Soil processes drive the biological silicon feedback loop

Abstract

Summary 1.Soil is the primary source of plant silicon (Si) and therefore a key reservoir of the Si biological cycling. Soil processes control the stock of Si-bearing minerals and the release of dissolved Si (DSi), hence the Si fluxes at the Earth's surface. Here, we review the interdependent relationship between soil processes and the return of plant Si in soils, and their controls on the biological Si feedback loop. 2. Dissolution and precipitation of soil silicate minerals govern the bioavailability of Si. Plants affect Si biocycling through mineral weathering, root uptake, phytolith formation, return and dissolution in soil. Thus, soil processes and Si biocycling readily interact in soil-plant systems. 3. Rock mineral weathering and soil formation are driven by the five soil-forming factors: parent rock, climate, topography, age and biota. These factors govern Si fluxes in soil-plant systems since they impact both the mineral weathering rate and fate of DSi. The variability of soil-forming factors at a global scale explains both the soil diversity and high variability of the rates of Si cycling in terrestrial ecosystems. 4. Plants play a crucial role in soil evolution by promoting weathering and forming phytoliths (plant silica bodies). They thus act as Si sinks and sources. With increasing depletion of lithogenic (LSi) and pedogenic (PSi) silicates, the biological Si feedback loop progressively takes over the Si plant uptake from weatherable LSi and PSi minerals. With rising weathering, the soil becomes increasingly concentrated in phytoliths, phytogenic amorphous silicates (PhSi), which are constantly formed in plant and dissolved in soil. Paradoxically, the Si biocycling is thus more intense in soils depleted in primary LSi source. By converting soil LSi and PSi into PhSi, plants increase the mobility of Si in soil and alleviate desilication in the topsoil. Nonessential plant Si is therefore an essential link between mineral and living worlds. 5. The dynamics of Si in terrestrial ecosystems is thus largely governed by pedogenesis and its relationship with plant community and diversity. Consequently, the appraisal of soil constituents and processes is central to further understand their interaction with the biological Si feedback loop.

Biogenic silica accumulation varies across tussock tundra plant functional type

“…First, BSi likely accumulates in the organic layer; the more soluble nature of BSi compared to mineral silicates could lead to higher DSi concentrations in the top layer compared to deeper soil horizons. Moreover, relatively high concentrations of low molecular weight organic acids in the rhizosphere should drive down soil pH, which would also result in elevated DSi concentrations in top soil layers (Alfredsson, Clymans, Hugelius, Kuhry, & Conley, ; Cornelis & Delvaux, ; Drever, ).…”

“…The degree to which this “silica filter” (Struyf & Conley, ) influences export rates from terrestrial to aquatic systems varies enormously, with 20%–80% of DSi found in soil solution having passed through terrestrial vegetation (Clymans et al., ). In turn, shifts in vegetation land cover can alter the quantity of silica stored on land and change the rates of silica export to the coastal oceans (Carey & Fulweiler, ; Clymans, Struyf, Govers, Vandevenne, & Conley, ; Cornelis & Delvaux, ; Struyf & Conley, ).…”

Phytolith‐rich biochar: A potential Si fertilizer in desilicated soils

“…It is evident that, in highly weathered soils exhausted in LSi and depleted in PSi, the continuous loss of PhSi results in the increase in soil desilication. Adapted from Cornelis and Delvaux ()…”

The functional ecology of plant silicon: geoscience to genes

“…Deshmukh & Belanger (, this issue ) review molecular evidence of Si uptake, and Stromberg, Di Stillo & Zhaoliang (, this issue ) search for evidence of adaptive origins. Cornelis & Delvaux (, this issue ) examine the relationship between soil development and plant Si cycling, while Hartley & DeGabriel (, this issue ) review how Si uptake mediates the interactions between plants and their herbivores, focussing on natural ecological systems. Schoelynck & Struyf (, this issue ) interpret the findings from wetland studies, where structural function is best understood and Si accumulation varies with functional type.…”

“…Cornelis & Delvaux (, this issue ) examine both the influence of plants on soil weathering and conversely, the weathering status of soil on Si availability for plants. The authors argue that as soils develop, first lithogenic and pedogenic silicates are the source of Si for plants, but over time this source is depleted; instead, the Si in soils has mostly already cycled through plants and exists as phytogenic silicates.…”