Introduction
The impacts of invasive alien plants (IAPs) are generally considered context-specific and vary across species, regions, ecosystems and habitat types (Pyšek et al., 2020). In recent times, several studies have investigated the ecological impacts of IAPs in order to unravel the invasion patterns and the underlying processes (Vilà et al., 2011; Tekiela and Barney 2017; Uddin & Robinson 2017; Pyšek et al., 2020). Nonetheless, majority of these studies have focused on the impact of plant invasions on aboveground community characteristics and ecosystem processes (Ahmad et al., 2019b), while as the impacts of IAPs on belowground soil biotic and abiotic components are relatively less investigated, although known to play a pivotal role in the success of IAPs in non-native regions (Bell et al., 2020). Our limited understanding of microbial community response to IAPs in belowground soil system stems from the fact that soil microbial diversity and abundance is dependent upon a number of edaphic factors as well as the management history of the invaded landscapes, which has impeded our ability to generalize impacts of plant invasion on soil microbial communities and as such remains an important research gap in invasion biology (Custer & van Diepen 2020).
The soil microbial communities, being situated at the interface between plant-soil-atmosphere systems, are likely to play a crucial role in facilitating ecosystem response to the drivers of global environmental change, including invasive species (Bell et al., 2020). Invasive species, coupled with ongoing climate change, can disrupt the essential plant-microbe interactions from local to global scales (Cavicchioli et al., 2019, Rudgers et al., 2020). Therefore, a better understanding of climate-driven plant-microbe interactions over space, from individual plots to the entire landscapes is urgently required (Rudgers et al., 2020). Such studies, while considering plant-microbe interactions across broad spatial gradients (e.g. altitude), can aid in detecting potential nonlinear climate sensitivity functions and has the potential to provide a robust tool to predict the effects of increasing climate variability on belowground microbial communities (Cavicchioli et al., 2019; Rudgers et al., 2020). In an era of global environmental change, the belowground response to plant invasion is crucial because these soil microorganisms perform fundamental role in regulating key ecosystem processes, including the decomposition and nutrient cycling. In addition to these critical ecological processes, soil physicochemical properties determine the establishment and spread of IAPs (Nuñez et al., 2009; Dawson & Schrama 2016; Ricciardi et al., 2017). Given the crucial role that the unseen soil microbial communities play in regulating the ecosystem functioning and thereby facilitating and/or impeding the process of plant invasion, (Dawson & Schrama 2016; Ricciardi et al., 2017; Custer & van Diepen 2020) exploring the response of soil microbial communities to plant invasion merits adequate research attention.
Although, some research attention has been directed in the recent past to study the impacts of IAPs on soil biotic and abiotic communities and their potential feedbacks to the process of invasion (Dawson & Schrama 2016; Ricciardi et al., 2017; Custer & van Diepen 2020; Duchesneau et al., 2021), however the direction and magnitude of the associated changes are highly variable (Vilà et al., 2011; Zhang et al., 2019; Custer & van Diepen 2020). The differences in the response of soil biotic and abiotic communities to plant invasion may be an outcome of local environmental conditions, traits of the plant invader itself, and/or the interaction of both (Vilà et al., 2011; Hulme et al., 2013). Other than the vegetation type itself, the local micro-climatic conditions (e.g. temperature, precipitation) are considered to be the prime determinants of soil biotic and abiotic components (Massaccesi et al., 2020; Tang et al., 2020). Several environmental gradients like latitude and altitude (Cardelli et al., 2019) have been used to investigate the regional and global patterns in plant invasion impacts. Altitude has been often used as a useful proxy to unravel the effect of micro-climatic changes along environmental gradients on belowground soil biotic and abiotic components (Massaccesi et al., 2020; Tang et al., 2020). As invasion success of IAPs at high altitudinal ranges is attributed to broad climatic tolerance combined with enough residence time in a given region, mountains are more prone to invasions with rapid spread of these species under ongoing climate change (Pauchard et al., 2009; Pyšek et al., 2011). The altitudinal gradients in mountainous landscapes provide the natural experimental systems to investigate the patterns and processes of IAPs across multiple sites within a shortest spatial distance (Ahmad et al., 2019a, b). Such type of studies along increasing altitudinal gradient can provide novel insights regarding the expected up-climbing of IAPs in response to ongoing climate warming in mountains (Lamsal et al., 2018; Thapa et al., 2018). Towards this end, the recently emerging metagenomic tools offer opportunities to characterise soil microbiome at an unprecedented scale and taxonomic resolution (Fricker et al., 2019). Such an approach can be an important step forward in understanding the structural, functional and interaction aspects of this complex belowground soil system, and has huge implications in developing scientifically well-designed and integrated management strategies to restore the invaded landscapes (Ricciardi et al,. 2017; Custer & van Diepen 2020; Tang et al., 2020; Rudgers et al., 2020).
In recent times, rapid land-use and climate changes in mountains have made them highly vulnerable to the risks of plant invasions (Lamsal et al., 2018; Thapa et al., 2018; Pathak et al., 2019). The Himalaya – one of global biodiversity hotspot – is increasingly experiencing the introduction and invasion of alien plants with severe ecological and economic impacts (Ahmad et al., 2019a, b; Lamsal et al., 2018; Thapa et al., 2018; Pathak et al., 2019). Kashmir Himalaya –an important region of the Himalayan biodiversity hotspot – is recently experiencing a rising risk from the spread of IAPs (Khuroo et al., 2010; Ahmad et al., 2019a,b). One such invasive plant is Leucanthemum vulgare Lam. (Ox-eye daisy), which is fast invading the high-altitude landscapes of this Himalayan region and has reported to impact the native biodiversity, community diversity and soil system (Khuroo et al., 2010; Ahmad et al., 2019a, b, 2021).
The present study, employing metagenomic tools, investigated the composition and abundance patterns of soil microbiome (bacterial and fungal communities) between invaded and uninvaded plots at multiple sites selected along an altitudinal gradient in the Kashmir Himalaya using L. vulgare as a model invasive plant species. Simultaneously, we also studied soil nutrient pools between the invaded and uninvaded plots to explore the possible role of nutrients in shaping bacterial, fungal and L.vulgare invasion patterns along the altitudinal gradient. More specifically, we aimed to address the following research questions: (i) Does the species composition and relative abundance of soil microbiome (bacterial and fungal communities) differ consistently between the invaded and uninvaded plots across the multiple sites along the altitudinal gradient in Kashmir Himalaya? (ii) Do the patterns of alpha and beta diversity in soil microbial communities differ between invaded and uninvaded plots across the sites along the altitudinal gradient? (iii) Whether invasive L. vulgareexerts a consistent effect on key soil nutrient pools along the altitudinal gradient? (iv)Which environmental and soil physicochemical factors best explain the abundance patterns of soil microbiome between invaded and uninvaded plots along the altitudinal gradient? Considering plant invasion as one of the major drivers of global environmental change, and with soil microorganisms occupying a pivotal position in the terrestrial habitats, the present study while addressing the afore-mentioned research questions will be helpful in devising efficient management policies on plant invasions and effective restoration of invaded landscapes.