Discussion
Because most PSF research continues to
be performed on plant monocultures in greenhouse conditions, the extent
to which PSFs affect plant communities in the field remains unclear
(Crawford et al., 2019; Forero et al., 2019; Ke & Wan, 2020; Reinhart
et al. 2021). Our factorial experiment provided unusually comprehensive
information about PSFs in the field. We measured all possible PSFs for
nine species and found that plants, on average, created soils that
changed subsequent plant growth by 36%. However, because plants
realized both positive and negative PSFs, the net effect was that plants
grew 14% less on home than away soils. While most PSF studies simply
measure PSFs, we also tested the effect of these PSFs in plant
communities. Despite causing 36% changes in plant biomass, PSFs had
little effect on Null model predictions of plant community biomass
across a range of species richness. While somewhat surprising, a lack of
a PSF effect was appropriate in this site because species richness
effects in this study were caused by selection effects and not
complementarity effects (PSFs would appear as complementarity effects).
PSFs had little effect on Null model predictions for several reasons.
First, even though the absolute value of PSFs was reasonably large, the
net PSF effect was small because some PSFs were positive while others
were negative. Second, PSFs for the two dominant plant species were
small (-0.14 to 0.12). Third, because PSFs were, on average, smaller
than differences in intrinsic growth rates (36% versus 193%), they
were unlikely to change competitive outcomes between species (Kulmatiski
2016; Lekberg et al. 2018). Finally, A. elatius dominated across
all species-richness levels so ‘away’ soils had little effect onA. elatius growth regardless of species richness. Broadly, our
results demonstrated that large PSF values alone are not sufficient to
explain plant species coexistence or the diversity productivity
relationship at this site. In fact, overyielding at the site was caused
primarily by selection effects, so complementarity effects of any kind
(e.g., niche partitioning or PSF) were unimportant. Results do not
exclude a role for PSF as a mechanism of species coexistence and
productivity, particularly at other sites with larger complementarity
effects, rather results highlight that PSF effects must be considered in
the context of other factors affecting plant growth such as intrinsic
growth rates (Crawford et al. 2019; Lekberg et al. 2019).
The Curious Case of Plant-Soil Feedbacks and the Dominant
Species
We predicted that negative PSFs would cause overyielding because soil
pathogens would be ‘diluted’ in diverse communities relative to
monocultures (Kulmatiski et al., 2012; Maron et al., 2011; Schnitzer et
al., 2011). However, A. elatius was such a dominant species that
it maintained at least 75% relative biomass across all species-richness
levels in the current diversity-productivity experiment. From a PSF
perspective, an important consequence of this dominance is that A.
elatius effectively only grew on ‘home’ soils. Therefore, A.
elatius never benefited from pathogen dilution on ‘away’ soils.
Research examining potential PSF effects ‘in vitro’ often assume that
species are competitively equivalent (Bever et al 2003; Kulmatiski et
al. 2011). Performing this experiment in field conditions helps refocus
the role of PSF in the context of strong competitive imbalances among
species which are common in field conditions (Crawford et al. 2019;
Lekberg et al. 2018).
Are Neutral PSFs a Successful Strategy?
In addition to primarily growing on ‘self’ soils, the dominant speciesA. elatius realized a small PSF with little variability within or
across soil treatments (Fig. 2). It is possible that small PSFs and
small variability covary. It is reasonable to expect that, for a plant
species to dominate in many communities, it will grow well across soil
treatments and, therefore, demonstrate small and consistent PSFs. In
contrast, plant species with large positive PSFs may have difficulty
establishing in ‘away’ soils, while species with a large negative PSF
may have difficulty attaining large growth on ‘home’ soils (Levine et
al., 2006). Our results suggest that there may be a selective pressure
to maintain neutral PSFs with low variability to dominate plant
communities. Consistent with this idea, we found that competitive
species were associated with small PSF values (Fig. 3) while
sub-dominant species demonstrated large positive and large negative PSF.
This perspective may help explain why PSFs often show weak correlations
with landscape abundance (Reinhart et al. 2021, but see Mangan et al.
2010; Kulmatiski et al. 2017).
