Stable Isotopes (13C, 15N) and biomarkers as indicators to assess drainage history of European peatlands

Degradation of peatlands by land use changes and anthropogenic climate change produces climate-active gases, reduce biodiversity and flood prevention functions. Therefore, substantial effort is put into peatland restoration projects. However, the methods that are currently available to assess peatland hydrology are expensive and time consuming and require expert knowledge.
The aim of this thesis was to establish a cost- and time-efficient approach to assess peatland hydrology (undrained, drained, rewetted). We used a combination of stable isotopes (13C, 15N) and microbial-derived membrane fatty acids (mFAs). Microbial communities and substrate cycling adapt to a changing hydrology. These activities should be imprinted in stable isotope bulk values as a result of specific isotopic fractionation by different microbial groups, their metabolic pathways, and nutrient sources. As the measurement of stable isotopes is a routine technique today, it could act as a tool to efficiently obtain reliable information about the hydrological regime.
In a first study, we investigated five nutrient-poor peatlands. Here we hypothesized that typical depth patterns of stable isotopes (13C, 15N) exist and that they differ significantly depending upon hydrological regime. We found for all drained sites a distinct peak (“turning point”) of the δ15N bulk values in the center of the drained layer. To support our results and link them to specific microbial groups (fungi, bacteria), we conducted a mFA analysis on a few samples. Our results suggest a switch of fungal- to bacterial-forced metabolism in the drained layers. This switch is reflected by the δ 15N depth trend. The highest diversity of microbial-derived mFAs was indicated by the δ 15N turning point. Below the δ 15N turning point, oxygen is increasingly limited and concentrations of all microbial-derived mFAs decreased down to the onset of the permanently waterlogged anaerobic layer. Hence, we concluded that δ 15N stable isotope bulk values reflect microbial community composition, which differs between undrained and drained peatlands.
In the second study, we performed two sampling campaigns in two nutrient-poor peatlands to investigate (i) how microbial - especially bacterial - groups are shifting with changing hydrology and how they are linked to stable isotope depth pattern (δ 15N, δ13C) and (ii) whether rewetting is also imprinted in mFA quantities and stable isotope bulk values. We integrated a mFA analysis related to various microbial groups that are common in peatlands with stable isotope bulk values. Under waterlogged conditions, overall levels of microbial-derived mFAs were low. Drained layers showed simultaneous changes in mFA quantities and stable isotope bulk values. We found decreasing fungal-derived mFA quantities and increasing bacterial-derived, particularly acidobacterial-derived, quantities with depth. Interestingly, cores from recent rewetted peatlands show no depth trend of δ15N in the layers grown under rewetting conditions; this is congruent with relatively low microbial-derived mFA quantities. Hence, we concluded that stable isotope bulk values, especially δ15N, reflect changing microbial metabolic processes, which differ between drained and undrained - and especially for rewetted - peatlands.
In a third study, we sampled nutrient-rich fens in an east-west transect across Europe (14 peatlands; Belgium to Poland) and conducted the same stable isotope (δ13C, δ15N) and combined mFA analysis that we used previously in nutrient-poor sites. The aim was to prove that the influence on isotopic fractionation by microbial communities and their metabolic pathways also exists in nutrient-rich fens. We found also for these sites’ consistent changes in microbial-derived mFA quantities and stable isotope bulk values corresponding to the hydrological regime. In the uppermost layers, the highest quantities of microbial-derived mFAs were measured in undrained sites, and the lowest were found in drained sites. Fungal-derived mFA quantities were especially decreased in drained sites and deeper layers. Simultaneously, δ15N stable isotope bulk values were the highest in drained sites and in the uppermost layer and lowest in undrained sites and deeper layers. These trends contrast with what we found previously in nutrient-poor peatlands. We hypothesized that this discrepancy is due to the higher nutrient levels and, therefore, difference in the fungal abundance. For δ13C bulk values the patterns were similar to those of nutrient poor sites. The δ13C bulk values for all hydrological regimes increased with depth, especially in drained sites. In rewetted sites, the mFA quantities and stable isotope bulk values shifted from values similar to drained sites (less than 10 years of rewetting) toward the values of undrained sites (more than 25 years of rewetting). We concluded that stable isotope bulk values reflect specific microbial metabolic processes that differ with hydrological regime and, thus, could signal both drainage and rewetting also in nutrient-rich fens.
The findings of this thesis enable us to obtain reliable information on the hydrological regime in a more cost- and time-efficient manner with the help of stable isotope measurements.