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).