There is also a statistical reason that dominant species may demonstrate
small PSFs. It is more likely that plant species with small growth will
realize large proportional changes in growth (Pfisterer & Schmid,
2002). For example, a plant species that can grow to 50 g
m-2 on ‘home’ soils can easily be imagined growing to
0 or 200 g m-2 on ‘away’ soils, resulting in PSFs of
1.0 and -0.75, respectively. However, it is essentially impossible for
plant species to grow to 1,000 g m-2 on ‘home’ soils
and 4,000 g m-2 on ‘away’ soils because 4,000 g
m-2 is beyond carrying capacity in grasslands. As a
result, subdominant species are more likely to have large PSFs than
dominant species. We are not aware of other studies suggesting these
ideas and this is likely because PSF experiments rarely perform the
types of large factorial experiments needed to examine PSFs for many
species across soil types (Rinella and Reinhart 2018).
Diversity-Productivity Relationships
Species richness effects were similar to other biodiversity experiments
in more mesic sites (Cardinale et al., 2011; Hector et al., 1999).
However, the mechanisms driving this response differed between the
current and pre-existing experiments. In the pre-existing experiment,
polyculture biomass was driven by selection (21% of monoculture
biomass) and complementarity (14%) effects. In the current experiment,
overyielding was largely explained by selection effects (43%) and
countered by negative complementarity effects (-20%). A. elatiuswas more dominant in the current than the pre-existing experiment (Fig.
5; Clark et al. 2020). Community productivity in the Jena Experiment
varies widely among years due to different environmental conditions
(Weisser et al., 2017), so it is likely that climate or other
environmental conditions that differed between the two studies also
caused greater dominance effects in the current experiment (Marquard et
al., 2009; Guimarães-Steinicke et al., 2019). For example, a large
flooding event in 2013 may have increased A. elatius growth by
increasing nutrient availability (Wright et al., 2015). A.
elatius is strongly competitive for light and nitrogen, so greater
seeding rates in the current experiment may have exaggerated asymmetric
competitive effects (Lorentzen et al. 2008; Roscher et al. 2008). It is
interesting to note, that even though the mechanisms differed, the net
biodiversity effect was similar in the new and old experiments.
Species-Level vs. Soil-Level
PSFs
Because sample sizes increase exponentially as species are added to
factorial PSF-experiments, most studies measure PSFs for one to a few
target species (Smith-Ramesh & Reynolds, 2017; Van der Putten et al.,
2013). By measuring all 72 potential PSFs for nine species, this study
provided unusually comprehensive insights into how PSFs vary among soil
conditioned by different species. For the most part, PSFs were
consistent among soil treatments. It is not unreasonable to expect PSFs
to vary widely across differently conditioned soils (Bezemer et al.,
2006; Rinella & Reinhart, 2018; Smith-Ramesh & Reynolds, 2017). For
example, a plant species may grow well on a soil conditioned by a
N-fixing species and poorly on a soil conditioned by an
early-successional species that accumulated a large pool of generalist
soil pathogens (Chapin et al., 1994; Van der Putten et al., 2013).
However, we observed only one species that had a positive PSF on one
soil treatment and a negative PSF on another soil treatment (P.
pratense ). The fact that PSF values were consistent across soil
treatments suggests that PSFs in this system are determined primarily by
growth on ‘home’ soil.
Site Differences
It has been suggested that PSFs will intensify competitive effects in
nutrient-rich conditions and strengthen facilitative effects in
nutrient-poor conditions (Bever, 2003; Lekberg et al., 2018). Consistent
with this idea, we found that PSFs were more negative, and competitive
effects (selection effects) were larger in the current experiment,
performed at a mesic, nutrient-rich site relative to a similar recent
study performed at a drier and nutrient-poor site (Forero 2021). Both
absolute (0.36 vs 0.27) and net (-0.14 vs. 0.10) PSFs were larger at the
nutrient-rich vs. nutrient-poor site, respectively (Forero 2020).
Further, overyielding was smaller at the nutrient-rich site than the
nutrient-poor site (Craven et al. 2016; Forero 2021). Larger PSFs and
competitive effects in nutrient-rich conditions provide a potential
explanation for why the strength and trajectory of
biodiversity-ecosystem functioning relationships over time differ
between more and less fertile soils (Eisenhauer et al., 2019;
Guerrero‐Ramírez et al., 2019; Ratcliffe et al., 2017).