Dissolved Organic Carbon (DOC)

decomposition, and the second source is derived from the peat itself. However, Palmer et al. (2001) performed ¹⁴C analyses and showed that the pore water DOC in peat was significantly younger than peat organic carbon, which could indicate that the main DOC fraction is derived from fresh vegetation. Thus, most of the DOC may have been derived from fresh vegetation during the observation period of this study.

Furthermore, the seasonal increase in the DOC at a depth of 10 cm in the lagg zone (Figure 5.2) during the SEN period, which was likely caused by fallen leaves acting as an additional carbon source, indicated the importance of fresh plant tissue in DOC production (Hongve, 1999; Blodau et al., 2004). Carbon addition from trees during the SEN period was caused by litter from deciduous plants because coniferous litter and peat release DOC more evenly throughout the year compared to deciduous litter (Hongve, 1999).

The higher carbon content in the precipitation at the lagg site compared to that of the other sites can be attributed to stemflow and rainfall passage through the canopy. The rainfall DOC concentrations in the treeless bog, fen (1.4-2.9 mg L^-1), and lagg (8.7-9.3 mg L^-1) zones were comparable to those observed at a swamp in Canada, in which Dalva & Moore (1991) recorded both DOC concentrations of 2.0 mg L^-1 in the above-canopy precipitation and increased DOC concentration levels after passage through tree canopies as throughfall (9.1-14.6 mg L^-1) and stemflow (23.1-30.1 mg L^-1). Koprivnjak & Moore (1992) reported DOC concentrations of 1-2 mg L^-1 in the above-canopy precipitation, as well as much higher values in stemflow and tree throughfall (50-150 mg L^-1).

Factors that control the rates of production and export of DOM are still poorly understood for peatlands (Holden 2005). In the current study, one of the main environmental controls of the observed variations in the DOC concentrations among sites (Figure 5.2) was most likely the site-specific vegetation, which served as the main source of DOC. The main vegetation species in the lagg zone were Menyanthes trifoliata, Betula pendula, and Alnus incana. The fen zone was covered predominantly with Scheuchzeria palustris and Sphagnum fuscum, while the vegetation cover at the bog site was predominantly Sphagnum spp. Litter from Sphagnum spp. has low bioavailability because it has low nutrient content and is characterized by polyphenols, which strongly inhibit microbial decomposition (Verhoeven & Toth, 1995; Bragazza et al., 2006). The decomposition of deciduous vascular plant species’

litter was observed to be two times faster than that of bryophytic litter (Hobbie et al.,

2000), which can cause increased DOC production. In contrast to the Sphagnum litter, which has low bioavailability, the DOC that is leached from Sphagnum spp. is highly labile and disappears faster than the DOC leached from vascular plants (Wickland et al., 2007). Therefore, the higher decomposition rate of plant organic matter and the slower decomposition of the leached DOC at the lagg and fen zones can cause higher DOC concentrations than those of the bog zone.

An additional important factor that controls DOM production is the temperature. If the temperature continues to rise due to climate change, alterations in plant production may occur (Weltzin et al., 2003; Wiedermann et al., 2007), which may modify the quantity of DOM produced in the mire. However, the response would depend on the type of plant and the ecosystem. Furthermore, temperature increases will lead to a number of other process alterations such as changes in the bacterial activity, energy balance, and water table etc., which may influence DOM production.

Another factor that affects DOM production is the nutrient content. At the bog site, the lower nutrient content (Tables 5.2 and 5.3) may be a limiting factor in the transformation of particulate organic carbon (POC) to DOC and can cause decreased DOC concentrations. The lagg zone received the highest nutrient supply from the surrounding mineral soils, which may increase microbial decomposition of POC and higher DOC production in the lagg zone compared to the ombrotrophic site.

Furthermore, the height of the water table plays an important role by creating oxic or anoxic conditions. A high water table may lead to anaerobic decomposition, which is slower than aerobic decomposition. Consequently, DOC in different states of decomposition accumulated in the fen and lagg zones. In contrast, the bog zone had a thicker aerobic layer because of the lower water table, which led to faster DOC mineralization through oxidation and CO₂ emission into the atmosphere (Schneider et al., 2012). However, several studies have reported conflicting results on the influence of the water table on the DOC content. For instance, Tipping et al. (1999) found that low water tables increased peat-derived DOC production, while Blodau et al. (2004) found that the water table did not significantly affect DOC production. The predicted changes in temperature and rainfall patterns due to climate change will likely affect both the amount and the characteristics of the organic carbon transported downstream from boreal catchments (Köhler et al., 2008).

