4. MONITORING TECHNIQUES
4.5. Biogeochemistry
4.5.1. Peat and water chemistry
Measuring peat and water chemistry is important for peatland restoraition for a number of related reasons that are described in Anderson et al. (2006). The establishment of vegetation, associated input of OM and elevation of the water table will modify the conditions in peat and water that will impact on the nutrient balance, the carbon transformations (Francez et al., 2000), the physicochemical properties of the peat (De Mars & Wessin, 1999; Laiho et al., 2004) as well as on the size of the microbial community and its activity. Thus an understanding of changes in peat and water chemistry can aid our assessment of peatland restoration especially when assessed in conjunction with other biogeochemical parameters. Physicochemical parameters can give an indication of habitat suitability for species and can indicate breaches in environmental legislation or licenses such as through pollution. However, estimating the limits of acceptable change for physicochemical variables is complex and sometimes impossible. The tolerances of individual species to changes in nutrients, pH and even water levels are only well recorded for some species.
There are two key components to physicochemistry in peatlands: peat and water as substrates.
Upland blanket bogs and lowland raised bogs are generally ombrotrophic and therefore receive water via precipitation, whilst lowland fens can be minerotrophic by receiving water via groundwater, surfacewater and precipitation. However, with low nutrient contents, such as in poor fens, they are termed oligotrophic. This is important when considering the physicochemical monitoring techniques in different peatland types. Important peat and water chemistry parameters are given in Table 18.
Table 18 Important peat and water chemistry parameters
Group Indicators Techniques
Nutrients NO3-
, NH4+
, PO43+
Ion chromatography, atomic absorption
Climate Temperature Probe/meter
Acidity pH Probe/meter
Redox potential Anaerobic status Redox probe with platinum electrodes
As mentioned in section 4.2, Anderson et al. (2006) measured a variety of physicochemical parameters as well as microbal parameters to evaluate the success of restoration of a Sphagnum peatland. High N:P (>20) and N:K (>15) ratios indicated possible K and P deficiencies in restored and cutover sites, which was mainly associated with intense leaching and a high degree of decomposition of peat in these sites. Concentrations of NH4, P and K in the top layer of the restored site were closer to those of the natural site, which indicated a possible effect of restoration on the physicochemistry of the restored site. However, microbial biomass, N:P, N:K and C:P and NH4:biomass ratios of the restored peat showed a tendency to evolve towards values closer to those of the reference site as well as to those found in the literature for natural mires. They could be potentially interesting indicators to monitor during the years following restoration to detect nutrient deficiencies in a restored site, and to compare it to reference or cutover sites.
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The potential advantages and disadvantages of physicochemical monitoring techniques compared with flora and fauna monitoring are shown in Table 19 (from Bardsley at al. 2001).
Table 19 Advantages and disadvantages of physicochemical monitoring
Advantages Disadvantages
Methods often precise and repeatable May require expensive equipment, difficult to analyse and interpret
Can provide precise information and may be necessary to identify the cause of changes in communities
Long-term monitoring requires mechanisation which is expensive
May also give an indication of when things are going to change prior to that change happening which will allow remedial action to be taken
May require specialist knowledge
Bardsley et al. (2001) suggested that the efficacy of a physicochemical monitoring strategy will be dependent upon careful design and consideration of the following points:
where to sample – for streams target run-off points to assess the maximum concentration of pollutants, or analysis of peat extracts/Sphagnum tissues for assessment of nutrient enrichment/deposition;
to sample both upstream/gradient and downstream/gradient of the potential threat. This may allow the upstream data to act as a control;
to assess whether existing monitoring can be used to provide some or all of your data needs;
to use standard, repeatable techniques;
to seek advice on interpretation of data and sampling strategies prior to beginning the sampling programme.
4.5.1.2. pH
Quinty and Rochefort (2003) suggest that pH should be measured as it is an important factor for plants, and especially for Sphagnum species that are very sensitive to the level of acidity. Although pH may be measured in the planning phase of restoration, it is helpful to make additional measurements in permanent plots because restoration procedures, such as surface preparation, may cause some change. Also, pH data can contribute to the interpretation of vegetation data.
There are a wide variety of pH meters that can be used in the field or lab. pH is generally measured electrometrically. The electrometric pH reading is a product of complex interactions between the electrode and the soil suspension; differences in the soil or peat: water extraction ratio, the electrolyte concentration of the suspension, and the spatial placement of the electrode can all effect measured pH.
Water pH can be measured directly in the field using a portable meter by dipping the electrode directly in the water. Peat pH can be determined by mixing one part peat by volume with two parts
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distilled water (pH 7), waiting for about 10 minutes, and taking a reading. Stoneman & Brooks (1997) report the pH readings from intact cores may be 0.5-1 pH units lower than those in peat/water slurries due to dilution effects.
