Changes in Peat Humification due to Permafrost Thaw and Plant Succession

Abstract:

The potential release of carbon from thawing permafrost peatlands is a major global change uncertainty. In addition to releasing old carbon from permafrost, thaw can also induce changes in hydrology that affect the aboveground plant community, leading to changes in litter quality and organic matter degradability. Stordalen Mire is a peat plateau in northern Sweden where permafrost thaw has led to land subsidence and inundation, causing dry palsas with intact permafrost to be replaced by Sphagnum-dominated bogs followed by sedge-dominated fens. In this study, we examined trends in solid phase peat humification along this permafrost thaw succession using a combination of C/N ratios and Fourier transform infrared (FTIR) spectroscopy. C/N ratios decreased with depth in all sites except fens, likely due to N immobilization during decomposition. In fens, depth trends in C/N ratios were complicated by the presence of Sphagnum-derived peat at depth. However, %N (by weight) at all sites was positively correlated with peat humification indices (HI), which indicate the degree of decomposition and are defined based on FTIR spectra as the ratios of absorbance at wavenumbers 1515, 1630, 2850, and 2920 cm⁻¹ (aromatics and aliphatics) to the absorbance at 1030 cm⁻¹ (polysaccharides). Each of these HI was inversely related to the prevalence of the carboxylic acid peak at 1720 cm⁻¹, and this trend became weaker along the permafrost thaw gradient. This result suggests that decomposition at the early thaw stages is inhibited by organic acids, but this effect becomes less significant as thaw-induced plant succession leads to lower acidity. All of these trends in HI were strongest for the HI defined at 1630 and 1515 cm⁻¹ (representing aromatics), indicating that peat humification at Stordalen is primarily characterized by relative increases in aromatic compounds compared to carbohydrates, with less pronounced relative increases in lipids and other aliphatics (2850 and 2920 cm⁻¹).