Switch of fungal to bacterial degradation in natural, drained and rewetted oligotrophic peatlands reflected in <i>δ</i><sup>15</sup>N and fatty acid composition

Abstract

Abstract. For centuries European peatlands have been degrading along with drainage, land use and climate changes. Increasing pressure on peatland ecosystems calls for a more cost-efficient method to indicate the current state of peatlands and the success of restoration efforts. Metabolic pathways in peatland soils are imprinted in stable isotope compositions due to differences in microorganism communities and their metabolic pathways. Therefore, we hypothesize that depth profiles of nitrogen stable isotope values provide a promising opportunity to detect peatland decomposition or restoration. We studied five peatlands, namely Degerö Stormyr (northern Sweden), Lakkasuo (central Finland) and three mires in the Black Forest (southern Germany). At all locations, cores were taken from adjacent drained (or rewetted) and natural sites to identify δ15N trends that could indicate changes due to drainage and restoration. At all drained (and rewetted) sites we found a distinct peak (“turning point”) of the δ15N values in the center of the drained horizon. We did a fatty acids (FAs) analysis to link our results to microbial community composition. As markers, we distinguished between one fungal-derived FA (C18:2ω9c) and four bacterial-derived FAs. For bacteria, we looked for one general bacterial-derived FA (C14:0), two FAs for gram-positive bacteria (i-C15:0; a-C15:0), and one FA for gram-negative bacteria (C16:1ω9c). In accordance with other studies, our results suggest that fungi dominate the microbial metabolism in the upper aerobic peat horizon. This is reflected by depleted δ15N values. Moving downwards, the drained horizon conditions slowly switch to oxygen limitation. Consequently, fungal-derived FAs decrease whereas bacterial-derived FAs rise. The highest diversity of microbial-derived FAs is indicated by the δ15N turning point. Below the δ15N turning point, oxygen is increasingly limited and concentrations of all microbial-derived FAs are decreasing down to the onset of the permanently waterlogged anaerobic horizon. Peatland cores with restoration successes again show, above the formerly drained horizon, no depth trend of the isotopic values. Hence, we conclude that δ15N stable isotope values reflect microbial community composition, which differs between drained and natural peatlands.

Rewetting and Drainage of Nutrient-Poor Peatlands Indicated by Specific Bacterial Membrane Fatty Acids and a Repeated Sampling of Stable Isotopes (δ15N, δ13C)

“…For the rewetted site the main moss species is Sphagnum balticum (Nielsson et al, 2008). The water table is near the surface, HI is low (H2) and macro-residuals are preserved well in the upper horizon (Table 1; Groß-Schmölders et al, 2020). In contrast the values of the former mesotelm indicate degradation: HI is higher (H3), less macro-residuals are visible, CN decreased to 41 and the BD increased (0.06 kg m −3 ; Table 1; Groß-Schmölders et al, 2020).…”

“…In the undrained part the main moss species is Sphagnum majus Nielsson et al (2008) and the water table is near the surface (Table 1). The humification index (HI) after von Post is low (H1-H2) and macro residuals are highly visible (Table 1; Groß-Schmölders et al, 2020). Also, biochemical parameters indicate undrained conditions.…”

“…Also, biochemical parameters indicate undrained conditions. The carbon:nitrogen ratio (CN) in the acrotelm is 89, and the bulk density (BD) is low (0.02 kg m −3 ), both is typical for undrained nutrient poor peatlands (Table 1; Groß-Schmölders et al, 2020). For the rewetted site the main moss species is Sphagnum balticum (Nielsson et al, 2008).…”

“…Stable isotopes of carbon and nitrogen are known indicators of peatland hydrology (Alewell et al, 2011;Krüger et al, 2016;Groß-Schmölders et al, 2020;Kohl et al, 2015). For δ 13 C, Alewell et al (2011), Krüger et al (2016), Novak et al (1999), Hobbie et al (2017) and Biester et al (2014) report an enrichment of 13 C with depth due to an increasing degree of organic matter decomposition.…”

“…We define a sudden directional change in the stable isotope depth patterns as "turning points," according to Alewell et al (2011) and Groß-Schmölders et al (2020). The δ 15 N turning point is located in the middle of the mesotelm, where δ 15 N values are highest.…”