Discussion
Here, we show in a field experiment that compositionally different plant
communities create legacies in the soil that, in turn, alter the
composition of subsequent plant communities that establish in these
soils. Plant communities with different ratios of grasses and forbs
created unique soil microbiomes, and these effects were most notable in
the soil fungal community. These soil legacies, in turn, affected the
responding plant communities. Specifically, both grass and forb
abundances in the responding phase were negatively affected by their
respective abundance in the previous plant community and this effect was
mediated by soil processes. We show that manipulating the composition of
the vegetation in grasslands alters the microbiome in the soil, and that
this alters the succeeding vegetation.
Plant communities dominated by plants of a certain functional type
create legacies that negatively impact plants from the same functional
type. This result is very robust, as the same pattern was observed in
all six plant communities that were used to condition the soil in this
field experiment. This finding is also in strong agreement with previous
work from pot studies (Kulmatiski et al. 2008; Petermann et
al. 2008; De Kroon et al. 2012; Wubs & Bezemer, 2018). The
functional type a plant belongs to also alters community structure in
soil fungi (Kos et al . 2015; Heinen et al. 2018).
In the soil of plant communities that had more grasses, we found an
accumulation of fungal pathogens (dominated by grass-associated fungal
pathogens). Interestingly, the abundance of forb-associated pathogens
was very low and there was no relationship with the abundance of forbs
in the vegetation. Grasses are a phylogenetically close group of plants.
Forbs, on the other hand, are a broad phylogenetic group. The
specificity of grass pathogens may be one reason why grass-associated
pathogens may drive grass abundance in the field, but no such phenomenon
was observed for forbs. Specialized forb pathogens are unlikely to
accept hosts from all forb families, given the phylogenetic breadth of
the group and as a result their individual abundances may not drive
abundance of forbs as a group. Specialized pathogens may often attack a
range of hosts if they are closely related but some pathogens have
rather broad host range (Barrett & Heil 2012) . Importantly, our
results indicate that negative soil legacy effects on grasses observed
in mid-successional grasslands, can be, at least partially, explained by
accumulation of pathogens (Kulmatiski et al. 2008; Van der Puttenet al. 2013).
Our results further reveal that both bacteria and fungi in the soil both
respond to the conditioning plant communities that grow in the soil.
However, only the conditioning effects on fungal communities are
longer-lasting, and have knock-on effects on the subsequent responding
plant communities. We may conclude that soil bacterial communities,
although responsive to conditioning treatments, play a lesser role in
the community dynamics in responding plant communities. This finding is
in strong agreement with recent findings that soil fungal communities
are strongly shaped over time by plants, whereas bacteria are shaped
less by plants, but more by varying environmental conditions over time
(Hannula et al. accepted manuscript). Soil legacy effects in
natural communities are likely not driven by one taxon specifically, but
rather by the composition of the soil fungal community as a whole
(Semchenko et al. 2018; Bennett & Klironomos 2018; Mommeret al. 2018, but see Harrison & Bardgett, 2010). Importantly, we
show that conditioning effects of plant communities on soil biota,
outweigh the effects on soil abiotic parameters, and are drivers of soil
legacy effects on plant growth in the field.
One potential confounding factor in the results is that plant roots and
seeds originating from the conditioning plant community could have been
left behind in the soil after the conditioning community was removed and
that these may influence responding communities via this pathway. There
were some positive conspecific relationships between conditioningand responding plant species, but the observed effects were
community-specific. For instance, a positive conspecific relationship
was observed for Rumex acetosella . This species flowers very
quickly and produces many seeds. It is possible that seeds produced
during the conditioning phase, and that entered the seedbank, caused an
increased local abundance of this species in the responding communities.
Furthermore, we observed a positive conspecific relationship forClinopodium vulgare and Holcus lanatus . Both species
regrow from root systems in pot experiments (Heinen, pers. obs.) and
hence regrowth may drive the observed relationships. However, it is
unlikely that these effects have had a strong effect on theresponding plant community as a whole, as the strongest
relationships - observed between functional types in theconditioning versus the responding plant communities -
were negative and thus cannot be explained by regrowth or seed
production. We therefore conclude that soil legacy effects must be the
dominant driver of these effects.
In this study we find that at the plant species level, we find very few
indicators for conspecific plant-soil feedbacks and the effects are
strongly conditioning plant community specific. This is an interesting
finding as the field site that was used in this study has been the soil
donor site for countless plant-soil feedback studies over the past
decades. In the majority of these studies, plant species grown in soils
from this site have negative conspecific feedback effects (e.g. REF).
This indicates that individual plant-soil feedbacks as observed in
individual pot studies, may be counter-balanced by other plant species
that simultaneously grow in (and thus condition) the soil in natural and
diverse plant communities. We may speculate that conspecific plant-soil
feedbacks could play a larger role in less diverse or more disturbed
systems such as dune vegetations or in very early secondary succession.
However, future work is needed to investigate the role of plant
diversity or ecosystem complexity in plant-soil feedbacks in the field.
In conclusion, we show here that the ratios between plants of different
functional types within a plant communities mediate plant-induced
microbial soil legacies and that these legacies determine the
composition of plant communities in the field. Importantly, this means
that by managing current plant communities in the field, we can
influence the composition of future plant communities and the ecological
functions they provide. This opens new avenues for optimizing nature
management practices, which is vitally important in the face of global
change, for instance in making nature more robust to climate change or
invasions.