Results
Plant-Soil Feedback
At the species level (i.e., across
soil types) there were six negative, one positive, and two neutral PSFs
in 2017 (Fig. 1). The mean absolute value of species-level PSFs was 0.37
(the 95% confidence interval CI95 was 0.16 to 0.58). In
other words, plants created soils that changed subsequent plant growth
by 37 %. The arithmetic mean value of species-level PSF was -0.15
(CI95 -0.50 to 0.19). In 2018, five species demonstrated
a species-level (Fig. 1), though neither absolute (0.35) nor arithmetic
(-0.17) PSF values differed between 2017 and 2018 (tabs= 0.25; tart = 0.17; P > 0.05, DF =
8; paired t-test; Fig. 1).
At the soil-level, 27 of 72 PSFs were negative and eight were positive
in 2017 (Fig. 2). The mean of absolute soil-level PSFs was 0.40
(CI95 = 0.34 to 0.45), while the arithmetic mean value
was -0.14 (CI95 = -0.24 to -0.03). In 2018, absolute
values (0.36, CI95 = 0.31 to 0.42), and the arithmetic
mean (-0.15, CI95 = -0.25 to -0.05) were similar to and
did not differ from 2017 values (tabs = 1.50;
tart = 0.44; P > 0.05, df=71;paired t-test ) though only 13 of 72 soil-level PSFs differed from
zero in 2018 (Fig. 2).
Competitive species demonstrated small PSF values and poor competitors
demonstrated large positive or negative PSF values (Fig. 3). More
specifically, RCI values from both the current and pre-existing
experiments were correlated with absolute PSF values from 2017 (current:
F1,134 = 28.7, P < 0.05, R =
0.18; pre-existing: F1,142 = 29.0, P <
0.001, R = 0.17) and 2018 (current: F1,134 = 29.4P < 0.001, R = 0.261; pre-existing:
F1,142 = 50.06, P < 0.05, R =
0.18; Fig. 3).
Observed and Predicted Biodiversity
Effects
Polycultures produced 55 % (current) and 40 % (pre-existing) more
biomass than monocultures, respectively (Fig. 4; Supplementary Table 1).
In both experiments, selection effects were greater than complementarity
effects (Fig. 4; Supplementary Table 1). Selection effects increased
with species richness in the pre-existing experiments, but
complementarity effects were unrelated to species richness in either
experiment (Fig. 4; Supplementary Table 1). Between experiments,
community biomass, net biodiversity and selection effects were greater
in the current than pre-existing experiments. Complementarity effects
were smaller in the current experiment than the pre-existing experiment
(Fig. 4; Supplementary Table 1). Predictions of biodiversity effects
never differed between PSF and Null models (Supplementary Table 2). Null
and PSF model predictions did not differ from observed biomass or net
biodiversity, but Null and PSF model predictions were smaller than
selection effects and larger than complementarity effects in the current
experiment (Fig. 4, Supplementary Table 2).
In regression analysis of community biomass, Null and PSF models
explained 28% and 30%, respectively of the variation in the current
experiment and both explained 8% of variation in the pre-existing
experiment (Supplementary Table 3). Similarly, for species biomass, Null
and PSF models explained 42% and 40%, respectively of the variation in
the current experiment and 38% and 36%, respectively of the variation
in the pre-existing experiment.