Soil microbial communities shaped by plant diversity and drought
We observed pronounced impacts of plant species richness and soil
moisture treatments on community composition of soil bacteria and fungi,
indicating an effective conditioning process in Phase I. It has been
frequently observed that soil microbial diversity increases with plant
diversity, and the hypothesized mechanisms are that root exudates,
litter heterogeneity and microhabitats provide more niche opportunities
for soil microbes in more diverse plant communities (Youssef & Elshahed
2009; Millard & Singh 2010). However, our results demonstrated that the
diversity of microbial communities did not always increase with greater
plant diversity. We found that fungal overall diversity and AMF
diversity increased and fungal pathogen diversity decreased with plant
species richness under ambient conditions, while plant species richness
increased AMF diversity under drought conditions. We did not observe any
relationship between overall bacterial diversity and plant species
richness. Our findings were consistent with an increasing realization
that the relationship between microbial and plant diversity can exhibit
complicated patterns that might differ for different functional guilds
(Rottstock et al. 2014; Prober et al. 2015; Schlatteret al. 2015; Dassen et al. 2017; Mommer et al.2018; Jia et al. 2020).
The positive relationships between overall fungal diversity or AMF
diversity with plant diversity could reflect the strong host specificity
of these microbes and thus plant mixtures harboured a mix of fungal
guilds associated with different plant species (Rottstock et al.2014; Dassen et al. 2017). The neutral relationship between
bacterial diversity and plant species richness likely reflected
functional redundancy of soil bacteria in our model systems. Thus, high
plant species richness did not increase bacterial diversity nor resulted
in increased functioning of the whole bacterial community (Allison &
Martiny 2008; Louca et al. 2018). Under ambient conditions, the
relative abundance of fungal pathogens increased with plant species
richness, likely reflecting that increased root biomass and exudates
stimulated the growth of more generalist soil pathogens (Eisenhaueret al. 2017) However, high-density root systems likely impeded
species-specific soil pathogens in locating their hosts, and thus some
specialized pathogen species present in monocultures were absent in
plant mixtures, leading to decreased fungal pathogen richness with plant
species richness (Mommer et al. 2018).
Biodiversity declined under drought stress for soil bacteria, fungi and
AMF. AMF relative abundance decreased and fungal pathogen relative
abundance increased under drought conditions. These findings reflect
that drought likely limited activities of some guilds that were
sensitive to drought stress and selected for particular drought-adapted
guilds such as dormant pathogens and drought-resistant AMF (Mariotteet al. 2017; Meisner et al. 2018). Our results provided
evidence that plant diversity and drought can condition soil microbial
communities, leading to soil legacy effects that can alter subsequent
PSFs (De Long et al. 2019).
The more pronounced plant diversity -productivity
relationships in Phase II
In Phase I, the BE was not correlated with plant species richness in the
ambient treatments even though the values were positive, and the BE was
absent in the drought treatments, causing a non-significant plant
diversity-productivity relationship (Fig. 3a). In Phase II, however, the
BE increased with plant species richness, regardless of soil moisture
treatments (Fig. 4d). Our results provided evidence that the positive
plant diversity-productivity relationships were more pronounced over
time, and soil microbes were one mechanism behind this finding (Schmidet al. 2008; van Ruijven et al. 2020).
These findings were in line with several studies that attributed
long-term biodiversity effects to increasing niche complementarity in
resource use and facilitation (van Ruijven et al. 2005; Cardinaleet al. 2007; Fargione et al. 2007). However, our results
show that these temporal patterns in plant diversity-productivity can be
primarily driven by PSFs (Maron et al. 2011; Schnitzer et
al. 2011; Kulmatiski et al. 2012; Jing et al. 2015; Wanget al. 2019). We found that the positive CE-plant richness
relationships were more pronounced in Phase II compared to Phase I. A
possible explanation was that accumulations of species-specific soil
pathogens decreased community biomass in the monocultures, while the
community biomass of mixtures was promoted due to enhanced diversity of
AMF (van Der Heijden et al. 1998; Wagg et al. 2011; van
Ruijven et al. 2020). Future studies are necessary to test this
hypothesis through identifying species-specific fungal species and
recording their shifts in abundance as plant species richness increase.
Interestingly, we observed that the SE became more negative in higher
plant richness levels in Phase II (Fig. 4f), and this result was
consistent with previous findings that the positive selection effects
become negative over time (Fargione et al. 2007; Marquardet al. 2009). In the present study, unproductive plants, D.
aegyptium , increased their performance more than productive species in
high plant richness levels, leading to a negative selective effect (Fig.
S3). In other words, unproductive species might suffer from stronger
negative density dependence than productive species, and the possible
mechanism was that productive species could produce dense and large root
systems in mixtures that declined dilution effects of species-specific
soil pathogens (Mommer et al. 2018; van Ruijven et al.2020). Previous studies have suggested that negative PSFs can maintain
species richness by preventing productive species from dominating the
plant community (Bever 2003; Adler & Muller-Landau 2005; Petermannet al. 2008; Mangan et al. 2010). The implication of our
results is that soil conditioning can alter biodiversity effects by
increasing complementarity and decreasing selection effects through
PSFs.