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.