Temperature, pH, and UV radiation differences among different mire sites can lead to variations in the DOC content. For example, differences in the pH between

sites can influence DOC solubility, and a pH increase of 0.5 units in pore water can cause a 50-60 % increase in the DOC content (Tipping & Woof, 1990; Clark et al., 2005). The pH in the lagg zone was higher (5.6) than the pH of the bog (4.4), which should increase the DOC solubility in the lagg zone. Temperature directly affects the rates of many physical, chemical, and biological processes (Limpens et al., 2008).

Temperature increases of approximately 5 °C may cause the decomposition rate to double in high-latitude soils (Hobbie et al., 2000). In the current study, the summer water temperatures at the open bog and fen sites were higher than the temperatures at the lagg site. Therefore, during the hot summer season, both DOC decomposition and POC to DOC transformation rate at the bog and fen sites may be higher than that of the lagg site. Moreover, organic matter degradation in peatlands is affected by UV light. Treeless bogs and fens are more exposed to UV radiation and sunlight during the summer, which can lead to additional mineralization. Furthermore, because of its slightly elevated location, the bog site continuously discharges exported DOC toward the fen and lagg zones, which lowers the amount of carbon in the bogs and gives additional carbon inputs to the fen and lagg sites. In summary, differences in the DOC concentrations among sites were caused by multiple physical and chemical parameters across the lagg-fen-bog transect, and one single factor cannot explain the observed differences.

The DOC (48-52 mg L^-1) concentrations in the outflow draining the mire complex were higher than the upper values of the total organic carbon concentration (20-40 mg L⁻¹) in the streams from a snow-free boreal mire (Köhler et al., 2008). The high temperature during the summer reduced the flow rate of water from the bog and fen sites toward the lagg site, which halted the delivery of DOC to the adjacent river.

The reduced supply of DOC was also observed on a larger scale, as indicated by the lowered DOC concentrations in river samples located upstream from the study site, which were influenced by a mixed forest-mire landscape. River concentrations decreased from 8.2 to 4.5 mg L^-1 due to the drought, leading to a stronger relative contribution of deeper groundwater sources to the river water. The decreased export of DOM from the mire to the river during the dry summer may decrease the net aquatic primary production downstream and other biogeochemical processes, such as the transport of organic pollutants, colloid chemistry, and acidity regulation.

Figure 5.7 A conceptual model of possible peatland effects on fluvial system and atmosphere under climate change conditions. Comparing a. high and b. low water table scenarios. The model assumes no changes in vegetation cover.

High temp.

and low water table

Lower discharge into limnic

systems

Lower CH₄ and higher

CO₂ emissions

Lower contents of

DOM and nutrients

Lower transfer of metals and pollutants Higher light

attenuation

Lower acidity and

higher pH

Lower CO₂ emissions from limnic

systems High temp.

and high water table

Higher CH₄ and lower

CO₂ emissions

Higher discharge into limnic

systems

Higher contents of

DOM and nutrients

Lower light attenuation

Higher transfer of metals and pollutants

Higher acidity and

lower pH

Higher CO₂ emissions

from limnic systems

a b

A schematic representation of the possible responses to climate change under increased temperature and water level changes is presented in Figure 5.7. As presented in the conceptual model, changes in the temperature or water table may lead to multiple changes in the ecosystem. The DOC retained in the mire complex because of the low discharge is can be transferred to the atmosphere in the form of CO₂ and CH₄ emissions due to microbial activity. Pastor et al. (2003) demonstrated an exponential increase in the CO₂ and CH₄ emissions that coincided with the increased retention of DOC from boreal peatlands because of decreased discharge. Therefore, if the current temperature increase continues, more DOC will be retained within the mire and will not be transferred into the aquatic system, which would increase greenhouse gas emissions in the atmosphere from the terrestrial system. In particular, lagg zones can act as strong CH4 producers, similar to the wet forested zones (Fiedler et al., 2005; Christiansen et al., 2010; Grunwald et al., 2012).

However, all of these potential responses may be short-term changes. For example, further DOC decomposition could be limited by nutrient availability.

Moreover, under the protracted influence of global warming, the ecosystem could undergo alterations in species composition. Fenner et al. (2009) detected increased vascular plant coverage at the expense of Sphagnum coverage in a weakly minerotrophic peatland exposed to elevated CO2 levels. Hence, if CO2 levels in the air increase and cause changes in the species composition of the peatland, the release of DOC from the mire could potentially change.

5.5.2 Water-chemical gradients: pH, macroelements, and microelements