Silicon Cycling in Soils Revisited

Abstract

Silicon (Si) speciation and availability in soils is highly important for ecosystem functioning, because Si is a beneficial element for plant growth. Si chemistry is highly complex compared to other elements in soils, because Si reaction rates are relatively slow and dependent on Si species. Consequently, we review the occurrence of different Si species in soil solution and their changes by polymerization, depolymerization, and condensation in relation to important soil processes. We show that an argumentation based on thermodynamic endmembers of Si dependent processes, as currently done, is often difficult, because some reactions such as mineral crystallization require months to years (sometimes even centuries or millennia). Furthermore, we give an overview of Si reactions in soil solution and the predominance of certain solid compounds, which is a neglected but important parameter controlling the availability, reactivity, and function of Si in soils. We further discuss the drivers of soil Si cycling and how humans interfere with these processes. The soil Si cycle is of major importance for ecosystem functioning; therefore, a deeper understanding of drivers of Si cycling (e.g., predominant speciation), human disturbances and the implication for important soil properties (water storage, nutrient availability, and micro aggregate stability) is of fundamental relevance.

Silicon in the Soil–Plant Continuum: Intricate Feedback Mechanisms within Ecosystems

“…This section provides a short overview of the different forms of Si in soils and the main drivers for Si availability for plants. A more elaborate review of Si occurrence forms, speciation and cycling in soils is provided by Schaller et al [26] in another manuscript published within this Special Issue.…”

“…It is known that high pH Si fertilizers (such as fly ash or steel slag) decrease metal availability due to changing metal speciation and/or increased precipitation [93]. The bioavailability of aluminum may be reduced by Si fertilization owing to instant binding of Al to Si [26,94,95] to form hydroxyl aluminosilicates [96]. Such a co-precipitation with Si was also suggested for Cr, Cd, Pb and Zn [97][98][99].…”

“…This is because Si exports through harvested crops generally lead to loss of Si in agricultural plant-soil systems (i.e., anthropogenic desilication) [20,[323][324][325][326]. However, some agricultural practices might also increase Si availability in soils, e.g., human set fires [327], the application of Si-rich fertilizers [26], or liming (pH effect, [328]). On a global scale, about 35% of Si accumulated in vegetation is synthesized by field crops, and this proportion is going to increase with increased agricultural production within the next decades [256].…”

“…From their results, they concluded a weaker binding strength of Si (as compared to P and dissolved organic matter) to Fe oxides leading to Si mobilization by the input of P and dissolved organic matter. However, it should be kept in mind that the laboratory experiments of Klotzbücher et al [342] were performed under acidic conditions (pH 4), resulting in a potentially decreased binding strength of Si due to slowed polymerization of H 4 SiO 4 [26]. According to these findings, it can be assumed that mineral fertilization, as well as organic matter input by crop straw recycling, should increase Si mobilization in soils resulting in potentially higher Si accumulation in plant biomass [20].…”

“…Moreover, its uptake from the soil and eventual reincorporation into the soil through plant litter and herbivore feces also affects soil properties and Si cycling [19][20][21][22][23]. Thus, Si uptake, accumulation and cycling by plants is a key phenomenon in many ecosystems [12,21,[24][25][26], with direct and indirect implications for ecosystem properties and processes [1]. Here, we review the existing knowledge of Si in the soil-plant continuum, its roles in plant biology and ecology, in ecosystem processes, and the possible implications for various ecosystem services.…”