Comparison of different methods to determine the degree of peat decomposition in peat bogs

Abstract

Abstract. Peat humification or decomposition is a frequently used proxy to extract past time changes in hydrology and climate from peat bogs. During the past century several methods to determine changes in peat decomposition have been introduced. Most of these methods are operationally defined only and the chemical changes underlying the decomposition process are often poorly understood and lack validation. Owing to the chemically undefined nature of many humification analyses the comparison of results obtained by different methods is difficult. In this study we compared changes in peat decomposition proxies in cores of two peat bogs (Königsmoor, KK; Kleines Rotes Bruch, KRB) from the Harz Mountains (Germany) using C / N ratios, Fourier transform infrared spectra absorption (FTIR) intensities, Rock Eva® oxygen and hydrogen indices, δ13C and δ15N isotopic signatures and UV-absorption (UV-ABS) of NaOH peat extracts. In order to explain parallels and discrepancies between these methods, one of the cores was additionally analysed by pyrolysis gas chromatography mass spectrometry (pyrolysis-GC-MS). Pyrolysis-GC-MS data provide detailed information on a molecular level, which allows differentiation of both changes attributed to decomposition processes and changes in vegetation. Principal component analysis was used to identify and separate the effects of changes in vegetation pattern and decomposition processes because both may occur simultaneously upon changes in bog hydrology. Records of decomposition proxies show similar historical development at both sites, indicating external forcing such as climate as controlling the process. All decomposition proxies except UV-ABS and δ15N isotopes show similar patterns in their records and reflect to different extents signals of decomposition. The molecular composition of the KK core reveals that these changes are mainly attributed to decomposition processes and to a lesser extent to changes in vegetation. Changes in the molecular composition indicate that peat decomposition in the KK bog is mainly characterized by preferential decomposition of phenols and polysaccharides and relative enrichment of aliphatics during drier periods. Enrichment of lignin and other aromatics during decomposition was also observed but showed less variation than polysaccharides or aliphatics, and presumably reflects changes in vegetation associated with changes in hydrology of the bogs. Significant correlations with polysaccharide and aliphatic pyrolysis products were found for C / N ratios, FTIR-band intensities and for hydrogen index values, supporting that these decomposition indices provide reasonable information. Correlations of polysaccharide and aliphatic pyrolysis products with oxygen index values and δ13C was weaker, assumingly indicating carboxylation of the peat during drier periods and enrichment of isotopically lighter peat components during decomposition, respectively. FTIR, C / N ratio, pyrolysis-GC-MS analyses and Rock Eval hydrogen indices appear to reflect mass loss and related changes in the molecular peat composition during mineralization best. Pyrolysis-GC-MS allows disentangling the decomposition processes and vegetation changes. UV-ABS measurements of alkaline peat extracts show only weak correlation with other decomposition proxies and pyrolysis results as they mainly reflect the formation of humic acids through humification and to a lesser extent mass loss during mineralization.

Peat decomposability in managed organic soils in relation to land use, organic matter composition and temperature

“…The H / C and O / C ratios in forest topsoils were lower than of those under cropland and grassland and did not change with depth. Interpreting these lower H / C and O / C ratios in the forest topsoils as indicators of more advanced peat decomposition (Klavins et al, 2008;Leifeld et al, 2012;Biester et al, 2014;Wüst-Galley et al, 2016) would be in contradiction to our conjecture that land management effects on peat decomposition, revealed by SOC, bulk density and C / N ratio, are less pronounced for forests. We rather argue that the reason for the low H / C and O / C ratio in the forest soils is a higher abundance of lignin rich (wood derived) plant residues.…”

“…Lignins and polyphenols have molar O / C ratios in the range of 0.2-0.6 and H / C ratios between 0.9 and 1.5, while the respective ratios of carbohydrates range from 0.8 to 0.9 for O / C and from 1.4 to 1.8 for H / C (Kim et al, 2003). In line with the molecular and spectroscopic analyses mentioned before, both ratios were found to decrease with increasing depth in peat (Klavins et al, 2008;Biester et al, 2014;Wüst-Galley et al, 2016). On the other hand, both fresh plant residues and undisturbed peat usually have high C / N ratios (Loisel et al, 2012).…”

“…a lower intrinsic decomposability of soil organic matter (SOM) (Hogg et al, 1992;Scanlon and Moore, 2000;Wang et al, 2010;Hardie et al, 2011;Leifeld et al, 2012). Using solid-state 13 C-NMR, DRIFT/FTIR spectroscopy and pyrolysis-GC/MS, various studies of OM composition of undisturbed peat profiles have shown a gradual change with increasing depth towards a relative enrichment of compounds that are recalcitrant against decomposition under anoxic conditions, such as lignins and polyphenols (Freeman et al, 2004), while the contents of labile oxygen-rich compounds, such as polysaccharides, were found to decrease (Leifeld et al, 2012;Biester et al, 2014;Sjögersten et al, 2016).…”

Long term change in chemical properties of preindustrial charcoal particles aged in forest and agricultural temperate soil

“…The signal from inorganic soil components overlapped with peaks of small intensity corresponding to cellulose, carbohydrate and polysaccharide (at ca. 1200, 1030 and 890 cm À1 ; Biester et al, 2014).…”

“…Spectra were normalized on the basis of maximal deviation. Attribution of organic and inorganic components to bands was based on Lehmann and Solomon (2010) and Biester et al (2014).…”