A molecular approach to the assessment of peat organic matter – investigating ecosystem-driven differences in chemical composition

Peatlands contain massive stores of organic carbon (OC), which are vulnerable to loss following drainage and aerobic decomposition. To protect these sensitive C stores in a warming world, research has been increasingly invested in strategies for peatland restoration. While restoration projects can have many aims (re-establishment of biodiversity, biogeochemical function, etc.), baseline measurements and monitoring targets are greatly needed – particularly for rewetting projects seeking to restore peatland C sequestration services (i.e., an excess of net primary productivity relative to decomposition in the ecosystem).
The main objectives of this thesis were to investigate changes in peatland decomposition status through the use of different biogeochemical analysis tools. This objective was achieved through the identification of differences in peat organic matter (OM) chemical composition with changing hydrological status and depth – primarily using pyrolysis gas chromatography mass spectrometry (Py-GC/MS) chemical characterization techniques. This research thus describes the use of high-resolution Py-GC/MS “fingerprinting,” as an indicator for degradative conditions.
Before the experimental objectives were approached, the versatility and utility of the method were investigated. As no comprehensive compilation of Py-GC/MS studies on peat had previously been done, an extensive review was conducted of previous work using the method for peat OM chemical characterization. The review investigated peatland plant and microbial biomarkers analyzable by Py-GC/MS, as well as the chemical environment where typical OM components and proxies might occur within the ecosystem – with a focus on natural vs drained peat. It was noted that while plant biomarkers were well represented in the literature, microbial biomarkers were more limited.
Further, to confirm whether Py-GC/MS OM characterizations are a useful proxy for overall peat composition, peat samples were first investigated exclusively by pyrolysis to determine to what degree the volatilized OM entering the analysis instrument (GC/MS) was representative of the original sample. This study found that percent OM volatilized (pyrolysis efficiency) by Py-GC/MS was highly reproducible, and relatively insensitive to changing parameters such as instruments, settings, sample mass or composition (from an organic carbon standpoint). Moreover, a reference baseline for successful pyrolysis efficiency (in terms of percent OM volatilized successfully onto the analytical instrument) was established for researchers using the method for peat.
For the primary experimental objective, two peatland research sites were selected, each with contrasting hydrological management occurring in the same ecosystem (Lakkasuo drained and undrained; Degerö Stormyr undrained and rewetted). OM composition was investigated using Py-GC/MS pyrolysis products in both a broader exploration of its composition (through the relative abundance of OM chemical classes such as phenols, polysaccharides, lipids, N compounds, etc), and by a more targeted biomarker approach (e.g., lignin pyrolysis products and the Sphagnum biomarker p-isopropenylphenol). Principal component analysis separated the complex dataset into different environmental influences on peat OM chemistry.
In Lakkasuo, peat drainage for forestry in 1961 resulted in significant differences in peat OM chemical composition in the drainage-affected cores. Decreased relative abundance in phenols and polysaccharides were reflective of the aerobic decomposition of OM originating from previously deposited Sphagnum, and increased abundance of guaiacyl-derived lignin, N compounds, C-29 triterpenoid sterols, and sesquiterpenes were reflective of the Pinus silvestris onsite. In the Degerö rewetted site, the high phenol abundance in the surface peat corresponded well with the established Sphagnum riparium onsite, indicating that phenols are an early sensitive indicator for rewetted conditions in Sphagnum peatlands. However, altered chemical composition remained visible in the rewetted site both in surface samples (continued increased abundance in lignin, N compounds, benzenes, lipids, and polyaromatic hydrocarbons (PAHs) – despite the shift in vegetation composition), and in the formerly drained parts of the profile. Peat OM recovery after rewetting may therefore continue to be subject to residual impacts originating from the (permanently) degraded OM below – underscoring the importance of protecting this vulnerable OM from further decomposition.
Overall, this work demonstrates not only that peat OM chemical composition is an effective tool to assess degradative status (and is thus a good proxy for its estimation), but also that hydrology serves as a strong driver for changes in peat chemical composition. The sensitivity of the Py-GC/MS method permits meaningful changes to be detected in OM compounds that are both reflective of changes in environmental influences and also present in very low abundance - allowing for improved source identification and discernment of dominant degradation mechanisms. It is expected that these findings will be particularly useful to those seeking to identify indicators in ecosystems most likely to benefit from rewetting mitigation practices.