development. In the early and intermediate
stages of soil development, the positive re-
lationship between pedogenic-reactive and
plant-available Si supports the hypothesis that
geochemical processes drive Si availability
( 15 , 18 ). This is consistent with the global-
scale relationship between soil pH and plant-
available Si ( 18 ), because soil pH is related tosoil buffering capacity that is driven by
weathering processes ( 26 ). However, with in-
creasing soil age from ~120 ka to ~2000 ka
(stages 4 to 6), the pool of pedogenic reactive
Si disappeared entirely, while that of alkali-
reactive Si remained large and was dominated
by soil phytoliths returned to the soil via litter.
Along with this shift, and despite the decrease
of plant-available Si from stages 4 to 6 reach-
ing among the lowest values worldwide ( 18 ),
the mean foliar Si concentrations of the most
abundant plant species were highest in the
last stage along the JurienBaychronosequence.
This shows that the terrestrial Si cycling is
sustained by strong plant retention of Si in
highly desilicated soils. Given the abundance
of quartz in these soils, its dissolution must
contribute to Si availability ( 17 ). However, the
correlation between phytoliths and plant-
availableSiforthequartz-rich horizons dem-
onstrates that the order-of-magnitude greater
solubility of biogenic amorphous silica com-
pared with quartz ( 14 ) compensates for the
lower concentration of phytoliths in driving
plant-available Si.
We assume a negligible dust imprint on Si
dynamics in topsoils along the two chronose-
quences ( 27 ), such that the increase in plant-
available Si in the topsoil horizons of the
intermediate and old soils supports the strong
impact of phytolith dissolution in desilicated
environments. Whereas P, Ca, K, and Mg are
essential plant nutrients and associated with
organic matter inside cells, Si precipitates to
form prominent silica structures between cell
walls and the lumen and in extracellular and
intercellular spaces of leaf epidermis ( 28 , 29 ).
This implies that P, Ca, K, and Mg are released
more readily during litter degradation ( 30 , 31 ).
Conversely, phytoliths can be preserved in the
soil environment for months to millennia
( 17 , 32 ) and therefore provide a long-term
source of Si to plants ( 31 , 33 ). In addition,
unlike most nutrients, Si is not remobilized
during leaf senescence, implying that all Si
is returned to soil via litterfall. Our results
thus show that the return of phytoliths to
topsoil is a key process contributing to a
slow-release source of Si that sustains the
terrestrial cycle over geological time scales.
We expect these results to be relevant broadly
for other systems, given that the chronose-
quences span the approximate range of soil
ages worldwide and represent three globally
relevant soil domains known to occur during
long-term pedogenesis: carbonate leaching,
formation of secondary minerals, and the
subsequent loss of said minerals through
dissolution and eluviation ( 26 ).
The oldest soils of the Jurien Bay chrono-
sequence are among the most strongly weathered
and nutrient-depleted worldwide ( 21 ). How-
ever, unlike the major plant nutrients for which
foliar concentrations decreased markedly withde Tombeuret al.,Science 369 , 1245–1248 (2020) 4 September 2020 3of4
Fig. 2. Relationship between soil
phytoliths and plant-available
Si concentrations from the
appearance of quartz-rich hori-
zons.The dot color indicates the
chronosequence from which the
horizon originates (Jurien Bay,
blue; Guilderton, red). Black lines
indicate the regression line between
both variables. Shaded areas
represent 95% confidence interval
of the regression. Equation
regression, coefficients of determi-
nation (R^2 ), andPvalues are shown.
The inset graph shows the same
relationship with the addition of
Jurien Bay stage 6 litter (y= 2.25x +
0.83;R^2 = 0.75;P< 0.01).
Fig. 3. Estimation of phytolith dissolution features with depth in two soil profiles.Theyaxis indicates
the pedogenic horizons. Soil depth increases from the horizon O (litter) to E2 (70 to 140 cm) for Jurien
Bay stage 6 and from the horizon A (0 to 10 cm) to B2 (89 to 140 cm) for Guilderton stage 5A.
RESEARCH | REPORT