4. MONITORING TECHNIQUES
4.2. Peat physical integrity
4.2.3. Physical characteristics of peat
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commercial multichannel logger to monitor change in water content (Gaskin & Miller, 1996). Dry bulk density can be calculated as a function of the volumetric water content measured by a probe combined with either wet bulk density (1) or gravimetric water content (2) (Wijaya et al., 2003):
(1) Dry bulk density (g cm-3) = wet bulk density (g cm-3) – [VADR x density of water]
(2) Dry bulk density (g cm-3) = (VADR / gravimetric H2O (g H2O g dry peat) x density of water)
where density of water = 1 g per ml, and VADR is the volumetric water content measured using an ADR probe (cm3 cm-3 or ml ml). For the first calculation, the volume of peat collected must be known in order to calculate the wet bulk density. Thus this method suffers from the peat compression limitations above. For the second calculation the sample must be oven dried to constant weight although the peat volume does not need to be known. This method therefore may be more time consuming although time may be saved in the field. Wijaya et al. (2003) showed that the estimation of dry bulk density with wet bulk density was better than that with gravimetric water content. They suggested that the accuracy of the probe is a critical factor in estimating dry bulk density.
Ground penetrating radar can also be used to determine the density of peat and the Environment Agency regularly fly in lowland areas although at different times of the year. Also, the height from a bridge or ground anchors can be used as reference points to calibrate the elevation data collected.
However, caution was stressed as GPS and Ordinance Survey can conflict and be 5-10cm out and the height of raised mires can vary up to foot in a year depending on water input.
4.2.3.2. Humification
Peat humification is a measure of the decomposition and structure of the peat and higher degrees of humification indicate a more well-decomposed peat. Humification and peat decomposition are useful indicators of the preservation of the peat (low humification indicating good preservation) and the likely hydrological conductivity of the peat body (low humification indicating high hydrological conductivity).
Although sophisticated techniques exist to determine the chemical structure of humic compounds, such as gas chromatography mass spectrometry (GCMS), pyrolysis mass spectrometry (PyMS), and nuclear magnetic resonance (NMR), most often they are fractionated with a simple scheme based upon their solubility at different pHs (Stevenson 1986; Bridgham & Lamberti, 2009). A pyrophosphate extraction of polyphenolic humic substances is also used (Bridgham et al. 1998;
Bridgham & Lamberti, 2009).
Peat is commonly characterized by its degree of physical decomposition, either through its fiber content or with the qualitative von Post index (Clymo 1983). Organic soils (Histosols) are classified into three groups based on fibre content: fibrists, hemists and saprists (Mitsch and Gosselink, 2000;
Richardson and Vepraskas, 2001) that depend on the degree of decomposition. The degree of humification or breakdown can be determined by the Von Post scale as a field method (Von Post and Granlund in 1926; Bridgham & Lamberti, 2009). According to the von Post method, peats are ranked on a scale from H1 to H10 relative to their degree of humification (Table 4). Within the organic soil horizon are the fibrists (little decomposition; H1-H3), hemists (intermediate decomposition; H4-H6) and saprists (high decomposition; H7-H10).
