INTRODUCTION
Soil bacterial communities are highly diverse and are involved in many ecosystem processes (e.g., nutrient cycling, soil formation, plant health; Van Der Heijden et al., 2008). Our understanding of soil bacterial communities is increasing due in part to their links with human and ecosystem health (Liddicoat et al., 2019; Roslund et al., 2022; Singh et al., 2023) and advances in genomics (Berg et al., 2020; Liu et al., 2021). While it is understood that plants and soil bacteria have a close relationship, further research is required to fully understand the breadth of connections and mechanisms involved. Filling this knowledge gap will contribute to an improved understanding of soil bacterial ecology, with potential implications for improving ecosystem integrity and soil microbial monitoring approaches.
Land cover type (e.g., forests, lawns) has a strong effect on soil bacterial communities, as demonstrated in many studies where more natural land cover, such as remnant vegetation, has been compared to nearby human-dominated land cover types (e.g., urban parks, agricultural lands; Delgado-Baquerizo et al., 2021; Liddicoat et al., 2019; Wan et al., 2021). While studies have reported higher soil bacterial heterogeneity across samples from remnant vegetation compared to cleared or urban lawns (Delgado-Baquerizo et al., 2021) and the occurrence of notable bacterial functional groups in soils (e.g., butyrate-producers) (Liddicoat et al., 2020; Roslund et al., 2020), further research is needed to improve knowledge on how aboveground biodiversity and land cover types influence soil bacterial communities. Indeed, different land cover and vegetation communities may have subtle impacts on soil bacterial communities that have been largely overlooked.
Plants display diurnal cycles due to endogenous circadian clock genes and zeitgebers - external cues that regulate organismal processes to a circadian rhythm or light-dark cycle (from the German terms Zeit “time” + geber “giver”; Hörnlein & Bolhuis, 2021). While many circadian clock genes function independently of light, zeitgeberssuch as light or temperature are crucial for maintaining light-dark cycles (Hörnlein & Bolhuis, 2021; Staley et al., 2017). While some bacteria, such as cyanobacteria, display true circadian rhythms via circadian clock genes, many bacterial taxa do not possess these genes (Kondo et al., 1993). Despite this, all bacteria may still be subject to the influence of zeitgebers such as light, temperature, plant processes via plant-soil feedbacks, and bacteria-bacteria interactions (Hörnlein & Bolhuis, 2021; Kelly et al., 2019; Van der Meer et al., 2007). Plants and soils, for example, are intertwined primarily through the rhizosphere – the interface between roots and soil (Haichar et al., 2008). In often commensal relationships, plants make exudates available to bacteria (Badri & Vivanco, 2009; Haichar et al., 2008), while bacteria contribute to plants via increased access to soil nutrients and reduced presence of pathogenic bacteria, and these processes can fluctuate over short timeframes (Canarini et al., 2019; Doornbos et al., 2012). Therefore, while not all soil bacteria exhibittrue circadian rhythms, soil bacterial communities may undergo light-dark cycles in their community composition due to wider ecological processes and zeitgebers.
Light-dark cycles of soil bacterial communities have been studied in greenhouse experiments with model and agricultural plant speciesArabidopsis thaliana , rice, and barley (Baraniya et al., 2018; Lu et al., 2021; Staley et al., 2017). These studies have generally indicated that plant rhizodeposition of carbon and other exudates into the soil affects soil bacterial community growth, composition, and the expression of various genes over light-dark cycles (e.g., rhizodeposition is generally greater during daylight hours), but we found only one study on the effect of light-dark cycles on soil bacterial community composition outside of these model plant systems (Landesman et al., 2019). Improving our understanding of light-dark cycles in natural soil bacterial communities and their interactions has the potential to impact soil bacterial ecology as their composition may change in a time-dependent way.
Accordingly, we aimed to better understand how soil bacterial composition and network complexity change across different land cover types and light-dark cycles. First, we hypothesised that bacterial community composition, diversity (α- and β-diversity), and network complexity would be affected by land cover type. Second, we hypothesised that bacterial community composition (β-diversity and taxa abundances) and networks would be different depending on the time of day because of the influence of plant-soil feedback loops and the strong effect light-dark cycles have on plants (i.e., light and temperature).