Results
In control plots, there was no change over time in species richness
(mean slope -0.08 species/year, -0.27 to 0.09 95% CI, Table S2) or
aboveground biomass (4.97, 95% CI: -4.37 to 13.97, biomass
g/m2/year) (Figure 2 a, b). Similar numbers of species
were lost (-0.19, 95% CI: -0.28 to -0.11, species loss (s.loss)/year)
and gained (0.12 95% CI: 0.04 to 0.21, species gained (s.gain)/ year)
(Figure 3a, b). In control plots, biomass loss associated with species
losses each year (-0.56, 95% CI: -0.97 to -0.26, SL
g/m2 associated with species loss/year) was slightly
less than the biomass gain associated with species gains each year
(4.02, 95% CI: 2.6 to 5.86, SG g/m2 associated with
species gain /year) (Figure 3c, d). Biomass change associated with
persistent species showed considerable variation, but no directional
change (-4.47, 95% CI: -10.76 to 1.84, PS g/m2associated with persistent species/year) (Figure 3e).
In plots treated with NPK, species richness declined (-0.47, 95% CI:
-0.66 to -0.22, species/year, Fig 2a) and biomass increased (19.99, 95%
CI: 7.14 to 31.11 g/m2/ year, Fig 2b) over time (see
Figure S2 for site level means between year 0 and the most recent year
of measurement). The response to NPK addition of species richness change
over time and biomass change over time were not correlated (-0.29, 95%
CI: -0.63 to 0.19; Fig 2c, Supplementary Information). Compared to
controls, NPK treatments increased species loss over time (-0.38, 95%
CI: -0.51 to -0.26 species/year Fig 3a), whereas the rate of species
gained did not differ from zero (-0.01, 95% CI: -0.08 to 0.06
species/year, Fig 3b). In NPK plots, biomass loss was associated with
species loss over time (-7.44, 95% CI: -10.18 to -4.92
g/m2/year, Fig 3c). Species that were gained in NPK
plots were associated with positive biomass change over time (7.36, 95%
CI: 5.27 to 9.77 g/m2/ year, Fig 3d). Finally, change
in biomass over time associated with persistent species exhibited
considerable variation in NPK treatments (3.05, 95% CI: -6.14 to 11.88
g/m2/year, Fig 3e). Combined, biomass gains associated
with species gained, and biomass increases associated with persistent
species over time contributed to overall biomass gained in NPK plots.
Effects of each component on species and biomass change through time
between control plots and NPK plots can be compared directly (Figure 4).
While species losses were greater than species gains under
fertilization, species gains contributed positively overall to biomass,
along with an increase in biomass of persistent species. Species losses
and gains due to nutrient addition were largely uncorrelated (0.29, 95%
CI: -0.03 to 0.58, Table S5), as was the net change in biomass from
losses and gains (-0.07, 95% CI : -0.38 to 0.23). Biomass change
associated with species losses and biomass change in persistent species
responses to NPK were also uncorrelated (-0.24, 95% CI: -0.55 to 0.09),
as was the relationship between biomass changes from species gains and
persistent species (-0.06, 95% CI: -0.39 to 0.29).
Whilst the additive nature of the partition means that changes in
species richness are the sum of the species gains and losses, and
overall changes in biomass are the sum of biomass changes associated
with species gains, losses, and persistent species, we found some
differences between our estimates of the rate of change for the whole
(e.g., species richness) versus the parts of compositional change that
contribute to richness change (e.g., gains and losses). The overall
model-estimated effect of NPK on species richness change per year
(-0.47) (Figure 2c) was qualitatively similar to the sum of the overall
model-estimated effect of NPK on species loss and species gains (-0.38
(s.loss) + -0.01 (s.gain) = -0.39) (Figure 4). However, the overall
model-estimated effect of NPK on biomass (19.99, CI: 7.14 to 31.11)
(Figure 2c) was much higher than the estimated overall effects of NPK on
each partitioned component of biomass change (-7.44 (SL) + 7.36 (SG) +
3.05 (PS) = 2.97) (Figure 4). We attribute this difference largely to
the uncertainty and variation in biomass change associated with three
components, and statistical regularisation associated with partitioning
plot-level biomass change into three separate parts across 59 sites, 3
blocks, and ~6 plots per site. This regularisation
introduced on each partitioned component leads to a more conservative
estimate of the rate of biomass gains overall when compared to the
overall estimates of strip biomass (Figure 2c, Figure 4).
There are four combinations of richness and biomass responses to NPK
addition for the experimental sites in this study. Most sites (41)
experienced an overall decrease in species richness and an increase in
biomass (-richness, + biomass), nine sites experienced a decrease in
species richness and a decrease in biomass (-richness, - biomass), seven
experienced an increase in species richness and an increase in biomass
(+richness, +biomass), and two sites experienced an increase in richness
and a decrease in biomass (+richness, -biomass), but these negative
biomass responses were very close to zero (Figure 2c, Figure 5, see
Supplementary Table S1, and Figure S7 for site level estimates). Sites
with differing overall community responses in terms of species richness
and biomass have notably different species gains and losses dynamics
(Figure 5). Under fertilization we expect biomass to increase and
species richness to decline (+ biomass, -species richness), but in
~15% of sites, biomass is decreasing and richness is
increasing (-biomass, -richness) (Figure 5, Figure S7). This variation
in overall biomass and richness responses was driven by a combination of
biomass loss associated with species loss (SL) and a biomass decrease
associated with persistent species (PS) (Figure 5). In contrast, in
sites where biomass increased and species increased, few species were
lost (s.loss) and there was little biomass loss associated with these
losses (SL), biomass increases were associated with species gains, and
biomass associated with persistent species was relatively stable overall
(PS) (Figure 5).