Root-associated branched tetraether source microorganisms may reduce estimated paleotemperatures in subsoil

• Branched GDGTs analyzed in root systems from Hungarian loess and sand profiles
• Branched GDGT source microorganisms closely associated with the root surface
• Deep-rooting plants in subsoil may reduce GDGT-based temperature estimates
• Sample pre-treatment can have an influence on GDGT-derived parameters

Branched glycerol dialkyl glycerol tetraethers (GDGTs) are complex lipids of high molecular weight, recently discovered in soils and suggested to be produced by still unknown bacteria. The relative distribution of these compounds was shown to depend on environmental parameters, mainly temperature and pH. Over the last years, an increasing number of studies have focused on the application of branched GDGTs as paleoclimate proxies, but only a few were performed in terrestrial archives. In this study, branched GDGTs were analyzed in calcified and non-calcified living and dead roots, in surrounding soil and sediment and in reference material distant from the roots. Samples were mainly collected from subsoil of two forest sites near Sopron (Hungary), where soils developed on fluvial sand and loess deposits, respectively. Branched GDGTs were more abundant in root samples and/or surrounding rhizosphere compared to reference material, suggesting that branched GDGT source microorganisms are closely associated with the root surface. In subsoil, the GDGT-based temperature estimates from former roots and surrounding sediments were mainly lower than those from reference material in both loess and sand profiles. This is likely due to the post-sedimentary incorporation of branched GDGTs deriving from microorganisms that fed on root organic matter in terrestrial sediments. In contrast, branched GDGT-derived temperatures do not seem to be influenced by the presence of roots in topsoil, which may be related to the much higher density of recent roots in topsoil than in subsoil. This argues for a more homogeneous distribution of root-associated microorganisms especially in densely rooted topsoils. In addition, we show that sample pre-treatment may have an effect on the abundance and distribution of branched GDGTs. Indeed, washing root samples with ultrapure water might lead to a decrease in GDGT abundance and an increase in temperature estimates, likely due to the removal of particles adhering to the root surface. Decarbonatization of root and surrounding sediment had only a limited effect on GDGT-derived parameters. Taken together, these results suggest that paleoenvironmental data obtained from branched GDGTs in terrestrial archives might depend on the way the samples were collected and prepared and should be interpreted with caution, especially in loess-paleosol sequences where the frequency of calcified roots can be locally very high.