1. INTRODUCTION
Desert
ecosystems, the arid areas (approximately 45.6 million
km2, covering at least 35% of Earth’s land surface)
most environmental sensitive to changes in climate and human
disturbances, are suffering from increasing land degradation and
biological diversity loss (Bastin et al., 2017; Schimel, 2010; Wang et
al., 2019a). In arid and semiarid regions,
fragile
steppe grassland ecosystems are especially prone to desertification,
which results in the destruction of plant communities, soil physical
texture, and nutrient losses. Restoring and rehabilitating
desertification grassland has been a serious issues in ecology
(D’Odorico et al., 2019). Soil microorganisms have a predominant
potential to reveal and regulate the changes in soil ecosystem functions
and services, so characterizing the microbial successional patterns and
their interactions with plants and soils is essential for increasing our
understanding of the mechanisms of ecological restoring and
rehabilitating, improving our capacity to predict the responses of
ecosystems to human disturbance, and optimizing the design of
large-scale restoration projects.
Soil multifunctionality has been widely used to quantify and compare the
ability of various ecosystems to support overall functionality (Manning
et al., 2018). Numerous studies have widely emphasized the significance
of multifunctionality in superficial soils (top ~20 cm)
due to their predominant ecological importance (Delgado-Baquerizo et
al., 2017; Delgado-Baquerizo et al., 2016; Wagg et al., 2014; Wagg et
al., 2019; Zheng et al., 2019), where the density, activity, and
diversity of microorganisms are higher than those in deep soils (i.e.,
below 20 cm) (Fierer et al., 2003). However, due to the large bulk of
soil profiles, deep soils may accumulate abundant soil nutrients,
leading to an enhanced ability to provide multifunctionality during
ecosystem development. For example, compared to superficial soils, deep
soils in typical forest and grassland ecosystems can store over
two-thirds the organic carbon and nearly equal amounts of phosphorus due
to the long-term input from plant roots and soil leaching (Balesdent et
al., 2018; Upton et al., 2020; Zhou et al., 2018). Indeed, increasing
evidence has confirmed that a large percentage of soil
multifunctionality is hidden in deep soils (Jiao et al., 2018; Upton et
al., 2020). Intriguingly, in desert ecosystems, the desertification
process can result in a decrease in the water-holding capacity of
superficial soils and a steady increase in the soil leaching capacity
(D’Odorico et al., 2013; D’Odorico et al., 2019; Neilson et al., 2017),
leading to the enhanced downward elemental (i.e., carbon (C), nitrogen
(N), and phosphorus (P)) transfer responsible for the persistent
accumulation of nutrients in deep soils. Thus, it is expected that deep
soil multifunctionality in desert ecosystems plays increasingly
important roles in regulating and buffering overall ecological service
functions.
Soil
microbial diversity and community composition drive and determine soil
multifunctionality in various ecosystems (Delgado-Baquerizo et al.,
2016; Wagg et al., 2014; Zheng et al., 2019). Different influences of
biodiversity and community composition on soil multifunctionality have
been investigated (Delgado-Baquerizo et al., 2017; Wagg et al., 2014).
For example, the bacterial and fungal diversity in superficial soils
(top ~20 cm) positively influenced multifunctionality
(Wagg et al., 2019), especially in arid ecosystems (Delgado-Baquerizo et
al., 2016), while particular taxa of bacteria and fungi but not their
total richness and abundance determined the resistance of superficial
soil multifunctionality in arid ecosystems (Delgado-Baquerizo et al.,
2017). Furthermore, although such vast soil microbes in superficial
soils may play a central role in driving nutrient turnover with
supportive effects on soil multifunctionality, microbial diversity and
particular microbial attributes in deep soils share indispensable
ecological drivers (Delgado-Baquerizo et al., 2019; Jiao et al., 2018).
Indeed, deep soil contains a significant portion (35% ~
50%) of total microbial diversity and biomass, suggesting its
non-negligible ecological functions (Eilers et al., 2012). Increasing
evidence, based mostly on results from ecosystems with typical
development (reforested ecosystems) and grassland ecosystems with high
rainfall, suggests that particular microbial attributes (i.e.,
individual species, particular communities and their cooperation) rather
than alpha-diversity levels (i.e., total abundance, species richness,
and the Shannon index) may play more
important roles in driving deep soil nutrient (i.e., C, N, and P)
cycling with direct feedback effects on soil multifunctionality (Jiao et
al., 2018; Upton et al., 2020), as particular microbial attributes
compose significant regulators of microbial growth and interactions and
predominant functional attributes (e.g., soil C cycling or N
mineralization) (Delgado-Baquerizo et al., 2017). In contrast to the
above ecosystems with typical development and less drought stress,
ecosystems undergoing desertification display enhanced environmental
stress gradients (i.e., decreasing plant productivity and soil nutrients
and increasing soil leaching), while the dynamics of soil microbiomes
and their contributions to regulating deep soil multifunctionality
remain largely unexplored, particularly the contributions of particular
microbial attributes.
Herein, we assess the importance of vertical soil microbiomes and their
contributions to multifunctionality during the desertification process
in desert ecosystems. We used three sites in different typical
desertification stages (potential, moderate, and severe desertification)
that represent the process of desertification. The multifunctionality
and diversity of soil bacteria, fungi, and archaea were investigated at
soil depths of 0-100 cm. We tested the hypotheses that (1)
multifunctionality
in
deep soils (20-100 cm) would, at least partially, be equally as
important as that of superficial soils (0-20 cm)
in
the desert ecosystem and (2) the combined effects of the particular
species richness of soil microbiomes would better predict
multifunctionality in desert ecosystems, especially in deeper soil
profiles.