Discussion
Total biomass of invasive plant species tended to higher than that of
co-occurring native plant species, and the growth rate over time was
also significantly higher than that of co-occurring native plant species
regardless of the presence of soil microbes (Fig. 2A & Fig. S1). It is
likely that invasive plants generally have a higher growth rate than
native plants (van Kleunen et al. 2010), the current test invasive plant
species also exhibited the trait as invasive plants grow larger than
native plant species over time.
Soil microbes tended to had a slight positive effect on total biomass of
the native plant species over time, while the effect of soil microbes on
invasive plant species as their total biomass tended to change from
promotion to inhibition over time (Fig. 2A). Consistent with our
predictions, these results suggest that soil microbes affected the
growth of invasive plant species and native plant species over time, and
presence of soil microbes inhibited the growth of invasive plant species
over time. This founding supports a prediction of declining enemy
release over time that invasive plants accumulated more microbial
natural enemies, and the increase in enemies (e.g., pathogen) may
inhibit the growth of invasive plants themselves (Callaway et al. 2013,
Diez et al. 2010, Mitchell et al. 2010, Stricker et al. 2016). Another
possible explanation is that invasive plant responses to mutualisms
weaken over time (Seifert et al. 2009), thus intensifies the inhibitory
effect on the growth of invasive plants. In addition, the reduction of
soil nutrients over time can also alter plant-soil feedback (Bennett et
al. 2019), especially the rapid resource acquisition of invasive plants
(Dukes et al. 1999), it may further amplify the negative feedback
effects caused by pathogen accumulation or reduced mutualists
dependence.
Local adaptations may be responsible for positive feedback in native
plant species. Native plant species may be able to quickly adapt to the
local soil microbial community (Kawecki et al. 2004). In generally,
native plants tend to be conservative, have stronger tolerance to
antagonists, and enhance adaptability to the local environment. Our
research results showed that soil microbes always promote native plant
species, which may be due to local adaptation in short-term, because it
is found that this promotion effect weakens over time (first to second:
7.3% to 2.7%; Fig. 2A). We speculate that the promotion effect may
also become inhibition in the future for a longer period, but the
current results cannot prove it. Although the growth performance of
invasive plant species was found to be different in individual analysis
of species in each genus, and only the three-factor interaction was
found in Paspalum (Fig. 2a-2), we speculated that temporal
variations exist in the direction and strength of interactions between
different plant species and soil microbes. Therefore, future studies may
text the growth of different species over continuous time, to determine
whether the strength and direction of invasive plant-to-soil feedback
emerges with a general pattern.
Harvest time significantly decreased proportional allocation of biomass
on roots (Fig. 2B), this result may be dominated by native plant
species. We found that native plant species allocated proportionally
biomass to their roots decreased over time, while an opposite pattern
was observed for invasive plant species that tended to increase over
time (Fig. 2B). It is consistent with plants growth responses, with
invasive plant species allocated more root biomass to resource
acquisition on growth. This is not surprising as more roots can help
plants obtain more nutrients, and plants will preferentially allocate
biomass to roots that can improve resource acquisition (Casper et al.
1997, Poorter et al. 2000). Although we found no significant three-way
interaction between the invasion status, harvest time and soil
treatments on root mass fraction (Fig. 2B), but the three-way
interaction was observed in Paspalum (Fig. b-2) andSolidago (Fig. b-3). While the two genera did not show a same
pattern, As guessed above, it is due to that the interaction between
plant species and soil microbes is strongly related to temporal
variations. In addition, we found that the presence of soil microbes
significantly reduced the proportion of root biomass allocation, which
indicates that soil microbes contain many pathogens, because soil-borne
pathogens damage roots (Packer et al. 2003, Reinhart et al. 2010).