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Table 4 The von Post Scale of Peat Decomposition (after Hodgson, 1997) Scale Peat type Peat Characteristics
H1 Fibrists Completely undecomposed peat; only clear water can be squeezed from peat H2 Almost undecomposed; mud free peat; water squeezed from peat is almost
clear and colorless
H3 Very little decomposition; very slightly muddy peat; water squeezed from peat is muddy; no peat passes through fingers when squeezed; residue retains structure of peat
H4 Hemists Poorly decomposed; somewhat muddy peat; water squeezed from peat is muddy; residue is muddy but it shows structure of peat
H5 Somewhat decomposed; muddy; growth structure discernible but indistinct;
when squeezed some peat passes through fingers but most muddy water passes through fingers; compressed residue is muddy
H6 Somewhat decomposed; muddy; growth structure indistinct; less than one- third of peat passes through fingers when squeezed; residue very muddy H7 Saprists Well decomposed; very muddy, growth structure indistinct; about one-half of
peat passes through fingers when squeezed; exuded liquid has a "pudding- like" consistency
H8 Well decomposed; growth structure very indistinct; about two-thirds of peat passes through fingers when squeezed; residue consists mainly of roots and resistant fibers
H9 Almost completely decomposed; peat is mud-like; almost no growth structure can be seen; almost all of peat passes through the fingers when squeezed
H10 Completely decomposed; no discernible growth structure; entire peat mass passes through fingers when squeezed
The von Post scale is a rapid field assessment technique. In this technique a sample of wet peat is squeezed through the closed hand and the colour of the liquid that is expressed through the fingers is noted, along with the proportion of the peat sample that is extruded and the nature of the peat/plant residues that remain in the hand. The scale is provided below (Table 4), and is a commonly used field technique to describe the nature of the peat, typically to characterise or compare between sample sites rather than monitor change over time (e.g. Holden & Burt, 2003;
PAA, 2003). In terms of monitoring, the scale could be used as a coarse but rapid field assessment for the long-term monitoring of peat decomposition over time as Malterer et al. (1992) found it to be a relatively reliable indicator of the degree of peat decomposition. In degraded situations, von Post scale scores can rise relatively rapidly over a few years. This scale can also be used to characterise peat at different depths, since degree of decomposition usually changes over depth which may inform monitoring conclusions about hydrology etc. However, it is not as accurate as empirical techniques due to personal bias and the water content, and does not permit statistical analysis.
E4:E6 ratio is the ratio of humic acid (E4) and fulvic acid (E6), and is commonly used to indicate the rate of humification in the peat and represent changes in the type of organic matter being mobilised (Worrall et al., 2002). E4:E6 ratio is determined by filtering samples through 0.45 μm GF/A filter paper and measuring the absorbance at 465 nm(E4) and 665 nm (E6) on a UV-VIS spectrometer (i.e.
Jonczyk et al., 2009; Chen et al.,1977). Percent transmission is also commonly used as a proxy for peat humification and relative biochemical composition (Blackford & Chambers, 1993). The degree of peat humification can be determined using a spectrophotometer to measure transmission of light at 540 nm through a solution containing a mechanically homogenised peat sample digested in NaOH
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solution. Well humified samples have more humic acid and, therefore, lower transmission. Caseldine et al (2000) report that luminescence excitation and emission wavelengths suggest that high molecular weight acids (humic acids') are altered by the NaOH extraction procedure to form lower molecular weight acids ('fulvic acids'), amino acids and polysaccharides. Percentage transmission is principally related to luminescence emission wavelength and thus to molecular weight of the compounds present. Luminescence emission shows much more sensitivity to peat composition and demonstrates that different plant species may be affected to different degrees by the NaOH extraction process (Caseldine et al., 2000). The findings broadly support the underlying principle of colorimetric determination of 'humification' whereby transmission levels decrease with increasing plant breakdown, but show that it is based on an inadequate understanding of the chemical processes occurring in peat decay and preparation procedures. Luminescence spectroscopy provides a technique for resolving these issues (Caseldine et al., 2000). McMorrow et al. (2004) report that this technique can be costly and time consuming. Klavins et al. (2008) suggest that humification describes the transformation of organic matter to humus, and therefore propose that the degree of humification should be expressed in terms of the quantity of formed humic substances as a fraction of the total amount of organic matter.
McMorrow et al. (2004) reports on progress towards using HyMap data at 3m spatial resolution and laboratory spectroradiometry to estimate physico-chemical properties of exposed peat, notably the degree of humification. The strong relationship of HyMap SWIR reflectance and derived indices with transmission provides a possible basis for estimating peat humification across extended areas, but the confounding effect of moisture content cannot be ignored. McMorrow et al. (2004) suggest that it is possible that higher moisture content were reinforcing the lower SWIR reflectance observed in poorly humified peats, especially as poorly humified peats in the study area were associated with wetter sites.