two soil profiles selected for microscopic ob-
servations ( 22 ) (Fig. 3; from 6 to 8% in topsoil
to 40 to 42% in deeper horizons).
The mean foliar Si concentration of the
10 most abundant species per plot at Jurien
Bay increased continuously with soil age, from
0.5 ± 0.2 g Si kg−^1 in stage 1 to 4.2 ± 1.4 g Si kg−^1
in stage 6 (Fig. 4), where Si availability was
controlled by phytolith dissolution. By contrast,
foliar concentrations of the major rock-derived
plant nutrients (P, Ca, K, and Mg) followed
the opposite pattern, decreasing as soils aged
(Fig. 4). Results were qualitatively similar when
element concentrations were weighted by the
relative canopy cover of each species in the plot,
showing that these shifts in foliar chemistry
represented community-level responses ( 22 ).
The patterns of increasing foliar Si and de-
creasing major nutrient concentrations were
associated with changes in plant species com-
position across the sequence, with dicot woody
species in the Proteaceae and Dilleniaceaecontributing most strongly to the increase in
foliar Si concentrations on the oldest soils ( 22 ).
However, this pattern also occurred (except for
K) within the few individual species that were
sampled across multiple stages of the chrono-
sequence ( 22 ). Together, these results suggest a
selective advantage for plants that accumulate
more Si on nutrient-depleted soils.
Our results provide clear evidence for a
shift from pedological to biological control in
the terrestrial Si cycle during long-term soilde Tombeuret al.,Science 369 , 1245–1248 (2020) 4 September 2020 2of4
Fig. 1. Stocks of Si pools across the Jurien Bay and Guilderton chronosequences.Stocks of reactive pedogenic Si extracted with oxalate (A), alkali-reactive Si
extracted with Na 2 CO 3 (B), and plant-available Si extracted with CaCl 2 (C) for the upper 50 cm of soil. In (B), the bar colors and labels indicate the depth-weighted
mean ratio of alkali-reactive Si to alkali-reactive Al. Soil age increases with increasing chronosequence stage.
RESEARCH | REPORT