands have shown decreasing DOC concentrations after drainage (Åström et al, 2004).…”

“…While the governing processes and influence factors for nitrate are quite well known, the effects of drainage and re-wetting on DOC and dissolved organic nitrogen (DON) losses are less clear. Several factors determining DOC concentrations in the porewater and the groundwater of peatlands -such as the groundwater levels (Frank et al, 2014), the temperature (Koehler et al, 2009), the ionic strength (Evans et al, 2006) and the pH (Clark et al, 2005) -have been proposed, but effects are sometimes confounding or interacting. For example, higher temperatures might cause a water table drawdown, subsequent sulphate release and thus a change in both the ionic strength and the pH (Clark et al, 2005).…”

“…Overall, most authors suggest re-wetting as an effective measure to reduce DOC concentrations and/or losses (Frank et al, 2014;Wallage et al, 2006). However, some studies have found -sometimes temporary -increasing DOC concentrations or losses after re-wetting, especially for highly degraded peat soils (Kalbitz and Geyer, 2002).…”