The rhizobiome of herbaceous plants in Clovelly and Sterkspruit soils of the Stevenson–Hamilton supersite

The rhizosphere is characterised by high microbial activity and an array of complex and dynamic physical, chemical and biological interactions termed the ‘rhizosphere effect’ (Helliwell et al. 2019; Hiltner 1904). These interactions play a crucial role in plant nutrition (Raaijmakers et al. 2009; Sørensen 1997) and ecosystem functioning in terrestrial ecosystems (Singh et al. 2004). Microbes associated with the rhizosphere (i.e. the rhizosphere microbiome or rhizobiome) represent a vast reservoir of microbial diversity that includes fungi, bacteria, nematodes, protozoa, algae, viruses and arthropods (Berg & Smalla 2009; Bonkowski et al. 2000; Bonkowski, Villenave & Griffiths 2009; Bulgarelli et al. 2013). Plant species actively determine the composition of their rhizobiome by secreting root exudates that attract or repel specific bacteria from the surrounding bulk soil (Berg & Smalla 2009; Doornbos, Van Loon & Bakker 2012). The composition of the rhizobiome is thus determined by numerous edaphic variables of the immediately surrounding bulk soil such as the soil mineralogy, soil moisture content and pH (Bardgett et al. 2005; Fierer & Jackson 2006; Myers et al. 2001). However, comparative studies on the effect of specific edaphic variables on the composition of the rhizobiome are rare because of a lack of suitable study sites where the influence of different soil types on the same plant species in close proximity can be compared effectively.