Soil age alters the global silicon cycle

doi:10.1126/science.abd9425

GSTDTAP > 气候变化

DOI	10.1126/science.abd9425
	Soil age alters the global silicon cycle
	Joanna Carey
	2020-09-04
发表期刊	Science
出版年	2020
英文摘要	Silicon (Si)—the second most abundant element in Earth's crust—relies largely on geological factors to control its mobilization. Thus, Si cycling through Earth's systems was often believed to be buffered from human disturbance ([ 1 ][1]). However, research over the past several decades has awakened scientists to the central role of vegetation in regulating Si availability in the biosphere ([ 2 ][2], [ 3 ][3]). It is now beyond doubt that human disturbance affects Si biogeochemistry and its associated impact on carbon (C) sequestration rates. Attempts to decipher how human activities (namely deforestation and agricultural expansion) influence Si cycling have left scientists to reconcile conflicting data on the importance of geochemical versus biological controls on Si biogeochemistry ([ 4 ][4], [ 5 ][5]). On page 1245 of this issue, de Tombeur et al. provide new insights into this debate by demonstrating the importance of soil age in regulating Si cycling ([ 6 ][6]). The Si and C cycles are intricately linked at the global level. On geological time scales, the chemical weathering of mineral silicates consumes atmospheric carbon dioxide (CO2), thus regulating Earth's climate ([ 1 ][1]). On biological time scales, the uptake of CO2 by Si-requiring microscopic phytoplankton known as diatoms accounts for roughly half of the photosynthesis that occurs in global oceans ([ 7 ][7]). As such, the amount of Si exported from terrestrial uplands to marine waters can directly control the rate of photosynthetically driven CO2 uptake ([ 8 ][8]). However, Earth's biological Si cycle is not relegated only to aquatic systems. Terrestrial vegetation performs an integral function in Si biogeochemistry and provides yet another means by which Si and C are linked ([ 2 ][2], [ 9 ][9]). Like diatoms in aquatic systems, land plants consume large quantities of dissolved Si—roughly one-third of the amount consumed by diatoms in the oceans annually ([ 9 ][9]). All species of terrestrial plants consume Si to some degree as they transpire and deposit it in their tissues, often as siliceous structures known as phytoliths ([ 10 ][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 ][1], [ 5 ][5]). However, the phytoliths created by vegetation are orders of magnitude more soluble than mineral silicates ([ 11 ][11]) and thus are major suppliers of dissolved Si needed for organismal uptake ([ 4 ][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 lithology, climate, and hydrology ([ 1 ][1]), the main control over the Si mobilization from terrestrial systems? Or are land plants, which both stimulate chemical weathering and absorb Si within their tissues ([ 3 ][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 relative contributions of chemical weathering and biological processes to the sustaining of Si availability for biological uptake. Specifically, as soils age, the amount of Si released by chemical weathering eventually decreases as a result of Si losses from secondary minerals, whereas the perpetual recycling of siliceous plant material becomes 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 depleted in carbonates, clays, and iron oxides. However, the vast soil phytolith pool, sustained by continued plant production and decay, supplied ample Si to plants and corresponded to elevated leaf Si concentrations in the mature vegetation in these oldest soils (see the figure). Thus, as soils age, chemical weathering becomes less important for supplying Si to the biosphere, whereas terrestrial plants play an increasingly preeminent role. ![Figure][12] 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. GRAPHIC: X. LIU/ SCIENCE 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 observed 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 ][10]), but the mechanisms responsible for this variability have remained unclear. Whereas factors such as stress exposure (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], [ 13 ][14]). Across the chronosequence of soil weathering stages studied by de Tombeur et al. , mature leaves of the most dominant plant species displayed increasing Si concentrations, 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 composition toward plants that accumulate high amounts of Si, but also from increasing Si concentrations within individual species across the chronosequence. Si improves overall plant fitness, allowing vegetation to withstand a variety of external stressors ([ 10 ][10], [ 13 ][14]). Given the elevated quantities of Si found in some of Earth's most important 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 production ([ 5 ][5], [ 8 ][8]). Along that land-ocean pathway, terrestrial plants intercept Si and regulate its availability for biological uptake ([ 3 ][3], [ 4 ][4]). Armed with the knowledge that soil age influences the relative importance of geochemical and biological processes 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. 1. [↵][15]1. J. Gaillardet, 2. B. Dupré, 3. P. Louvat, 4. C. J. Allègre , Chem. Geol. 159, 3 (1999). [OpenUrl][16][CrossRef][17][GeoRef][18][Web of Science][19] 2. [↵][20]1. D. J. Conley , Global Biogeochem. Cycles 16, 68 (2002). [OpenUrl][21] 3. [↵][22]1. A. Alexandre, 2. J.-D. Meunier, 3. F. Colin, 4. J.-M. Koud , Geochim. Cosmochim. Acta 61, 677 (1997). [OpenUrl][23][CrossRef][24][GeoRef][25][Web of Science][26] 4. [↵][27]1. E. Struyf, 2. D. J. Conley , Biogeochemistry 107, 9 (2012). 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/293259
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Joanna Carey. Soil age alters the global silicon cycle[J]. Science,2020.
APA	Joanna Carey.(2020).Soil age alters the global silicon cycle.Science.
MLA	Joanna Carey."Soil age alters the global silicon cycle".Science (2020).