Hydrologic influence on the δ13C variation in long chain n-alkanes in the Dajiuhu peatland, central China

Abstract

Symbiotic methanotrophic bacteria are prevalent on the stems or in the hyaline cells of Sphagnum spp. in peat-moss ecosystems and can contribute a significant amount of isotopically light carbon for moss photosynthesis. Despite this, the influence of the methanotrophs on the δ13C values of Sphagnum-derived lipids is poorly known, especially in peatland lawn or hummock conditions that do not favor the growth of submerged Sphagnum. We have investigated the δ13C variation in individual long chain n-alkanes in samples of both fresh Sphagnum and surface peat collected under Sphagnum-dominated lawns in the Dajiuhu peatland, central China. The δ13C23 values of living Sphagnum negatively correlated with water level, suggesting that the carbon isotope fractionation of Sphagnum is mainly controlled by the diffusion resistance of CO2 in the hyaline cells rather than by the influence of symbiotic methanotrophs in this study area. In contrast, the δ13C values of n-C23 and n-C25 alkanes in the surface peat from two of the three sites correlated positively with water level, indicating the potential influence of symbiotic methanotrophic bacteria. The contrary results between peat and living Sphagnum probably result from surface peat receiving a more important contribution of n-alkanes from the lower parts than the middle and top parts of Sphagnum. In non-submerged conditions, the lower parts of Sphagnum normally have relatively high methane oxidation potential relative to the top parts. Our study reveals that the isotopic signals from symbiotic methanotrophs and the associated hydrological conditions can be preserved in peat deposits through the preferential accumulation of the lower parts of Sphagnum in peat layers.