Geochemical and ecological significance of soil lipids under Rhododendron ponticum stands

Abstract

The bio-geographical significance of Rhododendron ponticum spp. baeticum (Ericaceae) as a relict species is well recognized. However, out of its native habitat it is an invasive exotic considered a major threat to natural ecosystems in areas of Atlantic Western Europe. The studies on the impact of Rhododendron influence on soil organic matter composition and associated ecological implications, i.e. presence of bioactive compounds with ecological significance, are limited. This work describes the soil lipid assemblage in three sites under Rhododendron stands and adjacent sites with deciduous oak (Quercus canariensis), both in their native habitats in Southern Spain (Sierra de Luna, Cádiz). The results are discussed in terms of organic matter dynamics and the presence of molecules that may be associated with Rhododendron invasive success. The soils are acid Xerochrepts formed on siliceous sands. Composite soil samples were taken at two depths (0-10 cm and 10-20 cm) and soxhlet extracted with a dichloromethanemethanol mixture (3:1). Soil lipid assemblage was studied by GC/MS after fractionation and appropriate derivatization of extracts. The qualitative chemical composition of soil extractable lipids under Rhododendron is reported here for the first time. Our results show that soil n-alkane and fatty acid distributions are compatible with an input from plant epicuticular waxes, as well as with the occurrence of selective preservation of long-chain fatty acids with depth. The pattern of short-chain n-alkanes found in surface samples indicates an anthropogenic contamination threat from nearby industrialized areas of ''Campo de Gibraltar''. The presence of branched iso and anteiso C 15 and C 17 , b-hydroxy fatty acids and the sterol brassicasterol points to high microbial soil activity. Finally, the pentacyclic triterpenes taraxerone and taraxerol were detected in soils with Rhododendron but not with Quercus. These are known bioactive plant compounds and could be related with the effectiveness of Rhododendron as an invasive exotic species.

Early response of herbaceous vegetation toRhododendron ponticumsubsp.baeticuminvasion in European Atlantic forests

“…Though R. ponticum can grow over a wide range of soils, it mostly thrives on podzolic sandy soils (Rotherham, 1983) while it poorly performs on brown soils (Esquivia, 1993). However, R. ponticum is well known for its recalcitrant, acidifying litter (Horuz et al, 2014; Jones et al, 2019), which also contains allelopathic compounds (Simons, 1988; González‐Pérez et al, 2011) that strongly reduce soil microbial activity (Sariyildiz & Küçük, 2009) and soil invertebrates (Malo et al, 2012). Due to deep‐shade conditions below R. ponticum canopies and the release of allelopathic compounds by the decaying litter, litter decomposition may be considerably slowed down.…”

Spatial variability of soil lipids reflects vegetation cover in a French peatland

“…Several studies showed a link between biomarkers imprints in soil and plant diversity in the surrounding (Marseille et al, 1999;Trendel et al, 2010;González-Pérez et al, 2011;Zocatelli et al, 2012;Lavrieux et al, 2012;Ronkainen et al, 2013). In addition, the analysis of soil lipids revealed that biomarker imprints of soils can distinguish between pasture/grassland and forest (Zocatelli et al, 2012) and that the soil chemical legacy of former land uses can be preserved for decades (Lavrieux et al, 2012).…”