4.5.1.3. Redox potential
A key factor in determining chemical transformations in peatlands is the degree of aeration. In saturated peat, the pore spaces are filled with water and oxygen can diffuse only slowly through the peat. Conditions are therefore anaerobic and any oxygen present is rapidly consumed. The redox or oxidation-reduction potential is a measure of how readily a medium will donate electrons to (reduce) or accept electrons from (oxidise) any reducible or oxidisable substance. Solutions with high redox potentials are highly oxidising. With increasingly anaerobic conditions the redox potential decreases and a series of chemical transformations can take place as a result of bacterial activity (Fig. 13). This is important in waterlogged peat and soil because at low redox potential, nitrate is reduced to nitrogen, and sulphates are reduced to toxic H2S (Reddy & D’Angelo, 1994; Mitsch & Gosselink, 2000;
Charman, 2002).
Figure 13 Sequence in depth of chemical transformations with increasingly reduced conditions or increasing depth in peat (Reddy & D’Angelo, 1994).
Evidence in the literature indicates that restoration of peat-based wetlands by reflooding can induce the redox-mediated release of soil nutrients, thereby increasing the risk of diffuse water pollution (Niedermeier & Robinson, 2007). For the sake of improving management decisions, there is a need for more detailed studies of the underlying relationship between the hydrological and redox dynamics that explain this risk. This is particularly the case in agricultural peatlands that are commonly targeted for the creation of lowland wet grassland. Niedermeier & Robinson (2007) conducted a 12-month field study to evaluate the relationship between hydrological fluctuations and
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soil redox potential (Eh) in a nutrient-rich peat field that had been restored as lowland wet grassland from intensive arable production. They found that during the summer, alternating periods of aerobism (Eh > 330 mV) in the surface layer of peat coincided with intense precipitation events.
Redox potential throughout the 30–100 cm profile also fluctuated seasonally; indeed, at all depths Eh displayed a strong, negative relationship (P < 0.001) with water table height over the 12-month study period. However, Eh throughout the 30–100 cm profile remained relatively low (< 230 mV), indicating permanently reduced conditions that are associated with denitrification and reductive dissolution of Fe-bound P. Thus redox can serve as an important indicator of microbiological processes in peat as well as variation in the hydrological regime.
To measure the redox potential, a platinum electrode is connected to an mV meter. Many pH meters also have an mV scale. It is most convenient to use a combined platinum KCl electrode. Tables are available to relate the oxidation-reduction potential Eh to the ion forms of interest, but redox alone is a useful index of the extent of oxidation or reduction in the system (Jones & Reynolds, 1996; Mitsch
& Gosselink, 2000). Redox potential may give a first indication of the likelihood of CH4 emissions example, but may not be significantly accurate or precise enough to be used as a proxy for budgeting CH4 emissions.
4.5.1.4. Exchangeable ions
The immobilisation and transformation of nutrients and pollutants are common aims of peatland restoration (Trepel et al. 2000). To evaluate success of the restoration in this respect, mass balances have to be calculated (see Davidsson et al., 2000). This may be achieved by determining the difference in concentration and volume of water in and out of the peatland.
The determination of exchangeable ions in soil or peat requires that ions on soil exchange sites be forced into a solution in which they can be effectively measured. Generally this involves flooding the exchange sites on clay and organic surfaces of soil or peat with ions from an extractant,usually a strong salt solution. The extractant, now containing exchangeable ions in addition to ions from the added salt, is separated from the soil or peat by filtering or centrifugation and is then analysed for the ions of interest such as by ion chromatography or atomic absorption (Robertson et al. 1998b).
Choice of the salt for the extractant solution will depend on the target ions (Table 20). Extractant ions must effectively displace ions from exchange sites and must not interfere with subsequent chemical analysis of the extracted solutions.
Table 20 Types of extractants to use for specific target ions and issues (Robertson et al. 1998b).
Extractant Target Issues
KCl Inorganic N (e.g. NO3-
) K+ cannot be a target NH4OAc Total cations Does not cover anions BaCl2 K+ and NH4+
Expensive
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Peat and water chemistry measurements are straight forward and relatively cheap for the amount of information that they provide. Nutrient concentrations, pH and redox potential should be included on every monitoring protocol and monitored at least seasonally every year at different peat depths to provide important information that relates to both plant and microbial functional development on the site. Advantages of these methods are that they are precise and repeatable, provide information necessary to identifying cause of community changes, and give an indication of when things are going to change prior to the change happening permitting remedial action. Disadvantages are that the techniques can be expensive, difficult to analyse and interpret, long-term monitoring requires expensive equipment and specialist knowledge may be required. Subsequent analysis of nutrient ratios may be used as important indicators of rutrient deficiencies that may aid adaptive management decisions.
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