Science - USA (2020-09-04)

1162 4 SEPTEMBER 2020 • VOL 369 ISSUE 6508 sciencemag.org SCIENCE

INSIGHTS | PERSPECTIVES

yet another means by which Si and
C are linked ( 2 , 9 ). Like diatoms in
aquatic systems, land plants con-
sume large quantities of dissolved
Si—roughly one-third of the amount
consumed by diatoms in the oceans
annually ( 9 ). All species of terres-
trial plants consume Si to some de-
gree as they transpire and deposit
it in their tissues, often as siliceous
structures known as phytoliths ( 10 ).
Chemical weathering is the
primary means by which Si is
liberated from rocks and made
available for uptake by both land
plants and aquatic organisms ( 1 , 5 ).
However, the phytoliths created by
vegetation are orders of magnitude
more soluble than mineral silicates
( 11 ) and thus are major suppliers
of dissolved Si needed for organ-
ismal uptake ( 4 ). An unsolved goal
of the scientific community has
been to define the balance between
geochemical and biological factors
that control the supply of dissolved
Si along the land-ocean continuum.
Are factors that dictate chemical
weathering rates, such as lithol-
ogy, climate, and hydrology ( 1 ), the
main control over the Si mobiliza-
tion from terrestrial systems? Or
are land plants, which both stimu-
late chemical weathering and ab-
sorb Si within their tissues ( 3 ), the
source and driving force behind Si
availability and export?
Across a chronosequence of soil
weathering stages that span nearly
the full range of soil ages on Earth,
de Tombeur et al. discovered that soil
age accounts for variability in the rela-
tive contributions of chemical weathering
and biological processes to the sustain-
ing of Si availability for biological uptake.
Specifically, as soils age, the amount of Si
released by chemical weathering eventu-
ally decreases as a result of Si losses from
secondary minerals, whereas the perpetual
recycling of siliceous plant material be-
comes the major supplier of bioavailable
Si over time (see the figure). For example,
Si release from soil weathering was nearly
nonexistent in the oldest soils studied by
de Tombeur et al., which were ~2 million
years old, dominated by quartz, and de-
pleted in carbonates, clays, and iron ox-
ides. However, the vast soil phytolith pool,
sustained by continued plant production
and decay, supplied ample Si to plants and
corresponded to elevated leaf Si concen-
trations in the mature vegetation in these
oldest soils (see the figure). Thus, as soils
age, chemical weathering becomes less im-

portant for supplying Si to the biosphere,

whereas terrestrial plants play an increas-

ingly preeminent role.

Herein lies an additional key relevancy

nearly hidden from plain view within the

de Tombeur et al. study: Soil age appears

to be partially responsible for the extreme

variability of plant Si concentrations ob-

served within terrestrial vegetation. For

decades, scientists have known that the

range of Si concentrations in plants is the

widest of any element (0.1 to >10% by dry

weight) ( 10 ), but the mechanisms respon-

sible for this variability have remained

unclear. Whereas factors such as stress ex-

posure (for example, herbivory, desiccation,

heavy-metal toxicity) and Si accumulation

mode (passive versus active) influence the

variability in plant Si concentrations, these

factors do not fully explain why such large

differences exist, both within and across

individual plant species ( 12 , 13 ). Across the

chronosequence of soil weathering stages

studied by de Tombeur et al., mature leaves

of the most dominant plant species

displayed increasing Si concentra-

tions, which corresponded to the

elevated Si availability from the

phytolith-rich older soils (see the

figure). The enrichment of leaf Si

concentrations mostly results from

shifts in plant community compo-

sition toward plants that accumu-

late high amounts of Si, but also

from increasing Si concentrations

within individual species across

the chronosequence. Si improves

overall plant fitness, allowing veg-

etation to withstand a variety of

external stressors ( 10 , 13 ). Given

the elevated quantities of Si found

in some of Earth’s most impor-

tant agricultural crops (such as

rice, wheat, and barley), improved

knowledge of the mechanisms that

control plant Si concentrations is

crucial for global food security.

The new study has implications

for researchers who wish to more

fully understand fundamental Si

cycling and the linkages between Si

and C. The Si liberated from rocks

by weathering eventually makes its

way to the coastal oceans, fueling

diatom-driven primary produc-

tion ( 5 , 8 ). Along that land-ocean

pathway, terrestrial plants inter-

cept Si and regulate its availability

for biological uptake ( 3 , 4 ). Armed

with the knowledge that soil age

influences the relative importance

of geochemical and biological pro-

cesses in sustaining Si availability

in terrestrial systems, scientists

are now better prepared to elucidate the

controls on Si mobilization from upland to

downstream aquatic systems, as well as to

recognize the drivers of differential plant

Si uptake. j

REFERENCES AND NOTES

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4. E. Struyf, D. J. Conley, Biogeochemistry 107 , 9 (2012).


5. D. J. Conley, J. C. Carey, Nat. Geosci. 8 , 431 (2015).


6. F. de Tombeur et al., Science 369 , 1245 (2020).


7. C. Rousseaux, W. Gregg, Remote Sens. 6 , 1 (2014).


8. P. J. Tréguer, C. L. De La Rocha, Annu. Rev. Mar. Sci. 5 , 477
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9. J. C. Carey, R. W. Fulweiler, PLOS ONE 7 , e52932 (2012).


10. E. Epstein, Proc. Natl. Acad. Sci. U.S.A. 91 , 11 (1994).


11. F. Fraysse, O. S. Pokrovsky, J. Schott, J.-D. Meunier,
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12. M. J. Hodson, P. J. White, A. Mead, M. R. Broadley, Ann.
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13. J. Cooke, J. L. DeGabriel, S. E. Hartley, Funct. Ecol. 30 ,
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10.1126/science.abd

Soil age (weathering stage)

Si:Al ratio in

amorphous soil pool

>

Si:Al ratio in

amorphous soil

pool <

Higher Si–

accumulating plant

community

Lower Si–

accumulating

plant community

Oldest soils

(~2 million years old)

Youngest soils

(~100 to 6500

years old)

Carbonate

leaching

Formation and

loss of secondary

Si-bearing

minerals

Quartz

enrichment

Si concentration in mature

plants

Si ava

ilability from

plant disso

lution

Si ava

ilability from

wea

thering

Soi

l development

stage

Silicon, soil, and the biosphere

A conceptual model describes the relation between soil age and controls

on silicon availability for biological uptake. The silicon:aluminum (Si:Al)

ratio of an amorphous, alkali-reactive Si pool indicates the amount of Si

available from plant dissolution.

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