Field scale (study 4)

A possible restriction of the field study was the moisture and vegetation gradient within the peatland, so that the three treatment and control plots could not be treated as true ‘randomized’

replicates. Furthermore, the study was only partially successful in simulating an extreme drought event, as water table level in the treatment plots could be lowered by only about 20 cm compared to the controls (Fig. 2), presumably due to the moist weather conditions in 2007. Nevertheless, comparing the complementary plots C1 – D1, C2 – D2, and C3 – D3, similar effects attributable to the drought and rewetting treatment could be identified.

As observed on the mesocosm scale (Fig. 5), solute electron acceptor concentrations increased as a response to the temporary drought. Such an effect has already been described earlier for a poor fen, a temperate swamp and a gully mire (Bayley et al., 1986, Mandernack et al., 2000, Dowrick et al., 2006). Methane concentrations declined or were prevented to build up (Fig. 7). After rapid wetting, electron acceptors were consumed, but the time scale of electron acceptor depletion varied considerably among the treatment plots and was dependent on depth (Fig. 7). A suppressive effect of the presence of alternative electron acceptors on methanogenesis is thus suggested to have happened also on the field scale by the inverse concentrations patters of dissolved sulfate and methane in all treatments.

Overall, methane concentrations at the Schlöppnerbrunnen site were low compared to bogs, but within the range reported for other fens. In an earlier study of Paul et al. (2006), the authors described the Schlöppnerbrunnen site to undergo repeated redox oscillations. This could probably cause a long-term suppression of methanogens by continuous resupply of electron acceptors, causing comparably low methanogenic activity.

30 RESULTS AND DISCUSSION

Fig. 7. Concentrations of dissolved inorganic carbon (DIC), ferrous iron (Fe²⁺), sulphate (SO42-), and methane (CH4) in the plo9ts C2 and D2 (left) and C3 and D3 (right). All concentrations are given in µmol L^-1. The drought phase lasted from day 129 to 203, indicated with solid arrows. Open arrows denote major rain events (compare Fig. 2) and the thin line represents the approximate water table level over time and space.

5.3 Arsenic mobilization and immobilization under variable redox conditions in a temperate fen soil (study 5)

In the peats investigated, arsenic was predominantly bound to the solid phase, regardless of the prevailing redox conditions changing over depth and time. Coinciding with iron contents, arsenic contents peaked at ~25 mg kg^-1 in the upper layers and decreased with depth. Total contents were thus in agreement with previously reported data from the same catchment (Huang and Matzner, 2006), but far lower than reported for naturally more enriched minerotrophic wetlands (Gonzalez et al., 2006).

Nevertheless, dissolved arsenic concentrations locally reached values as high as 300 µg L^-1 in W-V and also mostly exceeded the common drinking water standard of 10 µg L^-1 in all treatments when the peat was saturated (Fig. 8). Arsenic concentrations thus exceeded values reported by Huang and Matzner (2006) by about one order of magnitude. Furthermore, As(III) was the predominant species in contrast to the predominance of organic arsenic species in the adjacent peatland studied by Huang and Matzner (2006). These findings document the potential of arsenic remobilization in organic soils during phases of anaerobiosis. The occurrence of hot-spots of arsenic release near the water table in the treatments with vegetation was likely triggered by the activity of roots, providing easily decomposable substrates and thus favoring the rapid evolution of anaerobic conditions (Wachinger et al., 2000).

RESULTS AND DISCUSSION 31

depth below surface (cm) 40 60 80 100 120 140 160 180 200 220 240

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Fig. 8. Temporal dynamics of total dissolved arsenic (µg L-1) in the permanently wet treatment W-V, the drying/rewetting treatment with intact vegetation DW-V, and the defoliated treatment DW-D. Black dots depict sampling points over time and space underlying the interpolation. The black solid line represents the approximate water table level. Note the difference in scale between the W-V and the DW treatments.

According to correlation analysis and the observed dynamics during drought and rewetting, arsenic was predominantly bound to the reactive hydroxide fraction. Arsenic was thus released into solution during phases of iron reduction and immobilized during drought and concomitant re-oxidation of ferrous iron (Fig. 9). This coincidence with iron reduction and oxidation dynamics further suggested that arsenic was predominantly associated with reactive iron hydroxides, readily available for microbial reduction. Although this phenomenon has already been reported (Masscheleyn et al., 1991, La Force et al., 2000, Fox and Doner, 2003), this relationship had so far not been verified for natural peatlands with natural arsenic background. Persistently elevated arsenic concentrations under reduced conditions furthermore indicated that adsorption on sulfides or precipitation of sulfides was only of little importance for arsenic immobilization.

The rapid release of arsenic into solution during phases of iron reduction was likely supported by production and accumulation of DOC. Especially in the wet treatment W-V, maximum DOC concentrations coincided with highest observed arsenic concentrations. Such a mobilizing effect of DOC on arsenic was demonstrated in batch experiments (Bauer and Blodau, 2006). Re-adsorption of arsenic may also have been interfered by competitive sorption with DOC. Nevertheless, an export of arsenic from the peatland seemed unlikely, as arsenic clearly had accumulated over the past decades or millennia in the Schlöppnerbrunnen peat, as observed for iron.

32 RESULTS AND DISCUSSION

Fig. 9. Depth integrated turnover of arsenic and ferrous iron for all treatments W-V, DW-V, and DW-D during the experiment. In treatment W-V, depth integration of iron and arsenic turnover was done only for the depths at and below the water table level. Values >0 indicate release into the pore water.

5.4 Using carbon stable isotope signatures to assess the effect of experimental drought