Microtopography matters for CH₄ formation in a peat soil: a combined inhibitor and ¹³C study

Abstract. Abstract. Peatlands’ microtopography units – hummocks and hollows – are mainly differing by hydrological characteristics (water table level, i.e. oxic-anoxic conditions) and vegetation communities. These factors affect the fluxes of key greenhouse gases (GHG) – methane (CH₄) and carbon dioxide (CO₂). However, the effects of microrelief forms on belowground CO₂ and CH₄ production and pathways of methanogenesis need deeper understanding. We hypothesized increasing CH₄ and CO₂ production potentials from naturally drier hummocks to more wet hollows during anaerobic incubation. GHG production in peat was expected to decrease with depth (decreasing inputs of recent plant-derived deposits) but the contribution of hydrogenotrophic vs. acetoclastic pathway to the total methanogenesis should be higher in deeper peat layers as compared to upper layers. To test the hypotheses, we measured CH₄ and CO₂ productions together with the respective δ¹³C values under controlled anaerobic conditions with- and without addition of specific inhibitor of methanogenesis (2-bromo-ethane sulfonate, BES) in a peat soil of hummocks and hollows of five depths (15, 50, 100, 150 and 200 cm). The concentration of BES (1 mM) aimed to block acetoclastic but not the hydrogenotrophic pathway of methanogenesis. As expected, CH₄ production was ca. 2 times higher in hollows than in hummocks, though no differences in CO₂ were measured between the microforms. With depth, CO₂ production rates decreased by 77 % (15 cm vs. 200 cm) in both microforms, whereby the highest CH₄ production was measured at 15 cm in hollows (91 % of total produced CH₄) and at 50 cm in hummocks (82 %). Noteworthy, at 15 cm of hummocks less than 1 % of total CH₄ production was observed. Decreasing GHG production rates with depth positively correlated to an increase in the extractable total N and NH₄⁺ concentrations. The hydrogenotrophic pathway of methanogenesis in deep vs. surface layers was depicted by lower (more negative) δ¹³C-CH₄ and higher δ¹³C-CO₂ values, respectively. Between the microforms, overall higher contribution of hydrogenotrophic vs. acetoclastic methanogenesis corresponded to hollows as compared to hummocks. Contrary to the expectation, the addition of 1mM BES was not selective and inhibited both pathways of methanogenesis. Concluding, peatlands’ microrelief is an important factor regulating the GHG fluxes. However, the effects of microforms on the production of CH₄ and pathways of methanogenesis were pronounced for the upper 50 cm layer. Finally, inhibition with BES appeared to be less effective tool for the partitioning between pathways of methanogenesis as compared with the isotope method.