Chemotaxonomic patterns of vegetation and soils along altitudinal transects of the Bale Mountains, Ethiopia, and implications for paleovegetation reconstructions – Part 1: stable isotopes and sugar biomarkers

Abstract. Today, on the Sanetti Plateau in the Bale Mountains of Ethiopia, only fragmented patches of Erica species can be found at high altitudes (between 3900 and 4200 m a.s.l.). However, it is hypothesized that during the later part of the last glacial periodand the early Holocene the plateau was extensively covered by Erica shrubs. Furthermore, it is assumed that the vegetation was later heavily destroyed by human-induced fire and/or climate change phenomena. The objective of this study is to contribute to paleovegetation reconstructions of the Sanetti Plateau by evaluating the potential of stable isotopes ( δ13 C and δ15 N) and sugar biomarkers for distinguishing the dominant plant species, including Erica, and the soils below the plants. In a companion paper (Lemma et al., 2019a) we address the same issue by evaluating lignin-derived phenols and leaf-wax-derived n -alkane biomarkers. The stable carbon ( δ13 C) and nitrogen ( δ15 N) isotope values of the plant samples range from −27.5   ‰ to −23.9   ‰ and −4.8   ‰ to 5.1  ‰, respectively. We found no significant δ13 C and δ15 N differences between the dominant plant species. Mineral topsoils (A h horizons) yielded more positive values than plant samples and organic layers (O layers), which reflects mineralization processes. Moreover, the δ15 N values became generally more negative at higher altitudes. This likely indicates that the N cycle is more closed compared to lower altitudes. δ15 N maxima around 4000 m a.s.l. point to fire-induced opening of the N cycle at the chosen study sites. Erica species yielded the lowest overall total sugar concentration (ranging from 58 to 118 mg g −1 ), dominated by galactose (G) and mannose (M). By contrast, Festucaspecies revealed much higher total sugar concentrations ranging from 104 to 253 mg g −1 , dominated by the pentose sugars arabinose (A) and xylose (X). Although a differentiation between Erica versus Festuca, Alchemillaand Helichrysumis possible based on (G  +  M)  ∕  (A  +  X) ratios, Erica cannot be unambiguously distinguished from all other plant species occurring on the Sanetti Plateau. In addition, plant-characteristic (G  +  M)  ∕  (A  +  X) sugar patterns change during soil organic matter formation in the A h horizons. This can be likely attributed to degradation effects and soil microbial build-up of galactose and mannose. In conclusion, soil degradation processes seem to render sugar biomarker proxies unusable for the reconstruction of the past extent of Erica on the Sanetti Plateau, Bale Mountains, Ethiopia. This finding is of relevance beyond our case study.